Limnol. Oceanogr., 27(2), 1982, 304314 @ 1982, by the American Society of Limnology and Orwmography, Inc.

Reproductive cycle and fecundity of Euphausia pacijica in Puget Sound, Washington1

Robin M. ROSS,~ Kendra L. Daly, and T. Saunders English School of Oceanography WB-10, University of Washington, Seattle 98195

Abstract

The length and timing of the spawning season and the fecundity of Euplaausia pacijicu in northern Puget Sound, Washington, were investigated by analyzing preserved samples and by observing spawning of euphausiids in the laboratory. Gravid females (stage IV) and ripe females (spawning within 24 h) first appeared in mid-April. Spawning of the overwintered females was most intense in April and May, but continued at a low level throughout most of June. No ripe females ~19 mm were found after mid-May. Larger females disappeared by the end of summer. Females born in April began to develop ovaries (stage I) in mid-June, and although a few matured to stage IV and released eggs in late July-early August, most of the population was stage II (with immature eggs in the ovaries) in the fall, overwintered as stage II, and began to mature again in late winter or early spring. The brood size, brood frequency, and length of the spawning season varied with female size. Small females released a maximum of 11-12 broods (- 1,000 eggs) over a l-month period from mid-April to mid-May. Over a 2-month period large females released a maximum of 2531 broods (-3,700 eggs). A comparison of laboratory and field data suggested a method which may lead to accurate estimates of fecundity from preserved samples.

Reproductive effort, the amount of en- ergy or material an individual invests in eggs or sperm over a lifetime, can be sig- nificant both to the energetics of the in- dividual and to predators which consume eggs and larvae. The reproductive effort of large marine pelagic malacostracans that broadcast their eggs has been esti- mated largely from preserved samples (Mauchline and Fisher 1969; Omori 1974). Unfortunately, preserved samples do not establish a time scale for the es- timated reproductive effort, so calculat- ing an egg production rate or an accurate total fecundity is impossible. The frac- tion of eggs in the ovary released in 1 day, the number of broods, and the length of the spawning season for an in- dividual are unknown.

Fecundity, defined as the total number of eggs released by an individual during

l This work was supported in part by a contract with the Washington State Department of Ecology. Support and facilities for preparation of the manu- script were provided by the Australian Institute of Marine Sciences and the Department of Neuro- biology, Australian National University.

Z Present address: Marine Science Institute, Uni- versity of California at Santa Barbara 93106.

a spawning season, has been estimated for several euphausiid species. In most studies stages of egg maturity were de- fined, and the number of eggs in the ovary of one or several stages counted (Zelikman 1958; Naumov 1962; Pono- mareva 1966; Brinton 1976). Mauchline (1968) defined fecundity as the number of mature eggs equal to a volume half that of the ovary. However, the time that an egg remains in each stage is unknown, so the number of mature eggs per ovary may be for the whole or for only a small part of the spawning season. Methods that in- volve different assumptions about the time scale result in estimates of the fe- cundity of one species, Euphausia pacif- ica, from 100 to 1,400 eggs per female (Mauchline and Fisher 1969; Ponomare- va 1966). However, preserved samples can be used to determine the timing of the reproductive cycle by using either the appearance of gravid females (with mature eggs) or eggs and larvae to indi- cate the beginning of the spawning sea- son and thus its timing, duration, and the times of recruitment (Ponomareva 1966; Hulsizer 1971; Smiles and Pearcy 1971; Brinton 1976).

We have estimated the fecundity of E.

304

Euphausiid fecundity 305

pacifica from northern Puget Sound, Washington, by using live animals brought back to the laboratory through- out the spawning season and allowed to release eggs. Both live animals and pre- served samples were used to determine the reproductive cycle. On the basis of comparison of the results from live and preserved animals, we suggest a more ac- curate way to calculate the fecundity of E. pacifica, and perhaps other euphau- siids, if only preserved samples are avail- able.

We thank B. W. Frost and L. B. Quetin for their advice and review of the manu- script. We also thank D, Thoreson and J. Jen for assistance in collecting the ani- mals.

Materials and methods Euphausia paci$ca was collected from

Port Susan, part of Puget Sound a few kilometers north of Everett, Washington, at about 48”6’N, 122’22’W with either a l-m net or an NIO net, both of 571-pm mesh, towed obliquely from close to the bottom. The samples were taken about every 2 weeks throughout the 1976 spawning season and for all but the last few weeks of the spawning season in 1977; brood size and the beginning and end of the spawning season were esti- mated from the combined data. On each sampling date about 200 adults, usually all those in a haul, were brought back to the laboratory, placed in 4-liter jars con- taining a suspension of cultured diatoms (20 animals per jar), and kept at 8°C in dim light or the dark. The euphausiids were inspected four times in the 24 h fol- lowing capture to identify females about to release eggs.

In E. pacifica the mature ovary (with eggs ready to be released) appeared as a bluish-purple band under the heart, but the eggs were pale pink after release, Occasionally females with purple ovaries did not release eggs; the reason is un- known. The blue pigment is probably an astaxanthin-protein complex, similar to the complex found in goose barnacle eggs and nauplii (blue) or lobster eggs (green). In lobster eggs the chromopro-

tein linkage changes shortly before hatching, liberating the astaxanthin and giving the larvae a red tinge (Goodwin 1960). In E. paci$ca the change in the linkage seems to occur close to the time when the eggs are released.

We isolated individual females with purple ovaries in l-liter beakers contain- ing a suspension of diatoms, kept them at 8” or 12”C, and checked for eggs daily in the morning until the purple color dis- appeared. When eggs were found, we re- moved the female, determined the dry weight, total length, or both, and counted the eggs. When only total length was measured, we calculated dry weight from the equation describing the relationship between dry weight (DW, mg) and total length (TL, mm) for ripe females: DW = 1.45 TL - 18.06 (r = 0.943, n = 41).

We determined the frequency of broods for six samples taken in 1977 by analyzing the female population ~12 mm; Hulsizer (1971) found that the smallest females carrying ripe eggs were 12 mm long. The proportion of females >12 mm that turned purple and released viable eggs within 24 h (the proportion of ripe females) was determined for all females and for three arbitrary size classes: small, ~17 mm; medium, 17 to ~19 mm; and large, 19-23 mm. We based the size classes on two observations: only females >19 mm released eggs on the last two sampling dates; and the length- frequency histograms on the first two sampling dates showed two possible modes, with a separation at 16-17 mm (Fig. 1). If there is no synchrony in the spawning of individual females and if all females are fertile, the proportion of the female population that releases eggs in a 24-h period is equivalent to the daily fre- quency with which an individual female releases eggs. The inverse of this fre- quency is the interval in days between broods (D). We calculated the brood in- terval and the total number of broods (N) for all females and each size class for each sampling date when eggs were re- leased as

N = 2 TID i=l

306 Ross et al.

18 APR

I& layer depth in Port Susan. We estimated layer depth, the depth of the highest con- centration of euphausiids, from echo- grams obtained with a Ross 200-A fine- line echosounder operating at 105 kHz with a 10” hull-mounted transducer. To- tal depth was 130 m in Port Gardner and 115 m in Port Susan. The samples were preserved in 10% buffered Formalin. g 2 MAY 1 I17MAY

LENGTH (mm) 6 JUN

10

5

L-AI 10 15 20

LENGTH (md

Fig. 1. Length-frequency histograms of all fe- male and ripe female (shown solid) Euphausia pa- cijka at five sampling dates during 1977 spawning season.

where .j is the number of sampling dates on which females were caught that re- leased eggs and T is the length of time during the spawning season when a brood interval was valid. We determined the time interval by finding the dates halfway between the sampling date on which the brood interval was determined and the sampling dates immediately be- fore and after, e.g. the brood interval of the 2 May sample (the third estimate) holds for 14 days, from 26 April-halfway between the second and third sampling dates-to 9 May-halfway between the third and fourth- sampling dates.

We collected samples of E. paci$ca from 1973 to 1978 in Port Gardner, part of Puget Sound a few kilometers south of Everett, at about 47”58’N, 122”16’W, and in Port Susan, with an electronically op- erated, opening-closing NIO net of 571- pm mesh. There were 23 collecting trips in March through August, and 14 in fall and winter. Replicate horizontal hauls were made at 0, 10, 20, layer depth, 60, and 100 m in Port Gardner, but only at

The preserved samples were split with a modified sand splitter (Cooney 1971) to a subsample size of about 100 animals. The total length of the euphausiid was measured in l-mm increments from just behind the eyes to the tip of the telson. Each individual was classified as <6 mm, juvenile (no internal or external sexual characteristics), male, or one of four fe- male sexual maturity stages. The stages of maturity were slight modifications of those of Hulsizer (1971) and Ponomareva (1966): stage I-ovary is small and flat, lies behind and above the heptopancrcas, and contains small, yellow, transparent rounded eggs; stage II-ovary is large, and closely packed polygonal eggs are of uniform size; stage III-ovary extends into the abdomen, eggs are tightly packed and no longer transparent; stage IV-ovary completely fills the thoracic cavity, pushing aside the other organs, several sizes of eggs are present, and spermatophores are often attached to the thelycum.

At Port Gardner the average number per cubic meter refers to the entire water column, at Port Susan only to the layer depth, On the basis of the vertical distri- bution of the different sizes of E. pacifica at Port Gardner, we judge that the catch data for only layer depth may underesti- mate the frequency of individuals <6 mm but probably is the same for the other cat- egories. We express concentrations of ~6 mm and juvenile individuals as a per- centage of the total number of individu- als caught and the four stages of maturity of female cuphausiids as a percentage of the total number of females.

Results Although euphausiids with purple ova-

ries were rarely observed during daylight

Euphausiid fecundity 307

hours, some captured in the afternoon had purple ovaries by midnight. About 70% of purple females released eggs, but only eggs released within 24 h after the ovaries turned purple hatched (85% of the broods). We will call ripe females those that released viable eggs within 24 h of the ovaries turning purple. Thus the proportion of the female population that turns purple the first night after capture and releases viable eggs is the daily fre- quency of ripe females, or the daily fre- quency with which an average female re- leases broods of eggs.

The size composition of ripe females was not the same throughout the 1977 spawning season (Fig. 1). There were small ripe females in early April, but none ~19 mm after mid-May. Large ripe females were found from mid-April to early June. To test the possibility that small ripe females disappeared because they grew, we calculated the increase in length attributable to growth for each sampling date, using the initial average length of ripe females and the growth rate (0.042 mm * d-l) found by Hulsizer (1971) for Port Susan euphausiids during their second growing season (Table 1). If the calculated increase in length and the mean length of the sampled ripe females was the same (based on 95% C.I.), growth alone was assumed to be responsible for the disappearence of small animals. If the lengths were different, then the seasonal size composition of ripe females must have been affected by additional factors, such as differential predation or mortali- ty, emigration, or a spawning season that varied in length with female size. During a spawning season of 70 days, small fe- males ~16 mm do not grow to > 19 mm but remain small or medium, only releas- ing broods for about 1 month. Medium females >17.75 mm grow enough in the first month to be classified as large for the rest of the spawning season, while the fe- males of 16.00-17.75 mm grow to bc large in the second month. Thus females >16 mm at the beginning of the spawning sea- son probably released broods for >2 months. Although the large females on 6 June were not significantly smaller than

Table 1. Average total length, 95% confidence intervals, and number of observations of ripe female Euphausia pad&a during 1977, and new size of the initial spawners attributable to growth between sampling dates.

14 Apr 18 Apr

2 May 17 May

6 Jun

Lyg& by’

<sanlpicci)’

16.38kO.60 17.18-cO.69 18.19+ 1.45 20.59+2.16 19.61k0.82

New length (mm)

(growth only) N (0.042 mm.d-I)

20 26 16.55

6 17.14 4 17.77 3 18.61

those on 17 May (t-test, P < 0.05: Sokal and Rohlf 1969), the lack of growth sug- gested that the largest females may be emigrating, dying, or ceasing to release broods.

From the frequency of ripe females of all sizes, the spawning season seems to last 10.5 weeks, with a brood interval of 2-3 days for the first 7 weeks and 10 days for the last 3.5 weeks (Table 2). But the brood interval, the length of the spawn- ing season, and thus the total number of broods produced in a spawning season, varied with female size (Table 2). Eleven to twelve broods were produced in 1 month by females remaining in the small and medium size classes. The brood in- tervals were 2.0-2.5 days for small fe- males but more variable for medium fe- males, 1.6-4.0 days (Table 2). Large females had a much longer spawning sea- son, producing 25-31 broods in the 2 months from mid-April to mid- June. Dur- ing the first 6 weeks of the spawning sea- son, the large females produced broods faster than the small and medium ones, but near the end of the spawning season only produced a brood every 6 days.

The relationship between brood size (E, number of eggs) and the dry weight (DW) of the female was linear (Fig. 2), E = 39.07 + 7.87DW. Only egg counts of broods released within 24 h of the ovaries turning purple were used to derive this equation. Although variability was large (the linear correlation coefficient was only 0.357), the regression was signifi- cant (P < 0.001). The relationship be- tween brood carbon and nitrogen and

308 Ross et al.

Table 2. Number of broods released by all sizes and three size classes of Euphausia pacifica: num- ber of broods, average length of female, brood in- terval (II), and the relevant time interval (T) for each estimate of D. Total number of broods re- leased over a reproductive season by each size class is calculated as sum of independent estimates from each sampling date. X is mean number of broods, calculated from mean brood interval and total length of reproductive season for a size class.

No. broods

14 Apr 2.61 18 Apr 4.66 2 May 4.42

17 May 5.54 6 Jun 2.52

Total 19.75 x 17.38

14 Apr 3.00 18 Apr 3.53

2 May 5.60 Total 12.13 x 12.34

14 Apr 18 Apr 2 May

Total x

2.00 5.49 3.50

10.99 10.07

18 Apr 7.20 2 May 7.00

17 May 12.00 6 Jun 4.33

Total 30.53 x 24.93

All 16.38 17.18 18.19 20.59 19.61

17.37

Small 15.73 15.70 16.83

15.80

Medium 17.92 17.67 18.39

17.85 2.88

Large 19.92 20.33 20.59 19.61

1.25 2.00 1.50 6.00

20.10 2.69

2.30 1.93 3.17 3.25

10.33

4.20

2.00 2.55 2.50

2.35

3.00 1.64 4.00

6 9

14 18 26 73

6 9

14 29

6 9

14 29

9 14 18 26 67

female body carbon and nitrogen was de- rived using the equations describing the relationships between dry weight and body carbon or nitrogen (pg C = - 1.985 + 0.401 pg DW; pg N = -6.136 + 0.117 pg DW) and the carbon and nitrogen contents of an egg (2.58 pg C; 0.50 pg N) found by Ross (1979). Thus brood carbon (B,) and nitrogen (BN) were linear func- tions of female body carbon (WC>’ and ni- trogen (W,): Bc = 100.90 +. O.O51Wc, and BN = 19.74 + O.O34W,). We assumed that the variation in brood size caused by fac-

b

260 NO. EGGS = 39.07 + 7.67 DW

.

200 j

60. l .

I . .

40 . l .

. ‘. “0 0

.

b

20 l

.* . .

l APR +MAY 0JUN

L,, . l . l *

3 5 7 9 11 13 15 17 19

$’ DRY WEIGHT (mg)

Fig. 2. Relationship between brood size (num- ber of eggs) and dry weight of female Euphausia pacijca.

tors other than female size was random, and that these equations could be used to estimate the average brood size of a female over the season. Since female size only explained a fraction of the variability found in brood size, further investiga- tions are needed before we have a rela- tionship that will predict the size of any one brood. Food availability may affect brood size, as suggested by the correla- tion found between upwelling and the maximum egg production of E. pacifica (Brinton 1976). However, recent research on the egg production rate of Calanus paci$cus in Puget Sound has shown that brood frequency but not size was affected by food availability (J. Runge pers. comm.).

Some of the variability may be because brood size changes during the spawning season. The brood size of small and large animals remained constant over the re- productive season, but that of medium

Euphausiid fecundity 309

Table 3. Seasonal distribution of mean and range (in parentheses) of brood size of small, me-

dium, and large Euphausia pacijka in 1977 (n- number of broods in sample).

Small Medium Large

14 Apr 48(10-86) 54(33-126) 0 n = 14 n=4

18 Apr 39(3-110) 120(3-204) 177(71-234) n = 11 n = 12 n=5

2 May 170(24-265) 122(59-190) 71 - n=2 n=3 n=l

17 May 0 0 130(83-204) n=4

6 Jun 0

H” 1.17

Mean 60+21 brood (95% C.I.) size

0 78(71-w) n=3

10.21-F 2.09

132a79

* Statistic for Kruskal-Wallis one-way ANOVA (Sicgcl 1956). t P < 0.01.

animals increased (Table 3) (Kruskal- Wallis one-way ANOVA, P < 0.05: Siegel 1956). The mean brood size for small an- imals was 60 eggs, that of large animals 132. Since these conclusions were based on small numbers of euphausiids, our re- sults might change with additional data. Nevertheless our results indicated that a change in brood size with season was possible.

We used the average number of broods released daily by animals in each size class (l/D), the number of eggs in a brood, and the relationships between the carbon and nitrogen content of the brood and the female to calculate the daily re- productive effort, the number of eggs and the percent body carbon and nitrogen re- leased daily, by small (16 mm, 2,023 pg C, 555 pg N), medium (18 mm, 2,939 pg C, 806 pg N), and large females (20 mm, 4,118 pg C, 1,130 pg N). Small females released 4.3 and 3.0% of their body car- bon and nitrogen, or 33 eggs per day, and medium females, 2.9 and 2.0%. In the first half of the spawning season large fe- males released 74 eggs, or 4.7 and 3.3% of their body carbon and nitrogen daily; however, at the end of the spawning sea- son these rates decreased by almost half

to 2.8 and 1.9%, or 44 eggs. If a female remained in the small and medium size classes and survived the entire reproduc- tive season she released about 121 and 83% of her body carbon and nitrogen as eggs during this time. A large female that survived until mid-June would release 3,670 eggs, or 234 and 160% of her body carbon and nitrogen.

The results of the analysis of the pre- served samples are presented as a com- posite of the sampling years (Fig. 3). Stage IV females were first found in the field in mid-April, coinciding with the appearance of the first ripe females, ex- cept in 1978 when stage IV females were found in late March. The percentage of stage IV females decreased from early May through June to close to zero in late July, then increased slightly in late July-early August. These late summer stage IV females were small first-year fe- males, not the population reproducing in the spring. The percentage of ripe fe- males (live samples) was never as large as that of stage IV females (preserved samples), and began to decrease before the stage IVs (Fig. 3a). By late June or early July few ripe females were found. The proportion of stage IV females was usually about twice that of ripe females, suggesting that stage IV females did not release viable eggs daily.

The highest percentage of furcilia (lar- vae < 6 mm) occurred about 1 month af- ter the peak in stage IV and ripe females (Fig. 3b). The proportion of the popula- tion <6 mm slowly decreased from 50% in mid-May to ~5% in mid-July, then usually remained at a low level through summer and early fall. The exception was in late August 1975, when recruitment was unusually high and almost 20% of the population was <6 mm.

Half of the animals were juveniles by mid-May (Fig. 3~). The number of juve- nilcs stayed relatively constant through- out June and July, dropped sharply at the end of July to ~5% of the population, and increased again in mid-August to about 30%, except in the May 1974 sample from Port Susan when neither furcilia nor ju- venile euphausiids were found (Fig. 3b,

310 Ross et al.

80 ok

0 (1978): * ’ 1, : (a)STAGE IV AND

/e \ RIPE 9 9

(b) e6mm

100 - (c)JUVENILES

80- : ; 60- c- *A---$

E 40. &-

A \ \

/

s 20- I I , I /

c’ o-G---b A (1974)

15M 24A 3J 13J 22

80

z 60 ti 140 s

20

80 o+

0

(d) STkGE I 9 9 /‘A\

(e) STAGE II ‘$ !$ t /i : \

A: A I I

, \ \ . (1978) 6:

\ ,/ .

’ \ I \ \ \

loo- (f) STAGE III ? 9

80- ot 8 y 60- 9 \ p 40.’ s 4

20. ‘\\\ \ l c-.--d-

oe l -+---Aw--.-&---- A l

IA 15M 24A 3J 3J 2: DATE

A

Fig. 3. Reproductive cycle of Euphuzdsiu puci$cu at two stations, 1973 through 1978: Port Garclner- 0; Port Susan-A. In panel a, A-percent ripe females (of total females), Port Susan, 1977.

c). However, early and late furcilia did The proportion of stage I females, first- not occur at the same depths. When fur- year females (6-12 mm during spring) cilia first appeared in May, they were just beginning to develop ovaries, in- concentrated in the upper 20 m, but a creased to about 90% in late June, then month later were at the layer depth for decreased to ~20% (Fig. 3d), as that of the whole population (40-60 m). The ear- stage II increased to almost 100% in mid- ly furcilia, therefore, usually occurred in August (Fig. 3e). The number of stage III the upper 20 m, not at layer depth, which females increased slightly in late July to was the only depth sampled in Port Su- mid-August, but was always < 15% (Fig. san. The lack of small individuals in May 3f). As stated above, in some years a small 1974 may be because of a late spawning proportion of these first-year females ma- or a slower growth rate that spring, either tured to stage IV and produced the fur- of which would delay the appearance of cilia and juveniles that appeared in late juveniles and late furcilia at the layer summer and early fall. All the large fe- depth. males (17-23 mm) that were over a year

Euphausiid fecundity 311

old disappeared by the end of summer. Almost all females were stage I or II in early fall, overwintered as stage II, then began to mature again in late winter and early spring. In mid-March the females were about half-and-half stage II and III, and by mid-April almost all stage IV.

The stations at Port Susan and Port Gardner are only about 15 km apart and tend to follow the same pattern of in- creases and decreases in in the distribu- tion of the maturity stages of the females and the proportion of the total population that is <6 mm or juvenile. However, in Port Gardner the first-year females seemed to mature slightly faster, and a few (~5%) were stage IV and 11-16 mm long by late July or early August. In mid- to late August, a few juveniles from this second spawning appeared. This late summer spawning of first-year females has not been observed at Port Susan.

Discussion The timing of the spawning season of

E. paci$ca varies throughout its geo- graphic range. In northern Puget Sound the spawning was most intense in April and May, whereas in Oregon coastal waters the major spawning period is late summer and early fall (Smiles and Pearcy 1971). In the colder waters of the western North Pacific Ocean, spawning occurs mainly from the end of May to mid-June (Ponomareva 1966). The southern Cali- fornia population includes stage IV fe- males and newly hatched larvae year- round, but recruitment is highest in May through July (Brinton 1976).

Einarsson (1945) suggested that factors associated with the increase in phyto- plankton are responsible for the onset of spawning. The northern Puget Sound spawning season was well correlated with maximum chlorophyll a concentra- tions, usually sometime from late March to early May (K. L. Daly and P. Walline unpubl.). In 1975 and 1978 the Chl a maximum was in late March, earlier than other years. In 1978, 100% of the female population was stage IV in late March, and the presence of f&-cilia and juveniles in April 1975 indicated that stage IV fe-

males must have been present in March 1975. In 1974 the Chl a maximum was in early May, and no furcilia or juveniles were found in April or in mid-May as they usually were. Off the Oregon coast and southern California, spawning and recruitment are most intense as a result of the upwelling regime which stimu- lates phytoplankton production (Smiles and Pearcy 1971; Brinton 1976).

The length and timing of the spawning season can vary even within a general geographical location, as shown by Hul- sizer’s (1971) comparison of the repro- ductive cycles of E. pacifica in Port Su- san and Carr Inlet (southern Puget Sound). She found furcilia in more months in Carr Inlet (April-October) than in Port Susan (April-August). Num- bers of larvae were highest in June in Port Susan, but in August in Carr Inlet. Year-old adults only survived until June in Carr Inlet, but until August in Port Su- san. These variations between locations may be because of differences in phyto- plankton production or in temperature, or inherent differences in growth or matu- ration between populations.

In our study the first-year females that were stage III and IV in August were not found in the fall samples. Either stage III and IV females died from physiological causes, or were subject to selective pre- dation or advection, or were sampled in- adequately. For the larger stage III and IV females in late July and August that have already overwintered, death from physiological causes was probable. But most of the individuals were smaller first- year females (12-17 mm). The first-year stage III females may have been unable to bring their eggs to maturity because of decreasing food availability and, instead of maintaining the eggs all winter with large yolk reserves, resorbed some of the material, decreasing the egg size to that of a stage II female. Makarov (1975) showed that Euphausia superba is able to spawn in two successive years, imply- ing that any one female can become ma- ture more than once. In the same way stage III and IV E. paci$ca females from Puget Sound in August may have re-

312 Ross et al.

gressed to stage II in fall and began the maturation cycle again in late winter. We think that lack of food rather than de- creasing temperature prevented matura- tion because temperatures at 40-60 m are still increasing from July through Sep- tember in Port Susan and Carr Inlet (Hul- sizer 1971). Photoperiod is unlikely to be a factor because late summer maturation does occur in Port Gardner but not in Port Susan, only 15 km north.

There are indications from other stud- ies of the reproductive cycles of euphau- siids that the size composition of ripe fe- males is not constant. Naumov (1962) observed that smaller E. superba females release eggs before the larger ones, and Mauchline (1968) found that small Me- ganyctiphanes norvegica females release eggs before the ovaries of the large ones are fully mature. The data on E. pacijica from northern Puget Sound were consis- tent with the view that large females may have taken slightly longer to mature than small females, but produced eggs for a longer period.

Previous studies of the fecundity of eu- phausiids assumed that multiple broods do not exist and that the spawning period of an individual is very brief. Newly re- leased eggs are present in plankton be- cause all individuals do not mature and release eggs simultaneously (Naumov 1962; Mauchline 1968). Assuming that once the ripe eggs are released the im- mature eggs in the ovary do not imme- diately ripen and lead to another brood, Mauchline (1968) proposed to estimate fecundity as the number of phase IV eggs whose volume equals half that of the ovary and calculated the fecundity of E. pacifica to be 100-200 eggs per female (Mauchline and Fisher 1969). Brinton (1976) found that the number of phase IV eggs in the ovaries of E. pacifica off southern California varied from 20 to 212, and linearly with female size, as was also found in two investigations of the num- ber of eggs in the brood pouch and the size of Nematoscelis dif$icilis (Komaki 1967; Nemoto et al. 1972). The linear re- lationship that we found between brood size and female size only explained a

small fraction of the variability, indicat- ing that all broods from one female were not the same size. Both the time during the spawning season (as in medium fe- males in our study) and the position of the brood in the reproductive cycle of the individual may affect brood size. Banse and Komaki (pers. comm.) found that the second brood released by female E. pa- cifica in the laboratory contains fewer eggs. But we do not know whether suc- cessive broods in the field contain fewer eggs, or whether size fluctuates randomly or in a pattern.

Several laboratory and field investiga- tions have shown that euphausiids pro- duce more than one brood in a season. There are two peaks in the egg produc- tion of M. norvegica from April to July, indicating that females are releasing eggs at two separate times, with an unknown number of broods each time (Mauchline and Fisher 1969). Brinton (1976) found pulses of maximum egg generation in populations of E. paci$ca every 2 months during the upwelling season off southern California, and he also suggested that eggs can ripen and females spawn more often than his monthly surveys showed because the number of phase IV eggs did not lead to enough larvae at the time of the next survey. Although he interpreted his data to mean that under optimal feed- ing conditions individual E. pacifica off southern California spawn three times, an alternative interpretation consistent with his results and our results from E. paci$ca in northern Puget Sound is that each pulse contains multiple broods from a female. Research on the egg production of C. paci$cus also suggests that the fre- quency of brood release is keyed to food availability (J. Runge pers. comm.), which off southern California would be correlated with upwelling events.

We found that the average number of broods for the whole population of E. pa- ci,fica was 20 during the spawning sea- son, closer to 30 for large females, and 11-12 broods for females cl9 mm. Our estimates of fecundity were close to those of Ponamareva (1966) who counted sev- eral stages of eggs and estimated the fe-

Euphausiid fecundity 313

cundity to be 1,200-1,700 eggs per fe- male. The egg production of E. pacifica is of much greater significance to the en- ergetics of the species itself, and proba- bly to predators on eggs and larvae, than has previously been assumed.

One of our purposes in comparing lab- oratory and field data on the reproductive cycle of E. paci$ca was to discover the relationship of stage IV females to initi- ation of spawning and frequency of broods, and to determine if there was a more accurate method of estimating fe- cundity from preserved samples than that of Mauchline (1968). The co-occurrence of stage IV females and ripe females showed that the presence of stage IV fe- males indicated the initiation of spawn- ing. On 16 March 1977 there were no stage IV females, but in mid-April 100% of the females were stage IV, about twice the percentage of ripe females (55%). Therefore the inverse of the frequency of stage IV females gives a rough estimate of twice the brood interval, although the precise relationship between stage IV and ripe females may vary with location and perhaps year. Concurrent samples taken to determine the frequency of ripe females and stage IV females might allow us to divide female IVs into two groups based on as yet unknown morphological differences between eggs that are nearly ready to be released and those to be re- leased immediately (24 h), and thus to get an exact relationship between a stage seen in preserved females and ripe fe- males.

Hulsizer (1971) counted mature (phase IV) and semimature (phase III) eggs in the ovaries of E. paci$ca in northern Pu- get Sound and found over twice as many eggs per female (150400 depending on the size of the female, 14-20 mm) as the number of eggs per brood in our study. Therefore her egg counts probably in- clude two broods, and phase IV eggs are eggs ready to be released within 24 h. By counting the number of phase IV eggs in the ovary (E = number of eggs per brood), determining the inverse of the frequency of stage IV females (twice the brood interval), and then calculating the

brood interval (D), and the relevant time interval (T), we can determine the fecun- dity (F) of the euphausiid during that time:

F = E(T/D).

The estimates of fecundity made by this method will of course be approximate, but closer to the actual number of eggs released than estimates of fecundity from Mauchline’s (1968) method. Further re- finements using concurrent samples ana- lyzed for frequency of ripe females and number of eggs per brood, and for fre- quency of stage IV females and the num- ber of mature eggs in the ovary, will in- crease the accuracy of this prediction of fecundity from preserved samples of eu- phausiids.

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Submitted: 8 September 1980 Accepted: 6 July 1981