Production of a fish taxocene in a Texas pond

Env. Biol. Fish. Vol. 2, No. 3, pp. 241-259, 1911

Production of a fish taxocene in a Texas pond

Fredrick V. Jones*, William D. Pearson & Lloyd C. Fitzpatrick

Department of Biological Sciences, North Texas State University, Denton, Texas 76203, USA

Keywords: Production, Fisheries, Population dynamics, Energetics, Fishes, Competition

Synopsis

Production rates of seven species of fishes were determined for one year in a 0.95 ha pond. Total annual production of all fishes in the pond was 209 g m-2y-1 (after the subtraction of 16.2 g m-*y-r of negative production). Over 90% of the production occurred in the months of June through Sep- tember, with young-of-the-year fishes contributing 84% of the total. When Y of Y fishes shifted to benthic diets in the spring, the small biomass of benthos available could not sustain the large biomas of fish. Severe competition for the available food then resulted in decreased production rates for the entire fish taxocene.

Introduction

Ivlev (1945) defined production as ‘...the sum of all organic matter added to the stock of a product (or other defined organic unit) in a unit of time, regardless of whether or not it remains alive (i.e. part of the stock) at the end of that time’.

Production dynamics of freshwater fish populations were first investigated in detail by Ricker (1946) and Allen (1950) who used Ivlev’s definition in dev- eloping the first methods for calculating fish production. Interest in production studies was greatly stimu- lated by the publication of IBP Handbook No. 3 (Ricker 1968). Methods for estimating production and published data on production rates in natural waters were reviewed by Le Cren (1972).

Most published reports on fish production in natural waters deal with a single species of fish (Gerking

* Senior author’s present address: Department of Wildlife and Fisheries Sciences, Texas A&M University, College Sta- tion, Texas 17843, USA

Received 8.11.1976 Accepted 9.6.1977

1962, Petrosky & Waters 1975) while a few have been concerned with two to six species (Backiel 1971, Mann 1971, Mathews 1971). Regier & Hender- son (1973) have argued that much of the future the- ory and management practices of fisheries science will be directed at the community taxocene level. As far as we are aware the only reports on the annual production of an entire fish taxocene in natural situation are those of Goodnight & Bjornn (1971) in a salmon stream, Balon (1974) in Lake Kariba, and Chadwick (1976) in a small, infertile lake in Ontario, Canada.

This paper presents findings on the production dynamics of a fish taxocene in a small pond ecosystem on the North Texas State University Campus. The study was part of a trophic-dynamic study conducted on the pond from 1973 to 1975, the overall objective of which was to model energy flow in the pond.

Methods and materials

The pond is located on the North Texas State Univer- sity Golf Course within the city limits of Denton, Texas. It has a surface area of 0.95 ha, a volume of 14,150 m3, and a mean depth of 1.5 m. The pond was constructed in 1947 to supply irrigation water for the golf course, and drawdowns in summer months sometimes removed 70% of the total storage volume. Natural runoff was supplemented by water purchased from the city of Denton and from a well 30 m southeast of the pond. During this study, prob- lems with the pumping mechanism caused drastic fluctuations in water level in summer and fall months. By late fall, 1973, the pump was working properly and drawdowns were less severe.

The Texas Parks and Wildlife Department stocked

241

bluegill (Lepomis macrochirus), largemouth bass (Micropterus salmoides), and channel catfish (Ictah- rus punctatus) in unknown numbers in 1948. Channel catfish were last taken from the pond in 1970, when two 4.5 kg specimens were captured, marked, and returned to the pond. The superintendent of the golf course added seven adult white crappie (Pomoxis annularis) to the pond in 1970. Other introductions have probably been made by fishermen releasing bait fishes into the pond.

During this study there were seven species of fish present in the pond: golden shiner (Notemigonus crysoleucas), black bullhead (Ictalurus melas), bluegill, longear sunfish (Lepomis megalotis), green sunfish (Lepomis cyanellus), largemouth bass, and white crappie.

Fish sampling

Fish were sampled (from 1 June 1973 to 1 June 1974 on a biweekly (May-September) or monthly (Octo- ber-April) schedule) with a 61 m by 2.44 m by 0.6 mm mesh seine. The seine was set, from a row- boat, in the shape of a semicircle with the shoreline as the base and then pulled to shore by hand. The mean number of seine hauls per sampling date was 4.1 (Range = 3-9) and was determined largely by the number of fishes captured in the first three hauls (four hauls covered l/4 of the pond area). Ten sampling stations were established on the pond for the use of all investigators employed on the ecosystem analysis (Fig. 1). Seine hauls were usually made at stations 1, 3, 4, and 7. Hauls were occasionally made at each of the other six stations, except station 5.

All fish captured in each seine haul were separated by species and held in galvanized tubs. (A random subsample of adults and young-of-the-year (Y of Y, lo-20 specimens each) of each species was preserved in 5% formalin and weighed and measured in the laboratory within 24 h after capture). Each fish re- maining (age class I +) was weighed to the nearest gram, measured to the nearest millimeter (total length), marked for later recognition, and returned to the pond. When catches of a single species exceeded 40-50 individuals and the individuals could be easily separated into cohorts by inspection, group weights for each cohort were taken. Each species was then separated into cohorts by a length-frequency analysis using all measurements combined. Adult white crappie, largemouth bass, and black bullhead were marked with spaghetti tags inserted under the dorsal fin with

a tagging gun (Floy Tag Co.). Adult sunfish and golden shiner were marked by clipping the right .pelvic fin. Young-of-the-year sunfish were so fragile that all of those captured were either preserved or returned to the pond unmarked. Largemouth bass and black bullhead Y of Y were marked only after they exceeded 40-50 mm (December) in length to prevent excessive marking mortality. White crappie Y of Y and yearlings (age class I) were marked during the summer months; however, none were ever recaptured and we observed some immediate mortality in these marked young crappie. Therefore, marking of young crappie was discontinued at the end of the summer. All fish captured and not preserved were held live’in a bag until the end of the sampling day and theti returned to the pond.

In February of 1973 marked sunfish were already in the pond as a result of projects conducted by ecology classes at North Texas State University, and these early marks were used to determine densities of sunfish during the sampling period from June 1973- June 1974. (On 23 July 1973 seven seine hauls were made in addition to the regular monthly hauls in an attempt to catch and mark as many fish as possible.

Fig. 1. Location of sampling stations (l-10) with 0.30 m contour intervals; golf cotuse pond.

242

On 14 August an additional six hauls were made sole- ly to catch and mark large numbers of fish.)

Density estimates were made by two general methods. The primary method was the capture-mark- recapture technique of Schumacher-Eschmeyer (Ricker 19.58) by which estimates were made for adult bluegill, green and longear sunfish, adult black bullhead, all cohorts of golden shiner and large Y of Y black bullhead and largemouth bass. Density estimates for those cohorts which could not be marked due to resultant mortalities were made with a modi- tied indirect or area1 method (Regier & Robson 1967). In this method the number of fish captured in a seine haul is multiplied by the surface area of the pond over the area of the seine haul. Of these two methods, the indirect always gave much lower estimates (usually 20-45% lower), probably due in part to seasonal movements of fish to the center of the pond where they were inaccessible to seining, and in part to mortality associated with marking, but especially to the escape of fish from the net. Setting the seine took 5-15 min and resulted in disturbances due to wading, rowing, and paying out the net. Some fishes may have learned to associate these disturbances with an unpleasant experience and moved from the area before the set could be completed. Beukema & deVos (1974) and Buck & Thoits (1965) found that marked fishes could learn to avoid seines, resulting in underestimation in mark-recapture experiments. An unknown number of larger fish also jumped over the float-line (in one instance a largemouth bass, estimated at 1.5 kg, jumped over both the float-line and the meter-wide boat used to set the net at approximately eye level of the startled investi- gator seated in the boat). Other fishes (particularly bullheads) probably escaped by sqeezing beneath the lead-line as it was elevated slightly in passing over stones and debris.

For these reasons, we believe the density estimates obtained in seining a known area will usually be low, that the error will increase with increased effort, and that its magnitude will vary with the habits of the fishes. White crappie tended to inhabit central areas of the pond at all ages and their numbers were probably underestimated to the greatest extent. There- fore, we assumed that the mark-recapture estimates, where available, were most accurate.

From those cohorts of green sunfish for which both estimates were available, monthly correction factors were obtained as follows:

- Cr = N SE/Ni 2

where NS~ = Schumacher-Eschmeyer estimate, Ni = Indirect estimate, and C, = Correction factor applied to each indirect estimate. The correction factor for green sunfish was used because the greatest number of mark-recapture estimates and the indirect estimates with the narrowest confidence limits were those for adult green sunfish. Green sunfish were known to inhabit inshore areas and defend solitary territories (Werner & Hall 1976) suggesting that our seine hauls would provide a reliable indicator of the density of this species. In this manner corrected estimates were calculated for white crappie (all cohorts), Y of Y sunfish, Y of Y black bullhead before Decem- ber, Y of Y largemouth bass before December, and for the adults of those species which were seldom recaptured (bullhead and longear sunfish). For the adults of those species where both mark-recapture and indirect estimates were available we have reported only the mark-recapture estimates.

Growth, mortality, and production rates

Instantaneous rates of growth and mortality, and monthly production rates were calculated indirectly, using Chapman’s (1968) methods. If a species spawned in May, 1973 (largemouth bass and white crappie) production was calculated from 1 June 1973, but if the 0 cohort was spawned during the sampling period (after 1 June 1973, the initial weight of a 0 cohort member was considered to be the egg weight (values from eggs stripped from ripe females). On a plot of the log of mean individual weight vs. time, we extrapolated back to the date when the mean weight of an individual equalled that of an egg (Pearson & Kramer 1972). Initial population size (total spawn) was similarly obtained by extrapolating back on a plot of the log of numbers vs. time to the date when the egg weight was obtained. Production was calculated from that time forward. The percentage of Y of Y production calculated with the extrapolated values ranged from 8% for crappie to 22% for bullhead.

Creel census

An active creel census employing a stratified monthly sampling regime was conducted as described by Best & Boyles (1956). At least two week days and one weekend day of each week were sampled each month. Using this method, at least one third of each month was sampled. Depending on the number of days

243

sampled, appropriate values were used to calculate monthly, weekly, and weekend fishing and catch rates.

Fish collected for research purposes (i.e. stomach analysis) were summed separately from the calculations of angler yield.

Physical parameters

Selected mean monthly chemical parameters of the pond were determined by Kelly (1975) using methods described in APHA (1970). Both ambient air and water temperatures were monitored with a two-pen Foxboro thermagraph. Secchi disk readings were taken on each sampling date between 1200 and 1430 CST at station 6 and volume and surface area were determined.

Results

Physiochemical parameters

Shallow ponds in the southwestern United States are subject to wide seasonal variations in temperature. The study pond temperature ranged from a maximum of 32.0” C in August to a low of 4.0” C in early Janu- ary. The pond froze over completely for several days in January, an unusual occurrence in this area.

Turbidity fluctuated widely throughout the year (Secchi readings ranged from a minimum of 8 cm in March to a maximum of 97 cm in October; mean = 29 cm), and was usually low during extended periods of rainfall, while periods of no ram showed the highest turbidity readings (September-October). During periods of little rainfall large amounts of dust from the adjacent well-traveled road may have settled in the pond.

Fluctuations in water level were drastic as a result of watering the greens and the subsequent refilling by rainfall or well water. The highest volumes observed were in mid-June 1973 (18,000 m3) and early Sep- tember (18,800 m3). Three major reductions in volume occurred. The first, (to 14,200-13,300 m3) from mid-July until late August, the second (to 10,900 m3) for a short time in early January, and the third beginning in mid-March to a low of 11,500 m3 in early April. In general, the volume of the pond declined gradually throughout the study. An analysis of water quality parameters in the pond (Kelly 1975) showed total alkalinity to be 2-3 times that of other

surface waters in northcentral Texas, and fairly con- stant throughout the year. Concentrations of phos- phates and nitrates were also higher than those of surrounding surface waters but fluctuated widely in response to season and water level associated with pumped well water.

Pond biota

Phytoplankton, zooplankton, and benthic organisms in the pond were studied by others working on the pond study and data supplied by them were used to help interpret some of the observations of fish population dynamics (Kelly 1975, Smith 1976, Benson 1976). Zooplankton populations (primarily Dzizphano- soma, Daphnia, Chydorus, Bosmina, and Diaptomus) in the pond fluctuated widely throughout the year (Appendix 1). Population peaks of 7.5-9.5 x log zooplankton were observed in June 1973 and March 1974. Zooplankton densities were lowest in August (2-4 x log) and remained low during fall and winter months (Smith 1976).

Benthos populations (primarily chironomid larvae, Ephemeroptera, Odonata, and oligochaetes) exhibited different seasonal fluctuations from the zooplankton populations. Total numbers in the entire pond were low (4-9,000) in summer months and increased to 34-45,000 during winter months (Appendix 1).

Two turtle species (Pseudemys scripta and Trio- nyx spinifer) inhabited the pond along with two large groups of frogs (Rana catesbeiana and Acris crepitkns). Although the turtles and frogs were important in the overall energy flow of the pond they did not appear to have important direct effects upon the fish stocks.

Schumacher-Eschmeyer density estimates

A total of 16,660 fish were collected, weighed, and measured. Bullheads, especially Y of Y, were the most abundant fish in the catch with a total of 9,015 individuals. Second was white crappie (2,464), followed by bluegill (1,649) largemouth bass (1,643), green sunfish (1,279), golden shiner (526), and longear sunfish (84). In total, 392 adult bullhdad, 1,255 Y of Y of the first spawn and 610 Y of Y bullhead of the second spawn were marked. Seven hundred and eighty-eight bluegill adults were marked along with 697 green sunfish, 238 Y of Y largemouth bass, 259 golden shiner, and 40 longear sunfish.

At least some mark-recapture data were collected

244

Tuble 1. Mark and recapture population estimates (with confidence limits) of centrarchids in the pond

bluegill green sunfish longear sunfish largemouth bass Date adults adults adults cohort 0

Number 95%C.L. Number 95% C.L. 1 Number 95% C.L. Number 95% C.L.

2-23-73 3,335 4 l-73 4,247 6- 2-13 4,810 6-21-73 5,305 7-14-73 5,981 7-23-73 4,644 8- 4-13 4,940 8-14-73 4,697 8-18-73 3,995 9- 8-73 4,085 9-22-73 4,284

lo- 3-73 4,231 ll- 3-73 4,330 12- l-73 4,194

l- 9-14 4,182 1-19-74 4,166 2- 2-74 4,200 3- 2-74 4,106 4- 6-74 4,154 5- 4-74 4,079 5-18-74 4,060 6- l-74 4,076

16,132 +9,527 4,644 +3,384 8,657 *7,403

10,912 +6,501

7,126 +7,280 5,226 *4,366 4,659 *2,994

4,328 -+2,238 3,931 21,958 3,822 +1,746 3,773 +1,611 3,320 ~1,234 2,893 + 822 2,506 * 559 2,528 f 475 2,633 f 444

*1,781 Al,769 +2,338 53,019 +2,541 *2,599 ?2,062 +1,735 +1,617 ~1,622 +1,405 +-1,406 ?1,250 +1,178 21,135 +1,112 +1,023 + 995 + 932 f 900 f 885

221 f 24

164 ~106 111 f 69

141 + 77 840 632 358

130 f 58 259 115 5 51 255

284 309 335

138 + 60 360

5136 ~256 +201 f 83 f 38 f 36 f 34 f 43 + 47

Table 2. Mark and recapture population estimates (with confidence limits) of bullhead and golden shiner in the pond

black bullhead

Date adults cohort 0, cohort 0, golden shiner adults

Number 95% c. L. Number 95% c. L. Number 95% c. L. Number 95% C. L.

6- 2-73 6-21-73 7-14-73 7-23-73 8- 4-73 8-14-73 8-18-73 9- 8-73 9-22-73

lo- 3-73 ll- 3-73 12- l-73

l- 9-74 1-19-74 2- 2-74 3- 2-74 3-15-74 4- 6-74 4-23-74 4-26-74 s- 4-74 5-18-74 6- l-74

1,430 f 60 126

f 60 3,708 -+3,101 3,934 +1,391 2,674 +1,124 2,244 +1,177

113

347 +355

641 k693 610 2469 861 k569 925 +561

1,051 t463 1,007 *469

966 +460 951 +370 754 +221

2,302 + 957

1,370 ?: 45

3,573 +3,456

5,824 *4,114 7,202 +4,648

12,769 t8,613 11,094 t2,655

8,422 t1,267 8,315 +1,035

3,149 *I,211 3,167 ~1,061 1,078 f 127 743 A193

2,751 f 967 3,215 f 938 2,315 f. 610

2,922 + 918 780 *204 3,723 + 665 879 +255 4,324 *1,205 842 *217

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for each cohort of fish in the pond except white crappie and Y of Y sunfish. Tables 1 and 2 present the Schumacher-Eschmeyer density estimates with 95% confidence limits.

One of the most completely marked cohorts of fish was Y of Y largemouth bass. Although marking of this cohort did not begin until January, an estimated 50% of the cohort was subsequently fin-clip- ped. The effects of fin-clipping on growth of Y of Y bass was slight since Student’s t tests showed no significant differences (0.05 level) in mean weights of marked and unmarked bass later in 1974. The proportion of other species marked was so low that marking is unlikely to have affected growth or mortality rates.

Production of the fish taxocene

Black bullhead dominated the fish taxocene in numbers and total production. At the beginning of the year 3,671 yearlings and 1,128 adult bullhead were present and they produced 360,000 Y of Y. The total annual production of bullhead in the pond was 1,277 kg (Table 3). Although there were only a few, perhaps 2-12, adult crappie in the pond they pro-

duced 7,771 yearlings at the beginning of the year and 23,617 Y of Y during the year. The total annual production of white crappie was 273 kg. The density and production of largemouth bass were similar to those of crappie although there were very few yearling bass present. From an estimated 29-50 adult and yearling bass (only sever. yearlings were taken during the study) 76,924 Y of Y were produced. Total bass production during the year was 322 kg.

Bluegill and green sunfish stocks seemed to have greater stability than bullhead, crappie, or bass as indicated by age class distributions. Adult numbers for both sunfishes were typically 4-5,000. Yearling numbers at the beginning of the year were higher for green sunfish (6,671) than for bluegill (374) while numbers of Y of Y were nearly the same (S,OOO-8,650). Annual productions for these two sunfishes (44 and 52 kg) were 14 to 20% those of crappie and bass.

There were only 2,000-3,000 individuals in both the golden shiner and longear sunfish. The shiners did not spawn successfully in 1973, while the age class distribution of the longear indicated a small, but stable stock. Annual production for each species was less than 12 kg, about 4% that of bullhead.

Table 3. Total production (kg) of the seven species of fish in the pond; June 1973 - June 1974

golden black largemouth white shiner bullhead bluegill green longear bass crappie CP

612173 1.92 49.15 4.11 -10.07 0.70 53.05 38.02 137.48 6121173 2.33 229.04 10.78 -2.36 0.75 33.62 44.31 3 18.47 l/14/13 2.61 294.28 16.20 13.87 0.21 104.87 63.14 495.84 814173 1.10 303.88 -6.15 7.16 0.63 29.33 47.99 383.94 8118173 1.11 136.09 8.93 8.00 0.60 61.27 -17.16 198.84 918173 2.25 116.78 -10.01 6.21 0.40 11.83 -11.39 116.07 9122173 -1.27 94.43 10.41 -1.86 0.19 22.63 25.20 149.73 1013173 0.83 85..04 18.12 -0.94 0.48 12.73 66.74 183.00 1113173 -0.63 30.20 8.61 7.36 0.41 -4.65’ 9.49 50.79 12/l/73 -0.23 -3.89 -14.71 12.04 0.50 -3.16 6.30 -3.15 l/9/74 -0.12 -21.58 20.19 -2.26 0.16 0.42 11.30 8.11 l/19/74 0.36 13.37 -24.16 -.99 0.25 -0.22 0.64 -10.75 212174 1.33 -26.12 9.80 0.58 0.59 0.30 -10.97 -24.49 312174 1.26 7.21 21.41 1.44 0.03 -0.28 10.39 47.52 416114 -1.46 -49.13 -36.89 -0.09 0.23 -1.11 -12.56 -101.01 514174 -0.55 15.97 23.46 7.29 0.07 .96 2.90 50.10 5118174 0.93 1.69 -13.94 -1.23 -0.04 0.31 -2.08 -14.36

zp (+) 16.03 1,377.73 158.08 63.95 6.26 331.32 327.02 2,280.39 SP C-1 4.26 100.72 105.86 19.80 0.04 9.42 54.16 294.26

YZP 11.77 1,277.Ol 52.22 44.15 6.22 321.90 272.86 1,986.13

g mm2 y-l 1.2 134.4 5.5 4.6 0.7 33.9 28.1 209.1 % of total 0.6 64.3 2.6 2.2 0.3 16.2 13.7

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Table 4. Production values for black bullhead

Date

612173 6121173 7/14/73 814173 8/18/73 918173 9122173 10/3/73 1113173 12/l/73 l/9/74 l/19/74 212174 312174 416174 514174 S/18/74 6/l/74

Mean individual

weight w cd

0.10 0.29 3.00 4.34 6.02 7.12 9.93

13.36 17.13 18.00 17.11 15.50 17.13 16.06 19.46 18.90 18.94 18.72

Cohort 0, Cohort 0,

Mean Stock Production individual Stock Production

number weight number A Pe(kg) = (g) N P (kg)

340,000 264,596 36.76 246,909 189.03 182,612 274.21 103,845 258.57 0.01 20,000

33,845 102.68 0.82 18,120 0.08 23,076 79.81 1.54 17,693 9.65 16,921 68.02 2.78 13,558 16.34 14,999 56.18 4.08 11,193 14.53 13,461 12.94 5.23 9,231 11.39

9,613 -12.22 6.00 7,539 6.58 8,844 -16.14 5.46 6,847 -4.21 8,075 13.63 5.72 6,308 1.73 6,921 -8.87 4.44 5,347 -9.03 6,152 21.42 6.27 4,462 8.16 5,383 -3.35 8.34 3,712 8.01 4,614 5.39 10.19 3,501 6.23 4,230 -0.98 10.22 3,385 0.10

+1,118.64 +82.80 -41.56 - 13.24

x = 1,077.08 EP = 69.56

Date Mean

individual weight w (g)

Cohort I

Stock number

fi

Production

p ocg)

Mean individual

weight w Cd

Cohort II+

Stock number

fi

Production

P (kid

612173 40.77 3,671 6121173 41.54 3,557 l/14/73 50.77 3,437 814173 56.15 3,390 8118173 67.69 3,304 918173 75.38 3,209 9122173 81.54 3,076 10/3/73 85.38 2,947 1113173 91.15 2,804 12/l/73 94.23 2,671 l/9/74 95.77 2,576 l/19/74 95.77 2,548 212174 95.38 2,538 312174 93.07 2,437 416174 84.23 ’ 2,390 514174 66.92 2,380 5118174 65.00 2,285 6/l/74 65.77 2,228

2.84 29.67 17.63 35.92 24.00 18.99 11.59 16.38

8.59 3.93 0

-1.02 -6.09

-22.98 -46.03

-4.67 1.66

+171.20 -80.79

ZP = 90.41

118.46 127.69 138.08 140.77 151.54 162.69 173.07 171.15 168.46 164.61 161.15 159.23 157.69 154.23 155.38 140.77 162.31 164.61

1,128 1,036

936 898 862 830 802 774 722 680 638 622 604 564 512 448 404 344

10.15 10.34

2.44 9.39 9.33 8.33

-1.52 -2.05 -2.72 -2.18 -1.23 -0.97 -2.13

0.61 -7.76

9.02 0.91

+60.52 -20.56

EP = 39.96

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Total positive production of all fish in the pond was 2,139.9 kg (Table 3) offset by 153.8 kg of negative production, giving a net production of 1,986.l kg. Most (95.1%) of the total production occurred during the summer and fall of 1973. Most of the negative production occurred during winter and spring months of 1974. Young-of-the-year fishes accounted for 83.8% of the total net production in the pond. Production rates of Y of Y fish and adults were both high in mid-summer, decreased in September, and increased again in October-November. Both were low during the winter months, while in the spring Y of Y production rates remained positive and the adult rates often became negative.

Black bullhead

There were four cohorts of bullhead in the pond. The 0 cohort consisted of two groups (Oi and Os), one spawned in late May before sampling began, the other in August. Four to six swarms of the May group were seen in June, but none were seined until August.

However, several hundred (mean weight 0.3 g; Ta- ble 4) were dip netted in mid-June. Rapid growth occurred through November and peaked in December at 18 g. For cohort I and II+ growth was slow but followed a pattern similar to that of cohort 0, increasing steadily to January, declining again to May and increasing again into June 1974. Adult bullhead (II+) were the only cohort in the pond in which the winter decline in weight was accompanied by a decline in mean length. This reduction in size probably resulted from size-specific mortalities imposed by anglers.

There were an estimated 340,000 Oi bullhead compared to only 20,000 0s individuals. Annual mortality rates of the Oi and 0s classes were high (98.8 and 83.0%). The annual production of Y of Y bullhead was the highest for any year class of fish in the pond, total net production being 1,077.l kg for the Oi class and 69.9 kg for the 0s (Table 4). Bullhead Y of Y net production made up 57.7% of the total Y of Y net production.

Annual mortality rates of yearling and II+ bullhead were much lower (39.3 and 69.5% respectively) than those of Y of Y. The combined production of

Table 5. Production values for white crappie, cohorts 0 and I

Cohort 0 Cohort I

Mean Mean Date individual Stock Production individual Stock Production

weight number weight number w k) A p 0%) w (d A p 0s)

6/21/13 0.65 6121173 1.50 7/14/13 5.48 8/4/73 7.85 g/18/73 9.00 9/8/13 9.21 9122113 10.24 1013173 10.54 1113173 17.70 12/l/73 18.12 I/9/14 19.50 l/19/74 22.00 212114 20.17 312174 18.97 416114 24.35 s/4/74 21.23 5/18/14 23.44 6/l/14 24.26

23,611 13.11 20,231 15,694 13,925 12,308 10,924 9,502 8,385 5,848 4,116 2,924 2,616 2.231 1;540 1,232

924 847 693

40.20 31.06 15.01

3.25 10.05

2.11 45.87

2.31 5.51 6.90

-3.28 -4.12

7.26 -4.09

1.95 0.69

+185.94

20.01 7,111 23.48 7,616 24.00 7,340 28.86 7,188 33.80 7,033 31.03 6,800 28.00 6,431 31.69 5,926 35.44 4,047 37.25 2,726 31.50 1,817 40.00 ' 1,526 42.67 1,340 37.84 1,126 40.78 941 32.67 718 34.00 602 29.64 478

24.91 4.11

32.68 32.98

-20.41 -21.44

22.43 20.87

7.18 0.79 4.40 3.92

-6.85 3.13

-8.47 0.95

-2.77

+158.35 -59.94

TZP = 98.41 -11.49

BP = 174.45

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adult (I+) bullhead was the highest (130.4 kg) of any adult fish in the pond, accounting for 36.8% of the adult production, but only 6.6% of the total production of all age classes combined (Table 4).

White crappie

There were three cohorts of white crappie in the pond. The 0 class was spawned in late May, and when sampling began in June, Y of Y crappie weighed 0.7 g. Yearling crappie were present in the pond in high numbers but were sexually immature, so larger adults must have been present in the pond. One adult crappie weighing 1.4 kg was caught by a fisherman in early July and another marked crappie weighing 0.9 kg was captured in June 1973. The spawn of crappie consisted of an estimated 23,617 individuals in June 1973 and was probably produced by more than one female. A few large crappie were marked late in 1972 and early 1973 by ecology classes but only the one mentioned previously was recaptured. It is probably that several other large crappie were present in the pond but were not caught in the seine.

Growth rates of Y of Y crappie were initially high, and they achieved a mean weight of 17.7 g by November, but growth during winter months was slow and by June 1974 Y of Y crappie were 24.3 g (Table 5). Yearling crappie had a slower growth rate, increasing from 20.0 g in June 1973 to 42.7 g by February. After February the mean weight of year-

COHORT 0 A

COHORT I 0

I I 1 I I / I I I t I 1 I

JJASONDJ FMAMJ

MONTHS Fig. 2. Mean lengths, with 95% confidence limits, of white crappie cohorts 0 and I; June 1973 to June 1974.

lings declined markedly until by June 1974 the mean weight of yearling crappie was 29.6 g, only 5.4 g more than that of Y of Y, and confidence intervals about the mean weights overlapped. This loss in weight could have resulted from either starvation (which we shall call negative production) or size- specific mortalities which resulted in higher mortalities for larger individuals. A comparison of the mean length of crappie of cohorts 0 and I between sampling periods (Fig. 2) shows clearly that the mean length of individuals in both cohorts declined little, if it all, during the winter. Therefore, we attribute the decline in mean weight during winter months to starvation rather than size-specific mortality. Similar comparisons of mean length versus time were made for other fishes, and winter losses of weight were never accompanied by significant declines in mean length (with the exception of adult bullhead). Yearling crappie of 1973 were laden with eggs by June 1974 and probably delivered a small spawn.

Mortality rates of Y of Y crappie were high, and of 23,617 Y of Y present in June 1974, only 4,116 survived to January 1974, and by June 1974 only 693 remained (annual mortality = 97.1%). Yearling crappie had lower mortality rates, especially during the summer of 1973. Of 7,771 individuals estimated in June 1973, 6,800 survived to September, a mortality of only 12.5%. After September, mortality increased, leaving only 478 survivors in June, for an annual mortality of 93.9%.

The annual net production of Y of Y crappie was 174.5 kg. Yearling crappie had the highest net annual production (98.4 kg) of any yearling cohort in the pond. The production of all cohorts of crappie combined (272.9 kg) was exceeded only by that of black bullhead and largemouth bass.

Largemouth bass

There were only two evident age classes of largemouth bass in the pond. The 0 class was spawned in late May and consisted of an estimated 76,942 individuals (Table 6). Although 11 adult bass weighing 0.3-16. kg had been marked between October 1972 and February 1973, only 9 adult bass (2 of them recaptures) were captured during the study. There ap- peared to be at least two size classes of larger bass. The smaller group (40-80 g) was probably made up of yearlings from a relatively unsuccessful spawn in 1972. Another 11 bass captured by fishermen ranged in weight from 680-l ,927 g. If these catches are

249

summed, at least 29 adult bass were captured and probably more were in the pond.

Young-of-the-year bass had high growth rates during the summer of 1973, and by October the mean weight had reached 11.5 g. Growth ceased during winter and by May 1974 weight losses had reduced the mean weight to 8.5 g. A small increase to 12.5 g occurred by June 1974. Mortality rates of Y of Y bass were the highest in the pond. From an estimated June population of 76,924, only 7,694 survived in October and mortality remained high through the rest of the year, leaving only 336 individuals in June 1974, a total annual mortality of 99.6%. Young-of-the-year bass had the second highest annual net production (331.3 kg) of any cohort in the pond (Table 6).

Sunfishes.

Each of the three sunfishes produced comparatively small spawns (8,000 bluegill, 8,650 green sunfish, and 2,250 longear sunfish; Tables 7, 8, and 9). Mortality rates of Y of Y sunfishes were not as high as those of Y of Y of other species (range = 89.8-90.5%). Year- ling sunfishes, like those of bass, were present in very

Table 6. Production values for largemouth bass, cohort 0

Mean Stock Date weight number Production

w cd fi P 04

612173 6121173 l/14/73 8/4/13 8/18/13 918113 9122113 1013173 1113173 12/l/73 l/3/74 l/19/74 212114 312114 416174 s/4/14 5118174 6/l/14

.35 2.29 2.82 5.43 6.15 8.47 9.16

11.46 13.21 12.07 11.00 11.50 11.22 11.89 11.20

8.50 11.72 12.64

76,924 70,155 56,200 44,200 3 1,169 18,082 11,078 7,694 3,704 2,848

850 710 457 393 359 341 343 336

53.05 33.62

104.87 29.33 61.21 11.83 22.63 12.73

-4.65 -3.16

0.42 -0.22

0.30 -0.28 -1.11

0.96 0.31

+331.32 -9.42

XP = 321.90

Table 7. Production values for bluegilI sunfish, cohorts 0, I, and II+

Date

- MWI Stock

weight number Production w (9) N P 0%)

1013173 1113173 12/l/73 l/9/74 l/19/74 212174 312174 416174 5/4/14 $18/74 6/l/74

612173 6121173 7114173 814113 S/18/73 918113 9122173 1013173

z;:;;: l/9/74 l/19/74 212174 313174 416174 514174 5118174 6/l/74

b/2/73 18.74 b/21/73 19.51 7114113 21.52 8/4/13 24.96 8/18/13 24.02 918173 25.98 9122173 23.98 1013173 26.55 1113173 31.20 12/l/73 32.90 l/9/74 29.20 l/19/74 34.50 212174 29.22 312174 31.33 4/b/74 38.78 514174 29.88 5/m/74 36.67 6/l/74 32.50

.Ol 8,000 .40 6,000 0.04

.75 4,652 1.53

1.20 3,576 1.64

1.27 3,076 0.24

.92 2,652 -1.22

1.82 1,918 1.67

2.43 1,361 1.01

3.65 1,019 1.36

4.10 899 0.43

6.82 842 1.91

9.43 374 374 10.00 333 333 0.21 0.21

13.25 282 282 0.94 0.94

16.00 260 260 0.71 0.71

10.33 241 241 -1.79 -1.79

13.00 224 224 0.58 0.58

9.00 208 208 -1.06 -1.06

9.00 194 194 0 0

10.00 166 166 0.18 0.18

11.36 139 139 0.21 0.21

11.31 116 116 -0.01 -0.01

11.39 106 106 0.01 0.01

11.47 98 98 0.01 0.01

11.40 80 80 -0.01 -0.01

13.85 66 66 0.18 0.18

16.50 58 58 0.16 0.16

20.69 48 48 0.22 0.22

25.00 42 42 0.19 0.19

Cohort 0 -

+9.83 -1.22

ZP = 8.61

Cohort I

+3.60 -2.87

ZP = 0.73

Cohort II+

5,209 5,095 3.90

4,814 9.84

4,633 15.49

4,466 -4.36

4,333 8.35

4,228 -8.95

4,171 10.41

4,133 17.90

4,114 6.87

4,057 -16.34

4,019 19.94

3,975 -22.95

3,918 8.14

3,823 26.28

3,709 -38.41

3,652 22.81 -16.04

+149.93 -107.05

ZP = 42.88

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Table 8. Production values for green sunfish, cohorts 0, I, and II+

Date Mt%ll Stock

weight number Production = (g) N p cd

8/15/13 0.01 Y/8/13 0.41 Y/22/13 0.60 1013173 0.77, 1 l/3/13 1.07 12/l/73 0.83 l/9/74 1.41 l/19/74 i' 1.68 2lU74 1.08 312174 2.74 416174 1.97 5/4/M 2.68 S/18/74 3.54 b/1174 4.07

/ 6/2/d 6121173 7114173 814173 8jli173 Y/8/73 Y/22/73 10/3/73

::\:\77; l/9/74 l/19/14 212174 312174 416174 s/4/14 S/18/74 6/l/74

612113 19.18 6121173 17.31 7114173 16.31 a/4/73 18.39 8118173 22.08 Y/8/73 24.31 9122173 25.54 1013173 24.53 11/3/13 23.54 12/l/73 23.43 l/9/74 25.08 l/19/74 26.54 212174 27.47 312174 29.08 416174 29.38 s/4/74 26.30 S/18/74 27.53 6/l/74 30.23

7.52 7.23 7.53 9.23 8.45 8.93 9.46 9.37 9.23

11.73 15.00 13.00 13.16 11.43 13.00 11.50 14.51 13.16

Cohort 0

8pJ ,-7,550 0.02 7,050 1.19 6,842 1.07 5,671 1.75 1 -^n

4, ,LLI 1.53

3,842 2.07 3,437 0.95 9 nnc -2.47 J,U,'J 2,361 3.14 1,709 -2.13 1,209 1.05 . ,..A 0.90

Cohort I

6,671 5,899 4,747 4,266 3,880 3,557 3,247 3,057 2,766 2,576 2,418 2,361 2,323 2,228 2,171 2,114 2,095 2,076

+14.12 -4.60

CP = 9.52

-1.90 1.66 7.30

-3.45 1.79 1.82

-0.30 -0.44

6.09 7.44

-5.14 0.39

-4.27 3.30

-3.47 5.70

-2.97

+35.49 -21.94

EP = 13.55

Cohort II+

4,190 3,899 -8.17

3,361 -4.02

3,133 6.57

2,899 10.61

2,690 6.19

2,475 3.20

^ ^^< -2.63 L,Lc55 1,937

-2.25

1,633 -0.26

< IIn 2.53 I,,>‘ 1,228 1.93 1,114 1.09

890 1.71 709 0.27 ^ ^^ JO, 500 0.69 A11 1.29

+38.41 -17.33

ZP = 21.08

low numbers, indicating that the spawns of 1972 had not been successful. Sunfishes of age classes II+ were present in moderate numbers (5,209 bluegill, 4,190 green sunfish, and 158 longear sunfish). Total annual net production of the sunfishes ranged from 52.2 kg for bluegill to 6.2 kg for longear sunfish.

Golden shiner

The golden shiner population was next to the smallest in the pond. No spawn seems to have occurred during the study; however, Y of Y shiner are small and might have escaped through the mesh of the seine. Shiners were rather evenly divided into age classes I, II, and III+. The III+ class were mostly 3- and 4-year-olds with an occasional S-year-old. All cohorts exhibited moderate growth rates and moderate mortality rates (62.6-71.1%). Total annual net production of shiners was only 11.8 kg (Table 9).

Relative growth performance in 1972 and 1973

A comparison of growth rates of fishes in the pond can be made between the study year (1973) and the previous year by comparing the weight achieved by each cohort at the end of the study with the weight recorded for the preceeding year class at the beginning of the study year. Such comparisons are shown in Table 10 for six of the seven species in the pond (so few yearling and older bass were present that no valid comparison could be made).

The growth of Or bullhead in 1973 resulted in a final weight of less than half that achieved in the preceeding year, an indication of severely reduced growth due to overcrowding. Forney (1955) found bullhead in Iowa reaching 50 mm by August of their first year, while in the golf course pond they reached 65 mm. According to studies summarized in Carlan- der (1969) Y of Y bullhead reached 168 mm by December in southern USA ponds. In the study pond Or bullhead were just 121.9 mm by the end of the year, O2 were but 96.6 mm. The growth of yearlings was similarly depressed in 1973 compared to 1972 (Table 10). Although this comparison could not be made for adult (II+) bullhead, reduced coefficients of condition calculated for this age class in the summer of 1974 also indicated reduced growth in 1973 compared to previous years.

The end-of-year weights of 0 class crappie in 1972 and 1973 were nearly equal (Table 10). This might be

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Table 9. Production of longear sunfish and golden shiners

longear sunfish - P (kg) golden shiner - P (kg) Date

0 I II+ I II III+

6/2/13 0.45 0.25 0.66 0.75 0.51 6/21/13 0.38 0.31 0.81 -0.21 1.73 l/14/13 0.01 0.05 0.21 0.89 1.21 0.51 814113 0.45 0.05 0.13 0.10 0.94 0.06 8/18/73 0.41 0.01 0.18 -0.55 1.18 0.48 P/8/13 0.23 0.05 0.12 1.13 0.94 0.14 P/22/13 0.07 0.02 0.10 -2.16 0.73 0.16 10/3/73 0.10 0.08 0.30 0.40 0.19 0.24 1113173 0.15 0.10 0.16 1.05 -1.64 -.04 12/l/73 0.29 0.10 0.11 0.10 -0.02 -.31 l/P/74 0.13 0.05 -0.02 -0.28 0.27 -.ll l/19/74 0.14 0.05 0.06 -0.05 0.42 -.Ol 212174 0.47 0.13 -0.01 0.54 0.90 -.ll 312174 0.02 0.12 -0.11 0.73 0.43 0.10 416174 0.25 0.16 -0.18 -1.08 -0.33 -.05 514174 0.12 0.09 -0.14 -0.45 -0.05 -.05 5118174 0.14 0.07 -0.25 0.88 0.21 -.16

2.98 1.96 1.99 7.29 8.17 3.93 -0 -0 -0.71 -4.57 -2.25 -0.84

ZP = 2.98 xP = 1.96 BP = 1.28 TZP = 2.12 EP = 5.92 ZP = 3.09

TablelO. A comparison of mean individual weight (g) of cohorts on June 1973 and June 1974. Hyphenated pairs represent mean weight for successive age classes at comparable times.

0 I II

Ictalurus melns Pomoxis annulan’s Micropterus saltnoides Lepomis macrochirus Lepomis cyanellus Lepomis megalotis Notemigonus chrysoleucas

June ‘74

18.72 24.26 12.64

6.82 4.01

10.54

June ‘73 June ‘74 June ‘13

- 40.77 65.77 - 118.46 - 20.01 29.64

- 9.43 25.00 - 18.74 - 7.52 13.16 - 19.18 - 12.00 26.46 - 27.39

1.86 1.53 - 6.82

expected since the number of crappie spawned ap- peared to be nearly the same in both years. Zengerle (1972) reported 0 class crappie of 106 mm and I class of 171 mm at the end of the year in Texas. In the study pond 0 class were 136 mm and I were 155.6 mm. Whiteside (1972) and Schoffman (1964) reported somewhat smaller crappie at comparable ages in Oklahoma and Tennessee. Our interpretation is that Y of Y crappie grew equally well in both years (somewhat less in 1972 than in 1973) while yearling crappie growth was reduced in 1973.

Bass Y of Y were 110 mm in June 1974, which is greater than the 58-85 mm reported by Kramer and

Smith (1960) in Minnesota, about equal to the 102- 145 mm reported by Schultz & Vanicek (1974) in California, and considerably less than the 207 mm reported by Schoffman (1968) in Tennessee. So few bass were spawned in the previous year (1972) that no comparison of growth rates could be made between 1972 and 1973 Y of Y.

Growth rates of all three sunfish Y of Y were lower in 1973 than in 1972 (Table lo), perhaps in response to increased competition with bullhead and crappie Y of Y and crappie yearlings. Growth rates of yearling sunfishes were variable, but overall seemed little changed in 1973 compared to 1972. Growth

252

rates of Y of Y bluegill (69.2 mm) were similar to those (60.0 mm) reported by Brown & McCarly (1973) in Texas. Growth rates of age class I and older bluegill and green sunfish were somewhat lower than those reported in other parts of the USA by Carlan- der (1969).

Fish removals by fishermen and experimenters

Fishing in the golf course pond was prohibited for many years until the summer of 1973. Catches were high in June and July 1973, when 69.2% of the total numerical catch of 789 fish was made (all age class I or above, total weight = 61.3 kg). The number of fish caught decreased steadily during the rest of the summer and fall. In winter months the catch rate increased slightly and then fluctuated mildly during the late winter and spring. The number of fishermen also decreased as the year progressed. Of some 738 tisher- men visits at the pond, 415 were in June and July 1973 (Jones, Pearson & Fitzpatrick 1977).

Black bullhead made up 75.3% of the total catch by numbers (594 fish) and 83.2% (51 .O kg) of the catch by weight. Adult bass and crappie were the largest fish captured, with a 2.4 kg crappie being the largest fish captured. Only 10 adult crappie and 11 large bass (7.0 kg total weight) were caught. Al- though sunfish were the second most abundant fish captured, they accounted for only 2.1 kg of weight removed.

Many fish were removed during the study for research purposes (i.e. stomach analysis and respiration studies conducted by other students). The total weight of the 4,662 fish removed was 28.9 kg which was almost half as much as that cropped by fishermen (61.3 kg). Of the fish removed for research purposks, green sunfish accounted for the largest weight removed (7.7 kg), followed by bullhead and crappie (5.9 kg each). The total removals of bluegill, golden shiner, largemouth bass, and longear sunfish were much lower (0.5-4.0 kg).

Discussion

An examination of the results leads to three important topics for discussion: 1) how much confidence can be placed in the estimates of density and mean individual weight; 2) how might the physical environment of the pond, including man’s perturbations,

have influenced the production rates reported; and 3) the annual production rate for the entire fish taxocene appears to have been the highest yet reported for uncultured temperate zone waters, with 90% oc- curring in just 4 months and 84% resulting from Y of Y fishes.

Sources of error

The potential sources of error associated with mark and recapture techniques are well known (Ricker 1958, Buck & Thoits 1965, Regier & Robson 1967). Investigators seeking to verify mark and recapture estimates by making complete counts of the populations (after draining or poisoning) have usually con- cluded that mark-recapture techniques underestimate actual density (Carlander & Moorman 1956, Buck & Thoits 1965, Beukema & deVos 1974, and Chadwick 1976). Mark and recapture estimates were typically 10 to 90% of the actual population in these studies. Buck & Thoits (1965) attributed their errors of underestimation to the recapture of marked individuals in proportions greater than their true abundance in the population (3 to 1 for bluegill). This they attributed to either differential susceptibility to the gear (seines) or return of marked fishes to areas which were most efficiently sampled by the gear. Beukema & deVos (1974) found that catch per effort declined markedly with repeated seinings of a pond and that some marked fish seemed to be caught repeatedly.

We believe that our mark-recapture estimates were probably low, but the narrowing of confidence limits about the estimates as the study progressed gave us confidence that the error was probably in the range of 10 to 20% or less.

Indirect estimates could have been low if the fish were either especially adept at avoiding the net, or if they concentrated in the center if the pond where the seine could not be deployed. Adult bass and crappie populations may have been underestimated as a result of the former pelagic Y of Y and yearling crappie as a result of the latter.

Size of populations may have been overestimated if the marking procedure resulted in mortality of marked fish. Very high mortalities were associated with attempts to mark crappie, and we soon aban- doned these attempts. Growth rates of marked bass were not significantly different from those of unmarked bass, nor did we see other indications of the effects of marking (including mortality) in any other species. A second source of error resulting in overesti-

253

mation could have been a failure to recognize marked animals due to fm regeneration. Although some in- stances of regeneration were noticed in the spring of 1974 they seemed to be rather incomplete. The use of correction factors based on ratios of adult green sunfish estimates by the indirect and mark-recapture methods could have been a third source of overestimation. It might be expected that larger fish would be capable of escaping a seine more readily than smaller fish (assuming the smaller fish are not so small as to pass through the mesh). If this were true the correction factor based upon large green sunfish would result in overestimation of Y of Y of all species. One might also expect that Y of Y fishes would be more abundant inshore, while larger fishes, from which the correction coefficients were obtained, would be concentrated in the center of the pond. We have no way of knowing the magnitude of this third source of overestimation before December, but after December mark-recapture estimates for Y of Y bullhead, bass, and sunfish became available and were nearly the same as (variation = 1 to 19%) indirect estimates corrected by the green sunfish ratios.

Physical environment and perturbating factors

All fish were affected by the unusually low temperatures of the 1973 winter, especially bass Y of Y, in which food consumption decreased markedly. All species of fish had reduced growth rates, lost weight, or had an increase in mortality during the winter months, as one might expect (Winberg 1956).

Turbidity has been related to spawning success, protection from visual-feeding predators, food avail- ability, and loss of spawning sites (Buck 1956); although it rarely reaches a high enough concentration to be lethal (Horkel & Pearson 1976). The high turbidity of the pond probably afforded more protection against predation for Y of Y fish. Turbidity ap- peared to have had little effect on the reproductive success of fish in the summer of 1973, but the very high turbidities in the spring of 1974 may have affected reproductive efforts of early spawners (i.e. bass, crappie, and bullhead) since very few Y of Y fish were seined in the summer of 1974 by ecology classes at North Texas State University. Reproductive success could also have been reduced by the shortage of food and reduced growth rates of adult fishes, and the destruction of spawning habitat by repeated seining and water level fluctuations. Algal mats important in spawning of black bullhead (Dennison & Bulkley

1972, Forney 19.55) were much more extensive in 1973, than 1974.

Bennett et al. (1973) showed that drastic reduction in pond volume resulted in Y of Y fish being forced from protected areas where predation was more severe, thus increasing growth rates of piscivor- ous fish. Experiments in reducing population size (Beyerle & Williams 1972, Buck & Thoits 1970) have shown that decreasing standing crop can result in higher survival, growth, and production rates, in both Y of Y and adults. Fluctuations in volume of the pond resulted in reduced shoreline areas, reduced amounts of filamentous algae around the shoreline, and exposure of spawning habitats preferred by sunfish. The most drastic effects of the volume reduction were worked upon the benthic populations (Benson 1976). Benthic organisms utilized the algal mats as habitat, and as volume decreased the algal mats were destroyed and left the benthic organisms with very little cover. By spring most of the Y of Y fishes were feeding on bethic organism which further reduced benthos populations.

In the spring of 1974, production and growth rates of most fishes did not increase as expected with the increase in temperature. The reduced populations of benthos and the severe competition for food between members of the fish taxocene, as evidenced by declining coefficients of condition, prevented any one cohort from obtaining enough food to sustain the previous year’s production rate. Eventually the growth rates of most species declined and production rates remained low throughout the spring and into early summer, when the study was terminated.

The pond was first opened to angling in June 1973. Fishermen captured 789 fish weighing 61.1 kg during the study, which amounted to only 3% of the total fish production in the pond, or 28.5% of the total adult production. The effects of angling on fish communities have been extensively studied. Ricker (1969) and others have shown that even a small amount of fishing can alter the age class structure of a population and decrease the biomass of adult cohorts enough to allow the Y of Y to grow rapidly during the summer. This probably occurred in the bullhead population of the pond.

Although fishing was not a major factor in sunfish mortalities, removals of yearling and adult green sunfish for experimental work were extensive (nearly 500 were removed). Beyerle & Williams (1972) have shown that large removals of adults can result in higher production rates and better survival for younger cohorts. Bluegill Y of Y and yearlings had

254

very low growth and production rates. Green sunfish Y of Y and yearlings had much higher production and survival rates than did other young sunfish, and had the highest yearling sunfish production rate.

Production and biomass

Black bullhead was the dominant species in the pond. The production of 134 g m-2y-1 is higher than that reported for any other freshwater fish known to us. The two spawns of Y and Y bullhead made up 26% of the standing crop of fishes in the pond, and contributed 57.7% of the total production,

Yearling and older bullhead together had the highest mean standing crop in the pond, at 3 16.3 kg. Production was also high at 130.4 kg, or 6.6% of the total production rate. Whiteside & Carter (1972) reported that bullhead in Oklahoma ponds contributed little to the total production (0.3 to 4.7 g mm2y-‘).

Very few adult bass were captured during the study. An estimated 29 adults inhabited the pond in June 1973 and produced a large spawn (76,942 Y of Y) in May. This appears to have been one of the few large spawns of bass in the pond in several years. Growth of Y of Y bass was excellent during the summer and early fall, but by late fall growth ceased, and never increased again during the sampling year. The total production of Y of Y bass (33.9 g mb2y-‘) made up 16.2% of the total for all species. We were unable to find any reports’ on Y of Y bass production although production rates for adult bass of 0.4-8.5 g rnm2y-l have been reported (Cooper et al. 1963, Ben- nett et al. 1969, Whiteside & Carter 1972, Clady 1975).

The introduction of several large crappie in 1973 had significant effects on the fish taxocene. Only tdro of these adults were observed during the study, one captured by a fisherman and one by an ecology class, but two spawns of crappie occurred (in 1972 and 1973) and together they accounted for the third largest species production in the pond (28.7 g me2y-r or 13,7% of the total).

Bluegill did not spawn until October, although gravid females were picked up throughout the summer, and Swingle & Smith (1950) found bluegill re- producing from April through October, with peak production rates in May and June. The delayed spawn was probably due to fluctuations in volume which prevented the use of preferred spawning sites along the shoreline. Only an estimated 8,000 Y of Y were

spawned, a very small spawn considering the number of adults in the pond.

Bluegill had an annual production rate of only 5.5 g mm2y-’ (2.6% of the total). Gerking (1962) reported bluegill production of up to 9.1 g me2y-’ in Indiana, although Mahon (1976) has suggested that a more reasonable allowance for production of younger year-classes should increase this rate by a factor of six. Whiteside & Carter (1972) reported bluegill production rates of only 0.4 g rnv2y-’ in Oklahoma, while in Illinois Bennett et al. (1969) reported production of adult bluegill of up to 8.2 g mm2y-l. Cooper et al. (1971) reported surplus production rates ranging from -0.3-15.4 g mm2y-l, in Ohio and Beyerle & Williams (1972) reported production rates of 24.4 to 33 g rnm2y-l in Michigan. Production rates of bluegill in the pond were, therefore, lower than those reported in most other areas.

Total production by green sunfish was 4.6 g rns2y-l (about 2.2% of the total production of all fish species). Green sunfish produced 0.1-7.1 g rns2y-l (0.03-60% of the total production) in Oklahoma ponds (Whiteside & Carter 1972).

Longear sunfish and golden shiner contributed little to the total pond production. Longear contributed only 0.6 g mw2y-’ (less than 1% of the total) production. To our knowledge no previous studies of longear production have been reported. Golden shiner also contributed relatively little to production in the pond (1.2 g me2p1 ; less than 1% of the total, yet more than the 0.02 g m-2y-1 reported by Chadwick (1976) in Ontario).

The yearlings and older fish dominated the standing crop since Y of Y fish had large standing crops only in July and August during the fast growth period. Total fish production in the pond amounted to 208.9 g m-2y-1 (after subtraction of 16.2 g rne2y-’ of negative production) with 83.8% due to Y of Y, primarily bullhead, bass, and crappie. Mathews (1971) found Y of Y contributing from 39-73% of total fish production in the River Thames, and cited several other studies which had reported Y of Y con- tributions of 19-76% of total production for several species (primarily salmonids) of freshwater fish. Chadwick (1976) and Balon (1974) also reported that 0 class individuals contributed over 70% of the annual production in the fish communities’ of Red Deer Lake, Ontario and Lake Kariba, Africa. Clady (1977) reports Y of Y production of 77-84% of the total for smallmouth and largemouth bass.

Total fish taxocene production studies are rare. Mathews (1971) estimated the total annual produc-

255

tion of fish in the Thames River at 197 Kcal md2y-l, which, if we accept the conversion value of 1.0 assumed by Mann (1965) and Backiel(l97 l), is equiva- lent to 197 g m-2y-1. Chadwick’s (1976) estimate of the annual production of fish in Red Deer Lake (3.08 g rne2y-l) is the lowest yet known. The only annual production rate for an entire first taxocene which exceeds the 209 g mp2y1 of the golf course pond is the 346.8 g me2y1 reported for Lake Kariba, an impondment in tropical Africa (Balon 1974).* Studies dealing with selected species in one system have reported production rates of 0.4-97.2 g rnm2y-l (Backiel 1971, Cooper et al. 1971, Mann 1971, Hol- cik & Pivnicka 1972, LeCren 1972, Whiteside & Car- ter 1972, and Petrosky &Waters 1975).

The introduction of new species to a system has been known to increase total fish production tempo- rarily, while decreasing the contribution of each species to the total (Swingle 1946). Many fish culturists have observed higher production rates in mixed species culture than summed production of the same species held in monocultures (e.g. Buck et al. 1973 and Sarig & Marek 1974). The decline of catfish as a com- petitor with bullhead, and the introduction of white crappie may have been two major contributing factors to the high production rates observed. Also, the usually successful spawn of largemouth bass and the absence of yearling bass to prey on the Y of Y of all species may have enhanced high production rates of Y of Y. We believe that the annual production recorded was well above the long-term mean. Samples of fish collected in the fall and winter of 1974 by ecology classes at North Texas State University indicated that spawns of crappie, bass, and bluegill were all much less successful in 1974 than in 1973, and that standing crops and production rates of the fish community declined.

Procedures for calculating the annual production of a fish population (in Ivlev’s sense of the word) involve estimating the density and mean individual weight of each cohort in the population at least once during the year. The differences between procedures then come down to the number of empirical estimates made for these two parameters during the year and the method used to interpolate between the empirical data points or extrapolate beyond them.

Ricker’s (1946) method involves making a single annual assessment of the two parameters and then

* See corrected value (Mahon & Balon 1976) on p. 281, this issue [editor].

assuming exponential growth and mortality rates between successive cohorts. This method has been used by Mann (1965), Balon (1974) and Chadwick (1976). It will be in error as growth and mortality rates from the exponential function through the year. If several measurements are made of the two necessary parameters during the year the problem becomes one of connecting adjacent data points. These connections can be straight lines, smoothed curves, or exponential curves (Chapman 1968). We used 18 data points and integrated production between adjacent points algebraically using Ricker’s exponential method. We also calculated production based upon estimates of density and mean individual weight for each cohort on June 2, 1973 and June 1, 1972, again using Ricker’s exponential relationships. This method will approximate the one, large annual estimate (such as that of Chadwick) when age class structure of the population is assumed to be stable. Total production calculated between annual estimates was 680 kg compared to the 2,091 kg calculated by assessing expo- nentials between all 18 estimates. The major discre- pancies between the two calculations are in the O1 bullhead and 0 bass estimates. Since growth rates of these groups were not unusually high it would seem that the major error involved in estimating production between annual estimates in the pond results from overestimating mortality rates of Y of Y fishes in the first 4-6 months of life. There are few studies of the mortality rates of fishes in the first four months of life, however, if our observations can be applied generally then mortality rates of young fishes may be much lower than predicted by Ricker’s gene- ralized equation applied over an entire year.

In this paper we have routinely subtracted negative production (loss of weight by individuals) from the annual production values. Chapman (1968) has discussed the reasons for and against this procedure. An alternative procedure would have been to have substituted 0 for all negative values of production encountered during winter months. In Ivlev’s original definition, the phrase ‘... regardless of whether or not it remains alive (i.e. part of the stock) at the end of that time’ is clearly meant to include in production the weight of individuals which die during the interval considered. However, it is not clear whether weight loss due to metabolic or reproductive requirements is to be included. We have summed both negative and positive productions in each of Tables 3 to 9, and adding the negative values back into our totals would be a simple matter for those preferring this shade of definition. We would point out that methods of cal-

256

culating production based upon a single collection of aged fish will not detect negative production and in comparing our results to those produced by this method negative production should be restored to the net values given.

Conclusions

The combined spawning success of bullhead, crappie, and bass in the early summer of 1973 led to high Y of Y production rates during the summer of 1973. The reduction of adult sunfish and bullhead populations due to fishing and research removals probably enhanc- en the survival and growth rates of these Y of Y fishes. Zooplankters were the major foods of Y of Y fishes during the first summer of life, with an increase in size their food preference shifted from zooplankton to benthos. During the winter, consumption rates declined, and the standing crop of benthos increased slightly. As water temperatures increased, consumption rates increased and the benthic populations declined. Volume decreases and the destruction of algal mats also adversely affected benthic populations. The high number of Y of Y fish and adults relying on benthic organisms as food resulted in increased food overlap and severe competition for available food resources. This complex interaction of events prevented any one cohort from receiving enough food to sustain its biomas and production rates during the spring months and probably beyond.

There are several implications of our study which may be of interest to fishery managers: 1) the impor- tance and magnitude of production by Y of Y fishes has not been generally recognized, 2) the period in which most of the annual production occurred was late summer to early fall, 3) many important game species (e.g. bass, crappie, sunfish, catfish) feed upon benthic invertebrates at ages I and II, and the bent&c populations of many small ponds may not be capable of carrying large numbers of these fish through to adulthood, and 4) attempts to maximize fish production in southwestern ponds should include the introduction and propagation of fishes which feed on zooplankton throughout the life cycle.

Acknowledgements

The procedures employed in this study required that large numbers of fishes be handled quickly and returned to the pond alive in the usual hot Texas sum-

mer, and in the unusual cold Texas winter. This was made possible by the help offered us by many gradu- ate and undergraduate students at North Texas State University, but especially that of Dan Benson, Michael Childress, Joseph Fagan, John McIntosh, John Horkel, and Mary K. Jones. The unknown reviewers offered many helpful comments which increased both the clarity and scope of the interpretation. Financial aid was supplied by Faculty Research Grants at North Texas State University to William D. Pearson and Lloyd C. Fitzpatrick.

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ZOOPLANKTON 0

J J J A SOND J F MAMJ

MONTHS

Appendix 1. Total numbers of zooplankton and benthic organisms in the pond (data supplied by Smith 1976 and Benson 1976).

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Production of a fish taxocene in a Texas pond

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