North American Journal of Fisheries Management 11:167-176. 1991© Copyright by the American Fisheries Society 1991

Stepped-Oblique Midwater Trawling as anAssessment Technique for Rainbow Smelt

RICHARD A. KIRN* AND GEORGE W. LABARSchool of Natural Resources, University of Vermont

Burlington, Vermont 05405, USA

Abstract.—Discrete-depth midwater trawling for rainbow smelt Osmerus mordax at night inLake Champlain revealed a delayed vertical migration of smaller fish and size partitioning verticallywithin the water column. These behavioral observations led to the development and evaluationof a nighttime stepped-oblique midwater trawling technique. This latter method proved effectivein obtaining large catches of rainbow smelt (mean catch per unit of effort [CPUE], 215-655 fish/55-min trawl) and provided estimates of relative abundance with reasonable precision (CPUE ±13-50% from June through October), as well as estimates of length and age distributions unbiasedby vertical migration behavior.

Rainbow smelt Osmerus mordax serve as theprincipal forage species for stocked salmonids inLake Champlain (Kirn 1986) and many other NewEngland waters (Kircheis and Stanley 1981). Thedesirability of rainbow smelt as a food and foragefish, and the relative ease with which they aretransferred to other waters, is resulting in a con-tinually expanding distribution of this species(Maydenetal. 1987).

Knowledge of forage-fish population dynamicsis an important component of recent freshwatersalmonid introduction and restoration programs,especially in situations such as in Lake Champlain,where recruitment of salmonids relies almost ex-clusively upon hatchery production and is there-fore independent of forage abundance. The abilityto recognize fluctuations in rainbow smelt abun-dance before predator growth is affected is partic-ularly critical given the potential for extreme vari-ability in the abundance of rainbow smelt, whichapparently is unrelated to stock size, predation,fishing intensity, or disease (Smith 1972; Havey1973; Kircheis and Stanley 1981; Selgeby 1985).

Sampling rainbow smelt populations has beencomplicated by size- or age-specific seasonal dis-tributions (MacCallum and Regier 1970; Argyle1982; Plosila 1982) and diel vertical migrations(Ferguson 1965; Gray 1979; Heist and Swenson1983). Population dynamics of Great Lakes rain-bow smelt have been assessed from spring spawn-ing runs, during which concentrations of rainbowsmelt in tributaries are vulnerable to a variety ofsampling gears (Luey and Adelman 1984; Frie andSpangler 1985). These surveys, however, only pro-

1 Present address: Vermont Fish and Wildlife De-partment, Roxbury, Vermont 05669, USA.

vide information on the sexually mature portionof the population, limiting the value of such sur-veys for monitoring year-class dynamics. In ad-dition, this approach is not feasible for waters withsignificant lake-spawning populations of rainbowsmelt, such as are found in Lake Champlain (Plo-sila 1984). In recent years, hydroacoustic samplinghas proven to be a reliable technique for moni-toring the abundance of pelagic fish stocks (Argyle1982; Heist and Swenson 1983), but requires ex-pensive equipment and must be integrated withother sampling gear to determine species com-position and obtain biological samples.

Daytime bottom and midwater trawling in LakeHuron indicated that discrete portions of the rain-bow smelt populations inhabited both the bottomand midwater during the day, and were thereforeunavailable to a single trawling method (Argyle1982). Observations of size partitioning amongdaytime rainbow smelt distributions have beenassociated with bottom depth and possibly watertemperature (MacCallum and Regier 1970; Argyle1982; Plosila 1982). Length or age compositionsestimated under these conditions become a func-tion of trawling location and effort rather than arandom sample of the population at large (Argyle1982).

Midwater trawling at night, when diel verticalmigration of rainbow smelt provides a concen-trated, pelagic distribution, has shown promise asan effective sampling strategy. Rainbow smelt con-stituted 90-99% of the nighttime pelagic fish pop-ulation sampled with midwater trawls in the upper50 m of Lake Superior (Heist and Swenson 1983).Nighttime midwater-trawl sampling for ciscoCoregonus artedii in Lake Champlain has alsoproduced large catches of rainbow smelt (G.W.L.,unpublished data).

167

168 KIRN AND LABAR

Considering the influences of rainbow smelt be-havior on past sampling strategies, the specific ob-jectives of this study were to develop and evaluatea nighttime midwater-trawling technique for sam-pling rainbow smelt populations. Results frompreliminary discrete-depth trawls, also presented,led to the development of a stepped-oblique mid-water-trawling technique. This technique wasevaluated for consistency in estimating length andage distributions and relative abundance of rain-bow smelt. The influence of the temporal and spa-tial movements of rainbow smelt were also eval-uated and used to develop an overall samplingstrategy for monitoring the population.

Study AreaLake Champlain is a typically dimictic lake lo-

cated within a north-south, glaciated fault be-tween the Adirondack Mountains of New Yorkand the Green Mountains of Vermont. Meyer andGruendling (1979) provided a detailed account ofthe limnology of Lake Champlain. Lake dimen-sions are length, 172 km; maximum width, 19 km;maximum depth, 129 m; mean depth, 22 m; sur-face area, 1,140 km2. Plosila and Anderson (1985)estimated 45% of the lake to possess deep, coldwater with sufficient dissolved oxygen to supportsalmonids.

Potash et al. (1969) described five limnologicallydistinct water masses in Lake Champlain: SouthLake, Main Lake, Malletts Bay, Northeast Arm,and Missisquoi Bay (Figure 1). New York andVermont state fishery investigators have furtherpartitioned the lake into 15 management zones.Sampling during this study concentrated on zones3 A and 3B of the Main Lake (Figure 1), where thehighest densities of lake trout have been reported(Plosila and Anderson 1985). The Main Lake ac-counts for 82% of the entire volume and 60% ofthe total surface area of Lake Champlain (Potashet al. 1969). Maximum surface temperatures arereported to reach about 23°C, while summer hy-polimnetic temperatures average 6°C (Meyer andGruendling 1979). Summer stratification of theMain Lake is reported to generally begin in June(Meyer and Gruendling 1979). Thermal stratifi-cation was only observed in this study during Au-gust trawl sampling, when the metalimnion hadan upper boundary at 10-12 m and a lower bound-ary ranging from 17 to 25 m.

MethodsRainbow smelt were sampled with a 5-m x 5-m

midwater trawl. Mesh size (stretched measure) was

graduated and ranged from 20.3 cm at the head-rope to 1.3 cm at the cod-end liner. Although thisnet was capable of retaining some young-of-the-year rainbow smelt, many escaped. For this rea-son, young-of-the-year fish, distinguished by sizeand transparency, were not included in data anal-yses.

The net was towed at 1.1 m/s by the VermontFish and Wildlife Department's 9.4-m researchvessel Dore. Temperature and net depth weremonitored with an ultrasonic temperature-pres-sure transducer attached to the port bridle nearthe headrope.

Discrete-depth trawls.—Discrete-depth trawlswere performed during June, August, and October1984, and during June and August 1985. Prelim-inary trawls in June 1984 sampled sites in MallettsBay and the Northeast Arm (Figure 1). Subsequenttrawls were confined to the Main Lake.

Discrete-depth trawls consisted of a series of 20-min trawls at each of three predetermined depths:midway between the surface and the top of themetalimnion, at the top of the metalimnion, andat the bottom of the metalimnion. Because of apaucity of rainbow smelt (4% of the total catch inJune), trawls midway between the top of the meta-limnion and the surface were replaced by trawls10 m below the metalimnion after June 1984. Inthe absence of thermal stratification, depths of 15,25, and 35 m were sampled. All three depths weresampled before a second series of trawls was re-peated in sequence. Trawls conducted in June 1984were begun within 10 min after sunset. Trawlsconducted after June 1984 were generally initiatedwithin 90 min after sunset. Two series of discrete-depth trawls were conducted per night except inAugust 1984, when three series were conducted attwo of the four sites sampled, and in October 1984,when three series were conducted at 25 and 35m.

Stepped-oblique trawls.— Stepped-oblique trawlswere deployed at least 10 m above the lake bottom,or to a maximum depth of 35 m. Once set, thenet was fished for 5 min at each 3.3-m depth stra-tum, in stairstep fashion, until the net reached adepth of about 7 m, when it was completely re-trieved. During preliminary discrete-depth trawlsin this study and earlier work (G.W.L., unpub-lished data) in Lake Champlain, few rainbow smeltwere captured in the upper 7 m of the water col-umn. Although the actual net dimensions duringtrawling were unknown, it was assumed that the3.3-m depth strata were sufficient to ensure thatthe net sampled the entire water column. A rep-licate trawl was then fished along the same course

MIDWATER TRAWLING FOR RAINBOW SMELT 169

FIGURE 1.—Map of Lake Champlain (left) showing water masses and fishery management zones 3 A and 3B. Inthe large-scale map of zone 3 A and 3B (right), the lines connecting triangles, squares, and circles indicate shallow(<45-m), intermediate (45-75-m), and deep (>75-m) trawl sites, respectively.

in the opposite direction. Based on results fromthe discrete-depth trawls, stepped-oblique trawl-ing was begun at least 1 h after sunset to allowrainbow smelt to complete vertical migration.

Replicate stepped-oblique trawls were used toevaluate spatial and seasonal differences in rain-bow smelt abundance and length and age distri-butions. Sites were chosen within managementzones 3 A and 3B (Figure 1) in each of three depthranges: above 45 m, 45-75 m, and below 75 m.Replicate stepped-oblique trawls at these six siteswere scheduled for August and October 1984, andfor April, June, August, and October 1985. Smeltcatches from these sites and an additional MainLake site were also used to evaluate differences inabundance and length distributions between rep-licate trawls.

Fish processing and statistical analysis. —Whenavailable, a random sample of 100 rainbow smeltfrom each discrete-depth trawl and 200 rainbowsmelt from each stepped-oblique trawl were mea-

sured to the nearest millimeter (total length). Ad-ditional fish were counted.

Sites in the 45-75-m and below-75-m depthranges resulted in stepped-oblique trawling timesof 55 min (i.e., 11 "steps"). Due to the more shal-low deployment of the net at sites above 45 mdeep, trawl times were limited to 35 min (i.e.,seven "steps"). Therefore, to obtain a standard-ized total rainbow smelt catch per trawl (CPUE)as an index of relative abundance, smelt catchesfrom the shallow sites were proportionately ad-justed to represent a 55-min trawl (i.e., [catch/35]x 55). The time of deployment and retrieval ofthe net was not included in CPUE calculations.

The two-sample Kolmogorov-Smirnov (K-S)statistical procedure (SPSSX 1983) was used toevaluate differences between length distributionsof replicate stepped-oblique trawls and replicatediscrete-depth trawls, and between different depthstrata sampled by discrete-depth trawls. Beforeanalysis, rainbow smelt lengths were truncated to

170 KIRN AND LxBAR

the nearest 5-mm increment. Paired /-tests wereused to determine systematic differences in totalrainbow smelt catch between the first and secondreplicate stepped-oblique trawls from all samplesites and dates.

Variations in the relative abundance of rainbowsmelt due to management zones, sampling period,and water depths were examined by analysis ofvariance based on a hierarchical classification ofthe 1985 stepped-oblique trawl CPUE (Snedecorand Cochran 1980). To evaluate the precision ofabundance estimates, for rainbow smelt fromstepped-oblique trawling, 95% confidence inter-vals were calculated for mean CPUE for each man-agement zone when all three sites within the zoneswere sampled. Similarly, precision estimates formean CPUE of each' sampling period were cal-culated when all six sites were sampled (two man-agement zones, three sites per zone).

Age structure of rainbow smelt captured instepped-oblique trawls was used to evaluate thecatch efficiency of this technique (Ricker 1975).Otoliths were used for age determinations, as de-scribed by Kirn (1986). Complete annulus for-mation and thus recruitment of rainbow smelt tothe next age-class was considered to begin in June,the time of the first trawl sampling following rain-bow smelt spawning.

Age composition of combined replicate trawlsamples, determined from stratified sampling(Ketchen 1950), was examined statistically for dif-ferences due to location and water depth withPearson's chi-square analysis (Dixon et al. 1983).For each sampling period, comparisons were madeamong sites; among depths (i.e., <45 m versus45-75 m versus >75 m); within specific depths,between zones {e.g., <45 m, 3A versus 3B); andwithin zones, among depths. Length distributions(10-mm increments) were similarly tested.

ResultsDiscrete-Depth Trawls

In all, 77 discrete-depth trawls were conductedat nine sampling sites throughout Lake Champlainduring 1984-1985. Rainbow smelt dominated thecatch and accounted for 98% of the total; ciscoaccounted for 1% and were the next largest com-ponent.

Preliminary discrete-depth trawls in June 1984revealed a difference in the timing of vertical mi-gration for distinct size-classes of rainbow smelt(Figure 2). Smaller fish (<120 mm) were appar-ently less available for capture in earlier trawls,

Length (mm)FIGURE 2.—Length-frequency distributions of rain-

bow smelt captured in replicate discrete-depth trawlsfished at 15 m on June 22, 1984, in Inland Sea, LakeChamplain.

suggesting a delayed upward vertical migration ofsmaller fish. In June 1984, length distributionswere significantly different (K-S, P < 0.01) for allfour comparisons between replicate trawls (i.e.,same depth). Total catches of rainbow smelt werealso influenced by the timing of the trawl sample.In June 1984, rainbow smelt captured in the sec-ond series of trawls accounted for 87% of the totalrainbow smelt catch, which also reflected a delayedvertical migration of smaller fish into the watercolumn.

To reduce the effects of the delayed vertical mi-gration of small fish on estimates of rainbow smeltlength distributions, discrete-depth trawls con-ducted in August 1984 were started at least 60 minafter sundown. Length distributions differed sig-nificantly (P < 0.05) between replicate trawls (i.e.,same depth) in only 2 of 19 comparisons, indi-cating that the vertical migration of rainbow smeltinto the water column had generally been com-pleted by 1 h after sunset.

Subsequent discrete-depth trawls conducted inOctober 1984 and in June and August 1985 didnot reveal statistical differences (K-S, P > 0.05)between replicate trawls in 12 comparisons, al-though trawling was begun within 60 min aftersunset.

Size partitioning of rainbow smelt, such thatyearling smelt migrated higher into the water col-umn (Figure 3), was observed at all four sites sam-

MIDWATER TRAWLING FOR RAINBOW SMELT 171

120 140 160 180Length (mm)

FIGURE 3.—Length-frequency distributions of rain-bow smelt captured in replicate discrete-depth trawlsfished at 18 and 28 m on August 27, 1984 in Main Lake,Lake Champlain. Arrow indicates mean length of year-ling rainbow smelt.

pled in August 1984 and was again apparent inOctober 1984 (one site) and June 1985 (one site).Highly significant differences (K-S, P < 0.01) wereobserved for length distributions among depthstrata in these six cases. Trawls in August 1985(one site) did not reveal this pattern (K-S, P >0.25).

During periods of thermal stratification (i.e.,August in 1984 and 1985), greatest catches (82-98%) of rainbow smelt were observed from trawlsfishing the lower limit of the metalimnion and inthe hypolimnion.

Stepped-Oblique TrawlsAltogether, 67 stepped-oblique midwater trawls

were fished in the Main Lake during 1984-1985.Rainbow smelt again dominated the catch andaccounted for 97% of the total catch, followed bycisco, which accounted for 3%. Replicate stepped-oblique trawls provided reasonably consistentlength distributions of rainbow smelt. Only 6 of31 comparisons (19%) of length distributions fromreplicate trawls were significantly different (K-S,P < 0.05; Table 1). However, statistical differencesdid not necessarily imply practical biological dif-ferences, particularly at significance levels of 0.01< P < 0.05 (Figure 4). Only 2 of 31 comparisons(6%) were found to be significantly different whentested at P < 0.01 (Table 1).

85 105 125 145 165 185Length (mm)

85 105 125 145 165 185Length (mm)

FIGURE 4.—Length-frequency distributions of rain-bow smelt captured in replicate stepped-oblique trawlsduring June 1985 in zones 3A and 3B in water deeperthan 75 m (upper graph) and in water 45-75 m deep(lower graph) in Main Lake, Lake Champlain.

Total catches of rainbow smelt were also gen-erally consistent between replicate trawls, exceptin April 1985 when rainbow smelt were concen-trated for spawning (Table 2). Total rainbow smeltcatch was significantly different between replicatetrawls (paired /-test, P < 0.05; Table 3) only whenApril 1985 data were included.

172 KIRN AND LABAR

TABLE 1.—Probability values3 resulting from Kolmogorov-Smirnov comparisons of rainbow smelt length dis-tributions captured in replicate stepped-oblique midwater trawls in zones 3A and 3B of Lake Champlain, 1984-1985. Numbers in parentheses are the combined sample sizes of replicate trawls.

Date

Aug 1984

Oct 1984

Apr 1985

Jun 1985

Aug 1985

Oct 1985

<45 m

0.865 ,(365)

0.003**(394)

Zone3A

45-75 m

0.797(440)

0.096(329)

0.393(400)

0.810(400)

0.144(311)

Zone3B>75 m

0.388(398)

0.890(313)

0.022*(398)

0.010*(397)

0.911(401)

<45m

0.528(404)

0.955(330)

0.994(328)

0.387(295)

0.490(399)

0.984(405)

45-75 m

0.327(400)

0.050(396)

0.061(201)

0.013*(398)

0.975(401)

0.584(400)

>75m

0.000**(388)

0.021*(402)

0.322(315)

0.236(292)

0.964(400)

X"

1.000(394)

0.414(459)

a Asterisks indicate significance at P < 0.05* or P < 0.01**.b One site, designated "X," did not conform to any of the three depth categories.

Trawl catches of rainbow smelt, examined witha hierarchical analysis of variance, did not revealdifferences between management zones (P > 0.25)or among sampling periods within zones (0.10 <P < 0.25). A significant difference (P < 0.05) intotal catch was observed between depths withinsampling periods. This statistical procedure doesnot provide further information on the nature ofthese differences (i.e., which depths differed).

Trends in mean CPUE were associated with wa-ter depth and sampling period. Greatest catchesof rainbow smelt were generally associated withshallow sites (<45 m) throughout the study (Table2). Monthly mean CPUE ranged from 215 rain-bow smelt in April to 655 and 502 smelt in Augustof 1984 and 1985, respectively (Table 4). Larger

catches of rainbow smelt observed in August andOctober were associated with recruitment of year-ling fish to the catch (Table 5). However, due tothe association of CPUE with water depth, month-ly mean CPUE may not be accurate for samplingperiods in which all sites (and therefore depths)were not sampled (August 1984, October 1984 and1985).

Catches in April 1985 were the most variableand resulted in 95% confidence intervals (CI) forCPUE estimates of ±99% and ± 117% for zones3A and 3B, respectively, and ±74% for the com-bined estimate (Table 4). The most consistentcatches were in June 1985, when the 95% CI forthe mean CPUE estimate was ±13% and ±27%for zones 3A and 3B, respectively, and ±15% for

TABLE 2.—Mean catch per unit of effort (CPUE)3 and coefficients of variationb (in parentheses) of rainbow smeltfrom replicate stepped-oblique midwater trawls at various sites in the Main Lake of Lake Champlain during 1984-1985.

DateAug 1984

Oct 1984

Apr 1985

Jun 1985

Aug 1985

Oct 1985

<45 mc

30(37.4)317(26.2)825(34.2)

Zone 3 A

45-75 ffl

648(26.8)401(95.6)268(12.6)631(14.7)146(16.9)

>75m

729(15.6)154(15.6)284

(5.7)392(12.6)329

(6.9)

<45mc

1,008(24.6)264(42.5)569(84.0)232(33.9)638(30.5)680(16.2)

ZoneSB

45-75 m

312(10.2)531(22.4)117(13.7)318

(1.5)383(26.9)335(14.6)

>75m

407(18.8)470(35.1)15

(61.3)158(17.0)147

(4.4)366(21.3)

* CPUE = rainbow smelt catch per 55-min trawl. Catch does not include young of the year.b Coefficient of variation » 100 x SD/mean.c Trawls lasted for 35 min. Catch expanded to represent a 55-min trawl; CPUE = (catch/35) x 55.

MIDWATER TRAWLING FOR RAINBOW SMELT 173

TABLE 3.—Paired /-test comparisons of total rainbowsmelt catches from replicate stepped-oblique midwatertrawls in Lake Champlain, 1984-1985.

ComparisonAll trawlsAll trawls except

Apr 1985

N31

25

t2.14

1.50

Two-tailed>

0.041

0.150

the combined estimate. Trawling in August 1985provided the only additional complete sample,which resulted in a 95% CI for the mean combinedCPUE estimate of ±29%. October samples werecomplete only for management zone 3B in 1984and 1985, but revealed more precise estimates ofmean CPUE (±31% and ± 32%, respectively) thanAugust catches within the same year and zone(±50%, both years).

TABLE 4.—Mean catch per unit of effort (CPUE)3 formanagement zones 3 A and 3B in Lake Champlain fromstepped-oblique midwater trawls, 1984-1985. Ninety-five-percent confidence intervals (95% CI) are presentedfor each management zone, individually, and combinedwhen complete samples (i.e., two trawls per site, threesites per zone) were obtained.

Date andstatistic5

Aug 1984MeanSDN95% CI

Oct 1984MeanSDAT95% CI

Apr 1985MeanSDN95% CI

Jun 1985MeanSDN95% CI

Aug 1985MeanSDN95% CI

Oct 1985MeanSDAT95% CI

Zone3A

1,129

1

688.3130.0

4

195.0240.9

6±99%

289.546.76

±13%

615.8236.3

6±31%

237.0110.1

4

Zone3B

575.5357.3

6±50%

421.3162.7

6±31%

233.3339.8

6±117%

235.880.66

±27%

388.8240.7

6±50%

460.0182.2

6±32%

Combined

654.6387.7

7

528.1198.2

10

214.5281.6

12±74%

262.768.712

±15%

502.3256.3

12±29%

370.8189.1

10

TABLE 5.—Mean catch per unit of effort (CPUE)* andpercent composition (in parentheses) of rainbow smeltage-classes derived from stratified sampling of stepped-oblique midwater trawl catches in Lake Champlain, 1984-1985.

DateAug 1984

Oct 1984

Apr 1985

Jun 1985

Aug 1985

Oct 1985

1

352(54)171(32)88(41)11(4)

161(32)99(27)

2

210(32)225(43)74(35)107(41)188(38)110(30)

Ageb

3

83(13)106(20)37(17)106(40)120(24)122(33)

4

10(2)17(3)15(7)34(13)29(6)36(10)

>5

(0)9(2)

(0)5(2)4(1)4(1)

a CPUE = rainbow smelt catch per 55-min trawl. Catch doesnot include young of the year.

b N = number of trawls.

a CPUE = rainbow smelt catch per 55-min trawl. Catch doesnot include young of the year.

b Recruitment to next age-class was assumed to occur in Junesamples.

In an attempt to address short-term temporalvariability in CPUE at a specific site, additionalstepped-oblique midwater trawls were fished at theshallow (<45-m) and deep (>75-m) sites in man-agement zone 3B on four and five consecutivenights, respectively, in August 1987. Daily meanCPUE estimates were substantially more variableat the shallow site (Figure 5), possibly due to thegreater influence of physical structure on the dis-tributions of rainbow smelt. In this particular case,the shallow site transects the mouth and a portionof a bay, whereas the deepwater site transects amuch more uniform habitat of open water greaterthan 75 m in depth (Figure 1).

These data were also used to estimate the num-ber of trawls necessary to obtain certain precisionestimates for mean CPUE at individual samplingsites (Snedecor and Cochran 1980). To obtain aprecision of ±25% of the mean CPUE at the 95%CI, 3 and 11 trawls would be required at the deepand shallow sites, respectively. Mean CPUE pre-cision estimates of ±50% would only require oneand three trawls, respectively.

Rainbow smelt age composition and length dis-tribution varied significantly among sites andamong depths throughout this study (Table 6).However, there was no systematic association ofage or length distributions with depth or location.Consequently, random variation among sites wasassumed, and age structure was summarized ineach sampling period for all sites (Table 5). Ages1-3 accounted for about 90% of the total catch in

174 KIRN AND LABAR

LLJDa.o

270-

240

210

180

150

120

90en

< 45 m

3

4

? 1; ' t'

LLJ

CL0

270

240

210180

150

120

90

> 75 m

2 3 4

' * + - +

1 2 3 4Night number

4

5

FIGURE 5.—Mean catch per unit of effort (CPUE; catchper 55 min) of rainbow smelt captured in stepped-obliquemidwater trawls on consecutive nights in managementzone 3B of Lake Champlain in August 1987. Verticalline represents ±SE; the number above the bar representsthe number of trawls on a particular night.

all sampling periods. Although yearling fish con-stituted a substantial proportion of the total catch,rainbow smelt generally were not fully recruitedto the midwater-trawl gear until age 2.

DiscussionKoeller et al. (1986) stressed the need to address

species-specific behavioral characteristics (e.g., dielor ontogenetic vertical migration) when a surveydesign was developed for monitoring relativeabundance of pelagic fishes. Discrete-depth mid-water trawling in this study provided importantobservations of rainbow smelt nighttime verticaldistributions, although these observations pre-clude the use of this technique for population as-sessment. Delayed vertical migration of smallersize-classes of rainbow smelt was observed in June1984 trawl sampling. Size or age partitioning, ob-served vertically within the water column at nightin this study, has been previously reported for hor-izontal (MacCallum and Regier 1970; Argyle 1982;Plosila 1982) and daytime vertical distributionsof rainbow smelt (Ferguson 1965).

These behavioral observations must be consid-ered when a quantitative sampling technique isdeveloped for rainbow smelt. Sampling errors in-duced by delayed vertical migration of smaller fishcan be easily avoided by sampling after the upwardvertical migration has been completed. Size or agepartitioning of rainbow smelt in the water column,however, requires uniform sampling of each depthstratum to obtain unbiased estimates of length andage distributions. Stepped-oblique midwatertrawling, as described in this study, avoided po-tential sampling errors introduced by the verticalmigration behavior of rainbow smelt and providedconsistent catches and length distributions of rain-bow smelt, as well as sufficient samples for bio-logical information.

Spatial and seasonal variability must also beconsidered when an overall sampling strategy for

TABLE 6.—Chi-square analysis of rainbow smelt age compositions and 10-mm length distributions from stepped-oblique midwater trawls in Lake Champlain, 1984-1985. Significance levels are presented from comparisons ofage compositions and length distributions (in parentheses) among sites; among depths; within depths, betweenzones; between zones; and within zones, among depths.

DateAug 1984

Oci 1984

Apr 1985

Jun 1985

Aug 1985

Oct 1985

Amongsites

<0.001(<0.001)<0.001

(<0.001)<0.001(0.008)

<0.001(<0.001)<0.001

(<0.001)<0.001(0.023)

Amongdepths<0.001

(<0.001)<0.001

(<0.001)<0.001(0.001)

<0.001(0.003)

<0.001(0.001)

<0.001(0.006)

Within depth, between zones<45m

<0.001(<0.001)

0.179(<0.001)

45-75 m

<0.001(<0.001)<0.001(0.397)

<0.001(0.135)

<0.001(0.012)

<0.001(0.020)

<0.001(0.091)

>75m

0.222(0.160)

0.371(0.031)0.014

(<0.001)<0.001(0.460)

Betweenzones

<0.001(<0.001)

0.001(0.093)

<0.001(0.010)

<0.001(0.003)

<0.001(<0.001)<0.001(0.002)

Within zones,among depths

3A

<0.001(<0.001)<0.001(0.028)

<0.001(<0.001)

0.001(0.183)

<0.001(0.135)

3B

<0.001(<0.00l)<0.001(0.002)

<0.001(<0.001)<0.001

(<0.001)<0.001

(<0.001)<0.001(0.359)

MIDWATER TRAWLING FOR RAINBOW SMELT 175

monitoring rainbow smelt populations is devel-oped. Sampling several depth ranges in an area,as in this study, should help to avoid potentialsampling errors that may result from environ-mentally induced spatial or temporal distribu-tions. Length and age distributions were inconsis-tent among sites within sampling periods, althoughno systematic association with location or waterdepth was observed.

Although substantial catches of rainbow smeltwere observed at all sites, the largest catches ofsmelt were generally associated with shallow sites(<45 m) during this study. These findings are con-sistent with observations on Great Lakes rainbowsmelt populations (Ferguson 1965; Gray 1979;Crowder 1980; Heist and Swenson 1983). Heistand Swenson (1983) reported highest densities andabundances of rainbow smelt in shallow waters(<50 m) of Lake Superior, with a significant de-cline in densities with increasing bottom depth.They suggested that nearshore distributions ofrainbow smelt were associated with warmer tem-peratures and daily vertical movements. The noc-turnal spawning habits of rainbow smelt (Scott andCrossman 1973) probably resulted in the highlyvariable catches among sites and between replicatetrawls in April. The effect of shoreward spawningmovements on abundance measurements also ledArgyle (1982) to recommend fall surveys due tothe more predictable locations of rainbow smeltat that time.

Although June sampling provided the most con-sistent rainbow smelt CPUE among sites (95% CI= ± 15%), sampling in August and October wasmore advantageous, because in those months,trawls provided the greatest rainbow smelt catches,including large catches of yearling smelt. The sea-sonal peaks of rainbow smelt CPUE in Augustobserved in this study were also reported for LakeErie (Ferguson 1965). Increasing water tempera-tures and thermal stratification were cited as themajor factors initiating the return of rainbow smeltto deeper waters and increasing their tendency toform schools. During the summer months in LakeErie, both yearling and adult rainbow smelt wereobserved moving into the thermocline and becom-ing more vulnerable to trawling gear (MacCallumand Regier 1970). Thermal stratification of LakeChamplain was not observed during trawl sam-pling in 1985 until August, when increased catchesreflected recruitment of yearling rainbow smelt tothe trawl. Considering that young-of-the-year smeltas small as 40 mm were often captured in largenumbers, increased catches of yearling rainbow

smelt (>90 mm) were most likely the result ofimmigration rather than the fish attaining a gear-selective size threshold.

Gear restrictions limited trawl sampling to theupper 35 m of the water column during this study.To obtain representative samples of the rainbowsmelt population with stepped-oblique trawls, wemust assume that the majority of rainbow smeltwere confined to this stratum. If a substantial por-tion of the rainbow smelt population was not sam-pled in our sites deeper than 45 m, we would ex-pect larger rainbow smelt to be systematicallyunderrepresented in these samples when com-pared with samples from shallow (<45-m) sites(where the entire water column was sampled), dueto stratification of larger rainbow smelt in deeperwaters. This pattern was not observed.

The results of this study suggest that stepped-oblique midwater trawling at night is an effectivesampling technique for obtaining estimates of rel-ative abundance with acceptable precision levelsand unbiased estimates of length and age distri-butions of rainbow smelt. Replicate stepped-oblique trawls provided CPUE estimates for in-dividual management zones with precisions of± 13-50% for samples in June through October.The addition of a third oblique trawl per site wouldreduce these ranges an additional 5-9% per man-agement zone if similar variation between trawlsis assumed. This trawling procedure is mechani-cally "clean," unlike bottom trawling, and was es-sentially species specific for rainbow smelt, whichconstituted 97% of the total catch in Lake Cham-plain. Although a large vessel and net were usedin this study, the procedure and equipment couldbe easily scaled down for use with small boats toprovide an inexpensive and effective monitoringtechnique for state agencies with limited resources.

AcknowledgmentsWe thank Richard Furbush, Captain of the RV

Dore, who provided valuable assistance through-out the study. We also thank Kathy Newbroughfor her help in the laboratory, field, and office.Gary Badger and Michael Constanza provided ad-vice on statistical techniques, and Barry Johnsoncritically reviewed the manuscript. The VermontFish and Wildlife Department provided the use oftheir research vessel. The project was funded bythe U.S. Fish and Wildlife Service, Project AFS-6-R-2.

ReferencesArgyle, R. L. 1982. Alewives and rainbow smelt in

Lake Huron: midwater and bottom aggregations and

176 KIRN AND LABAR

estimates of standing stocks. Transactions of theAmerican Fisheries Society 111:267-285.

Crowder, L. B. 1980. Alewife, rainbow smelt and na-tive fishes in Lake Michigan: competition or pre-dation? Environmental Biology of Fishes 5:225-233.

Dixon, W. J., and six coauthors. 1983. BMDP statis-tical software, 1983 printing with additions. Uni-versity of California Press, Berkeley.

Ferguson, R. G. 1965. Bathymetric distribution ofAmerican smelt, Osmerus mordax, in Lake Erie.Proceedings, Conference on Great Lakes Research,13:47-60.

Frie, R. V., and G. R. Spangler. 1985. Dynamics ofrainbow smelt during and after exploitation in SouthBay, Lake Huron. Transactions of the AmericanFisheries Society 114:713-724.

Gray, J. E. 1979. Lake Ontario tactical fisheries plan:resource document,?; coldwater rehabilitation: (1)sea lamprey (2) alewife (3) smelt (4) sculpins (5)deepwater ciscoes. Ontario Ministry qf Natural Re-sources, Toronto.

Havey, K. A. 1973. Effects of a smelt introduction ongrowth of landlocked salmon at Schoodic Lake,Maine. Transactions of the American Fisheries So-ciety 102:392-397.

Heist, B. G., and W. A. Swenson. 1983. Distributionand abundance of rainbow smelt in western LakeSuperior as determined from acoustic sampling.Journal of Great Lakes Research 9:343-353.

Ketchen, K. S. 1950. Stratified subsampUng for deter-mining age distributions. Transactions of the Amer-ican Fisheries Society 79:205-212.

Kircheis, F. W., and J. G. Stanley. 1981. Theory andpractice of forage-fish management in New England.Transactions of the American Fisheries Society 110:729-737.

Kirn, R. A. 1986. Assessment of rainbow smelt (Os-merus mordax), as forage in Lake Champlain. Mas-ter's thesis. University of Vermont, Burlington.

Koeller, P. A., P. C. F. Hurley, P. Perley, and J. D.Neilson. 1986. Juvenile fish surveys on the Scotianshelf: implications for year-class size assessments.Journal du Conseile, Conseil International pour1'Exploration de la Mer 43:59-76.

Luey, J. E., and I. R. Adelman. 1984. Stock structureof rainbow smelt in western Lake Superior: popu-lation characteristics. Transactions of the AmericanFisheries Society 113:709-715.

MacCallum, W. R., and H. A. Regier. 1970. Distri-

bution of smelt, Osmerus mordax, and the smeltfishery in Lake Erie in the early 1960's. Journal ofthe Fisheries Research Board of Canada 27:1823-1846.

Mayden, R. L., F. B. Cross, and O. T. German. 1987.Distributional history of the rainbow smelt, Os-merus mordax (Salmonifonnes: Osmeridae), in theMississippi River basin. Copeia 1987:1051-1054.

Meyer, G. E., and G. K. Gruendling. 1979. Limnologyof Lake Champlain. New England River BasinsCommission, Lake Champlain Basin Technical Re-port 30, Burlington, Vermont.

Plosila, D. S. 1982. Lake Champlain bottom trawlingprogram 1977-1980. New York State, Departmentof Environmental Conservation, Albany.

Plosila, D.S. 1984. Spacial [sic] distribution of rainbowsmelt spawning in the New York waters of LakeChamplain. New York Fish and Game Journal 31:109-110.

Plosila, D. S., and J.Anderson. 1985. Lake Champlainsalmonid assessment report. New York State De-partment of Environmental Conservation and Ver-mont Fish and Wildlife Department, Albany, NewYork.

Potash, M., S. E. Sundberg, and E. B. Henson. 1969.Characterization of water masses of Lake Cham-plain. Internationale Vereinigung fur theoretischeund angewandte Limnologie 17:140-147.

Ricker, W. E. 1975. Computation and interpretationof biological statistics offish populations. FisheriesResearch Board of Canada Bulletin 191.

Scott, W. B., and E. J. Crossman. 1973. Freshwaterfishes of Canada. Fisheries Research Board of Can-ada Bulletin 184.

Selgeby, J. H. 1985. Population trends of lake herring(Coregonus artedii) and rainbow smelt (Osmerusmordax) in U.S. waters of Lake Superior, 1968-84.Report to Council of Lake Committees Plenary Ses-sion (March 20, 1985), Great Lakes Fishery Com-mission, Ann Arbor, Michigan.

Smith, S. H. 1972. The future of salmonid commu-nities in the Laurentian Great Lakes. Journal of theFisheries Research Board of Canada 29:951-957.

Snedecor, G. W., and W. G. Cochran. 1980. Statisticalmethods, 7th edition. Iowa State University Press,Ames.

SPSSX. 1983. SPSSX user's guide. McGraw-Hill, NewYork.