Ontogenetic Development of Growth Rates in Larval ...

Ontogenetic Deve opment of Growth Rates in Larva Herring, CIupea harengus pallasipMeasured with RNA-D NA Ratios in the Strait of Georgia, British Co Can. J. Fish. Aquat. Sci. Downloaded from www.nrcresearchpress.com by Hunan Normal University on 06/04/13 For personal use only.

S. M. C. Robinson Department of ZoolsgyiOceano~phy,University sf British Columbia, 6278 University Bbvd., $/ancower, B.C. V6T f W5

and DoM. Ware Department of Fisheries and Oceans, Pacific Biological Station, Nanaimo, B.C. V9R 5K6.

Robinson, S. M. C., and D. M. W r e . 1988. Ontogenetic development of growth rates in lawal Pacific herring, CBupea harerigus pallasi, measured with RNA-DNA ratios in the Strait of Georgia, British Columbia. Can. 1. Fish. Aquat. Sci. 45: 1422-1 429. During the spring of 1986, a cohort sf Pacific herring, Clupea hasengus pa%lasi,larvae was sampled for 36 d in the Strait of Georgia to measure growth rates using RNA-DNA ratios for individual larvae. Concurrent with the field study, a population of herring larvae was stawed from hatching in captivity for comparison with those caught in the field. The mean RNA-DNA ratio at hatching for the starved larvae was close ts 2 but it quickly rose to 3.4 by age 4 d. The mean RNA-DNA ratio subsequently dropped back to 2 and below by age 8 d, presumably reflecting the exhaustion of the endogenous food supply of the yolk sac. The mean RNA-DNA ratio at the calculated point-of-no-return was 2.06 which was very similar to the zero protein growth rate or what we define as the "critical ratio." Herring larvae from the field generally showed an increase in the RWA-DNA ratio over the 36 d from approximately 2 to 7 although the Cirst 18 d showed more variation than the latter. There was no evidence of mass starvation ('kritical period") for the 1986 year class but there was a noticeable drop in the growth rate during the change to exogenous feeding. We suggest that starvation probably only directly affected the developing larvae during a window of about 11 d. Frequency distributions of the RNA-DNA ratios are shown for larvae over time. Au cours du printemps de 1986, les auteurs ont 6chantiilonn6 pendant 36 d des Barves de harengs du Pacifique (Clupea hasengus paldasi) d'une meme cshorte se trouvant dans le detroit de Georgie afin d'en determiner les taux de croissance. [Is ont utilise, pour ce faire, le rapport entre I'ARN et I'ADN de chaque lawe. Au cours de la meme p&rio.de, une population de larves de karengs a &t4 consew& en captivite, sans &re nourrie, afin de perrnettre [a cornparaison avec les valeurs obtenues sur le terrain. Le rappyt moyen AWN-ABN A I'6closion des larves temoins apprschait 2, mais il a rapidement atteint 3,4 2 IP&e 4 d. A lp$ge8 d, le rapport avait diminas6 A la valeur de 2 su rnsins, ce qui refl6tait probablement la fin de l'utilisation des sources alimentaires endoghes du sac vitellin. be rapport ARN-ADN moyen au point calcul6 de won-r6tabliswment etait de 2,06. Cette vateur est tr&s semblable au taux de croissance z&ro par manque de proteines su de celui defini par les auteurs comme 6bnt le (( rapport critique D. Les larves etudi&s sur le terrain ont present& de fason g&n#rale pendant les 36 d de li6tude, une augmentation du rapport ARN-ADN. Ce rapport passait de 2 A 7 et %a variation etait plus apparente au cours des 18 premiers jocrrs. Rien n'indiquait que la classe de 1986 avait souffert d'un manque de nourriture grave (perisde critique) mais on a note une chute appr&ciable du taux de croissance loss du passage A I'alimentation exogsne. L a auteurs 6mettent I'hypoth&se que le manque de noarrriture nn' directement affect6 les lawes en croissance qu'au csurs d'une periode limit& A 11 d environ. hes distributions de fr6quenc.e~des rapports ARN-ADN des lames en fonction du temps sont pr&ent&es. Received September 10, F 987 Accepted May 11, 1988

QJ9421)

research has k e n conducted to establish whether larvae me n H 9 14, Hjort hypothesized that recruitment to a y e a class indeed starving in the field. This has been done f a herring of fish was d e k h n e d in the very earliest l m d stages and (Hempel and Blmter 1963; Chenoweh 1970; Ehdich et dBa. a critical period wcumed during the switch from endoge1976; McGwk 1985), cod (Koslow et al. 1985; Neilson et d. nous feeding on the yolk sac to exogenous feeding on plankton 1986), plaice (SheBboume 1957; E M e h et aB, 1976), northern where starvation-igBC%uCedmortality was high. Despite a proanchovy (09ConnelB 1976, 1980), and jack mackerel (Theifusion of studies designed to test this hypothesis, no fm conlacker 1978, 1986). clusion has k e n reached (see May 1974 for review). One of Three basic methods have k e n used to diagnose nutritional the major problems has k e n sampling a rapidly dispersing lac, and chernvd fish population with enough resolution to detect a s h ~condition in 1 m d fish: m o ~ h o m e ~histologic& icd. W i l e each has certain attributes md advantages, the change in the mortality rate over the short period of time in the chemical condition factors are probably the most sensitive, as feeding transition phase. Because of the resolution problem, 1422

Can. .?. Fish. Aquat. Sei., Vol. 45, 1988

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changes in the fish would occur fist at the cellular level, then the tissue level, md finally the organism level. Some studies report signifcant differences in certain chemical constituents' in as little as 2 4 d (Ehlich 1974; BucHey 1981). A chemical method used to measure the state of growth and nutritional eondition in l a n d fish is the ratio of ribnucleic acid (RP4A) to deoxy~bonucleicacid (DNA). Use of this ratio is based on the premise that while the DNA per cell is approximately constant, the mount of RNA in the cell is directly proportional to the mount of protein synthesis mcming. Several studies have shown that there is a linear relationship between the rate of protein synthesis md the MA-DNA ratio (Hines 1973; Buckley 1979, 1980, 1982, $984). Buckley (1984) found that if the WA-DNA ratio was combined with data on water temperatures, 92% of the variance in the protein growth rate could be explained. This technique is particularly appropriate for studying the short-term growth of larval herring as well as other larvae. Ehlich (1974) has shown that herring larvae produce p ~ d o ~ n m dprotein y md some cdohy&ate instead of t i glycerides during their early growth stages (up to 20 resolution of this method then dlows one to follow changes in growth rates on a fine time scale. The objective of this study was to follow the growth rates of a cohort of Pacific herring, Gkupea harengus pallmi, larvae over time in the field using RNA-DNA ratios on individual fish. The hypothesis being tested was that herring larvae experience a 'kritical pried" (sensu Hjort 1914) at the onset of exogenous feeding which will be reflected as a s h q drop in the mean growth rate. In addition, individual larvae will be mdyzed to determine the RNA-DNA ratios by morphologicd stage md the frequency distribution of RNA-DNA ratios over time.

Makrids and Methods Field Study This study was conducted from March 3%to May 6, 1986 (Calendar Days 90-126) in Lambert Channel (49"3OPN, 124"43'W) in the Strait of Georgia, British Columbia (Fig. 1). The area is at present one of the major spawning areas for Pacific herring in the Strait of Georgia (Wowston 1980). Spawning usually occurs on the eastern side of Denman Island, with occasional spawwings occurring on the northern md eastern side of Hornby Island. Three sapling sites were chosen to follow the emerging cohort of herring larvae (Fig. 1). This work is part of a larger study examining the dispersion md feeding cs of Hmal Pacific herring (S. M. C. Robinson, in Temprahre and salinity measurements were taken with a portable temprdwelsalinity probe (Autolab model 602) at each of the sapling sites. The mean temperature at a depth of 5 m increased from 8.4"C on Cdendar Bay 92 tt~9 -4°Con Calendar Day 126. This depth was chosen based on the observation that well over 50% of the larvae are found in the top $8m and dso the temperature gradient from 5 to 30 m was generally on the oder of 0.5"C due to the mixing of the water column at this time of year (S. M. C. Robinson, unpubl. data). The salinity at 5 m ranged from 27.3 to 28.9 ppt, with a mean of 28.3 ppt (SD = 0.4, sa = 15). L m a e were captured from a small (7 m) b a t usi diameter bongo nets with 3 5 0 - ~ mNitex mesh. Both nets were black to reduce net avoidance (LeBrasseur et al. Can. J. fish. Aqwwrt. Sei., VQ&.4.5, 1888

1967). Single cycle oblique tows (e.g . down md up) at a towing s p e d of 1.5 mas- were done to 50 rn or the bottom, whichevm was shdlower. Toks were done d g the daylight horn. A subsmple of captured l m a e (n = 6-23) were individually placed in numbered 1.5-mLn~icrocen~fuge vids containing 1 d.. of, 20-pm filtered seawater. These were then frozen md stored in liquid nitrogen for later mdysis. The. l m a e were alive when frozen md the whole capture md preserving procedure took less thm 10 min. L m d lengths were corrected for sfi&age in the liquid nitrogen before mdysis of the data.

A control study was done to compare the l m d herring RNADNA ratios observed in the field study with those from l m a e h o w n to be staved. Eyed eggs from natural spawn of Pacific herring were collected from Den Island (Fig. 1) md held in aerated seawater at 8°C until hdching. L m a e were then transferred to aerated 70-L containers filled with 20-km filtered seawater. Temperature was regulated with m external water bath to reflect tempratwe patterns found in the field. The sdinity was constant at 29.0 ppt. At approximately 3-d intervals, subsmples (n = 6-15) of the l m d ppuwion were taken md treated as above for later analysis of WA-DNA ratios. Laboratory Analysis All previous studies measuring RNA-DNA ratios in larval fish (Buckley 1984; Wright md Martin 1985; Fukuda et al. 1986; Buckley and b u g h 1987; Clemesen 1987) have used the modified Schmidt-'hahauser technique which requires a minimum of 800 pg dry weight~smple- I . An 8W-kg Pacific herring larvae would be approximately 28 d old (S. M. C. Robinson, unpubl. data); therefore, estimates of individual variation in the RNA-DNA ratio are lost until well into the initid exogenous feeding stages. A more sensitive technique, suitable for measwing small mounts of nueleie acids, uses the LePecq and Paoletti (1966) fluorometric procedure, modified by Karsten md Wollenberger (1872, 1977). This technique is based on the enhanced fluorescence of nucleic acids after introduction of the dye ethidium bromide (2,7-dihno-10-ethyl-9-phenylphenmthidinium bromide) a d is sensitive to small amounts of nucleic acids at concentrationsdown to 0.05 p,g=mL-lfor DNA and 0.1 p g . d - Bfor RNA. For this study, an individual l m a was thawed, measured for standard length under a dissecting microscope to the nearest 0.2 , and staged mophologically according to Doyle (1977). The lama was then placed in 3 d., of ice-cold phosphate-buffered saline and homogenized and sonicated with a Polytron tissue grinder at 24 000 rpm for two $0-streatments. Two replicates of 0.5-mL diquots were taken and processed for determination of DNA md two for the total nucleic acids. Fluorescence of the smple was measured with a FOCI Ratio Fluoromekr-2 (Fmmd @tical Company Limited) on smple mode. Two modifications were made in the Karsten and Wollenbrger technique. Incubation time was increased from 20 to 30 min and the RNase concentration was increased from 50 to 100 ~ ~ T I I Lfor- ~added assurance tha essentially all the RNA would be hydrolyzed by the enzyme. Stmdafd curves were done for both DNA md RNA. Rwessing time of the samples by two persons was one fish every 5 min after the initial time. lag of 30 min due to the iB86uba~onperiod. Samples w chosen each day h m h e liquid nitrogen storage tematic errors would not affect a particular size group.

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FIG.1. Study site showing the h critical Ratio Buckley (1984) examined the relationship between the h s m m e o u s protein growth rate (the dependent variable), the water ternWA-DNA ratio (the indep~dentvariable), perature. The relationship was linear (9= 0.92) and was given as (1) G, = 0.93T 4.75RNA-DNA - 18.18 where Gpiis the protein growth rate (percent per day), a" is water (degrees Celsius), a d MA-DNA is the M A Buckley's study involved tempemm m g e s from 2 to 2092 a d eight species of rn e fish larvae including Atlantic herring, Clupea k e n g u s harengus.

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1424

e sampling mas.

to

(2) WA-DNA = R,, = (18.%We93n/4.75 where R, is what we call & c i ~ ~ dand the 0 t h vd ~ ables are the same as ah % criticalrho is the &eme~cd WA-DNA ratio where there is no wet protein growth in a larval fish at a specified temprame.

~

~

~

~

l

b

Shrv8tion Controls There was a significant increase in the mem WA-DNA ratio over the Tist 4 d from 1.82 to 3.37 (p < O.OOl, Student's 8Can. $. Fish. Aqua$. Sci., VsB. 45, 1988

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adults (Pig. 3). Traces sf larvae from a second spawning wave

ratio for the wi

ratio, and by about xhibiting a loss in

Field Study

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16: @ > 8.05, Student's t-test). The mean stmchrd length &so increased consistently with each mophollogicd stage. In the early yok sac phase (stages la and I b), about 50% of the larvae were below the critical ratio, but this percentage dmppd to zero by the end of the yok sac stage 1c. The percentage sharply increased at stage 2a and then fell to much lower levels at successive stages. Initially, the shape of the frequency dis~butionsof the RNADNA ratios of the thee areas was skewed right (Fig. 5). Because of logistic sampling problems, C d e n h Day 90 was only composed of Areas B md C, and two perids (Calendar Day 805 + 107 and Calendar Day 124 126) had to be cornbined in order to show d l thee m a s . By Cdenda Day 1q5 887 the shape of the dis~butionwas slightly skewed left but shifted to a strongly skewed left and truncated distribution by Cdendw Day 124 9 126. The mode moved right over time a d the range of vdaaes initially increased and hen decreased. =+

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Discussion

S 85

95

S W a ~ o Controls n I

1 1

a

mm

(hlandar by?

FIG.4. R~gessiomof the RNA-DNA ratio, standard length, a d estimated protein growth rate (calculatedfrom equation (1)) of field-caught Pacific herring larvae over time from the thee sampling areas. The calculated critical ratio is plotted for reference. B a s represent 1 SB of the mean.

However, when this relationship was ex wed fog each s m pling period, except for one date (Calendar Bay 117), there were no significant correlations between standard length and the WA-DNA ratio (p > Oe05).W i l e there may be no direct demonstrable relationBhip between ratio and lengih, a past history of high ratios results in lager larvae. Larvae from Area A appeared lager at fist but as the ratios kom Area B md Area C increased over Area A the mean sizes in Areas B md C dso increased over those from Area A. The mean lXbL&-DNA ratios for all areas combined were found to increase with each successive morphological stage except for stage 2a, the first feeding stage, where it dropped by a b u t 223% compmd with stage 1c (Table 2). The mean RNA-DNA ratis sf stage 2a was significantly different fmm that of stage 2b (p < 8.05, Student's t-test) but not from stage

The calculated protein growth rates (percent per clay) of the l m a e quickly dropped ts zero after exhaustion sf their yolk sac, dthough the frequency at which the larvae were smpled was not high enough to demonstrate this unequivocally. The drop to zero of the growth rate was then followed by a reduction in length as the effects of inadequate nutrition presumably began to occur (Fig. 2). This pattern of shrinkage in standard length was similar to that found by Blmter and Hempel (1963) for Atlantic herring larvae. The WA-DNA ratio vdues md the initial increasing and then decreasing panern of the ratio were very similar to those found by sen (1987) for Atlantic hening larvae; however, the the ratios of her fish appeared to happen a b u t day 4. The difference may be due to the higher rearing temperatures she used (14.9"C) or lower yolk reserves in the larvae. The duration of the yok sac stage in this study was comparable with several other studies on captive g (Schack 1981; McGurk 1984). Before day 8 the coefficient of variation (standard deviation x B$%O/mem) was approximately 58%; however, h m day 8 onwards the coefficient of variation ranged from 79 to 19196, suggesting that the herring larvae are experiencing starvation effects. Blater md Hempel (1963) defined the p i n t where a larva, deprived of food, is unable to feed even if suitable food becomes wailable as the PNR. For water temperatures used in this study, McGwk (1984) cdcaalated that it to& starved Pacific herring % m a eI 9 d to reach the P W . This vdue is comparable with findings on Atlantic herring larvae (Blaxter and Hempel 1963; Blaxter md Ehlich 1974). h our study, the mean RNADNA ratio at 11 d (from McGwk 1984) was 2.06, dmsst identical to the cdculated critical ratio (2.443) derived from Adantic herring Buckley (1984). Cle

TABLE1. Percentage of Pacific herring ~

~ having a RNA-DNA e ratios below the critical ratio ioR, during the sampling period in each of the h e sampling areas. - = no observation for that day. Calendar Day Area

90

94

98

99

1

105

187

113

118

124

126

Can. J. Fish. &WE.Sci., Vol. 4.5, 1988

TABLE2. Mean md s m d a d deviations of WA-DNA ratios md standard lengths fm land Pacific herring in d B areas combined by morphoBogicd stage (kom Doyle 1977). W, is the percentage of Imae below &e citicd r a h . Stage 1 may be described as yo& $zap: 1mase, stage 2 as p s t yolk sac p~flexionlarvae, md stage 3 as postflexion lwx.

n

f

2

SD

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Stage

SD

Calendar Bay

FIG. 6. Record of spawning dates md estimated mounts of spawn for Pacific herring in the Lm$efa C h m e l m a during 1986. (data fmm nt of Fisheries md Oceans, pen. c o r n . ) .

l m a e at different ages up to day 50 a d found the WA-DNA ratios seem to fdl to approximateIy the same levels observed in this study. A similar pattern was found for larval striped bass, Adorone saatilis, although the experiment only lasted for 14 d (Wright and Matin 1985)-Therefore, it a p p m that the critical ratio md h e calculated PNR both have ItNA-DNA ratios approximately equd to 2. More work should be done to clarify this possibility.

Day 90

B Day 126

0

2

4

6

8

RNA-DNA Ratio FIG. 5. Distributionof IZ.NA-DNA ratios of Pacific herring larvae over smpIhg areas c m b h d . Note that a few days had to be combhd (see text). Cm. Js Fish. Bg4mt. Sci., Bid. 45, 1988

Field Study Due to the daylight ichthyopldton tows md the net size used, we recognize that some net avoidance of larger larvae undoubtedly occurred. If weaker or smaller larvae were selected for, the observed growth rates would be underestimates of the population a d this study would represent a worst case scenario. However, the gowth rates (millimetres per day) of the captured larvae compared favorably with other studies on Pacific herring larvae (Stevenson 1962; Schnack 1981; McGurk 1987), md therefme, we feel that most of the samples were representative of the population. The one dominant mode and the almost imperceptible second mode in the length-frequency plots (Fig. 3) indicated that this study is representative of the growth md condition of the y e a class produced from this area. We have ignored the "daily" cohorts produced by the fist spawning wave md assumed that they are all one cohort. The existence sf a single dominant cohort is also supported by observations (B. Chdmers ,Department of Fisheries and Oceans, peas. eo .) s n the spawning dates and estimated m u n t s in this region (Fig. 6). Spawning was observed to mew over a period of 10 d at various locations with a peak at Calendar Day 73. No second wave was observed. Although the trend was not consistent, the mean RNA-DNA ratios of l m d herring in our study generally tended to increase (Fig. 4). This implies, during the first 36 d of life, that herring l m a e in the fieEd are not growing at a simple expnentid rate because the WA-DNA ratio, which is directly related to the instmtmmus protein growth rate, increases with time. The f e t that there was essentially no correlation of the RNA-DNA ratio with size at each sampling period suggests that early growth rates may be a function of l m d age andlor the progression of the spring plankton bloom. Tmasichuk and Ware (1987) have suggested hat higher p w t h rates of newly hatched larvae later in the season are why second spawning waves sf herring do not produce lager larvae as their theoretical spawning model predicts.

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n e main hatch sf l m a e was estimated to start about Cdendm Day 88 (inclement weather prokbited sampling from Calendar Bays 87 to 89) md probably continued for 7 or 8 d. If we assume that the hatch kgm on Cdendar Day 88 md the duration of the yolk sac stage was 6 d (McGwk 1984; this study), then the compulsory switch to exogenous feeding would start mound Calendar Day 94. All thee aeas showed a levelling off or a decrease in the WA-DNA ratio during the period Cdenda Days 95-102 after a previous period of rapid increase (Fig. 41, suggesting that the larvae me undergoing a period of increased nuhtiond diRiculties. This view of the l m a e experiencing problems in the switch to exogenous feeding is also supported by the mean ratios for the morphological stages where there is a consistent increase in the ratios at each successive stage except for stage 2a (the onset of exogenous fixding; Table 2). The difference between the mean WNA-DNA ratios of stages Ic and 2a is not statistically significant, but we feel that this is due to the low sample numbers for stage Ic. In ~ below the critical ratio in the addition, 28% of the l m were fist feeding stage (2a) compmed with 096 at the end of the yolk sac stage (Ic) (Table 2). As the mean RNA-DNA ratio in each area continues to increase, it appms that by about Calendar Day 105 the larvae are at a stage where their growth increases well above the critical ratio md shows no signs of h e earlier fluctuations. There were also no l m a e found below the critical ratio after this date (Table 1). This suggests that if stmation is a significant factor in regulating population numbers in this cohort of larval herly in a window of about 1H d. ring, it will have to act pri Herring would be affected from the end of the yolk sac stage on about Cdendar Day 94 (assuming a yolk sac stage of 6 d) to about Cdendm day 105. If the fish were unable to capture fmd, then the window of the PNW would be 6 d from @dendm days 99 to 105. A similar trend for l m a l Pacific herring, showing a brief initid period where larvae were diagnosed as s t m ing , was found by McGurk (1985) using a multivariate condition factor. 'This view of a larval escape from food restrictions, with respect to stmation, after a brief window is supported by ~ et a study on jack mackerel, Trachurus ~ m m e t r i c u(Hewitt al. 1985). They found that stamation was an important source of mortdity rate only during a window of about 7 d during the fist feeding stage. Ware md Lmbert (1985) also found that mortality in Atlantic mackerel, Scomber scombrus, rapidly increased during the first feeding stages md then decreased a few days later. This study supports some of the traditiond views of the early life history feeding dynamics of herring. We found evidence that Bmae do undergo some brief problems switching to a plmktivorws diet a d that this occurs during a smdl time window following exhaustion of the yolk sac. This is consistent with Hjort 'S (19 14) original hypothesis. However, for this cohort, we c m find no evidence to indicate -that a 66criticd period" is present where a large increase in mortality occurs due to stmation during the switch to exogenous feeding. The mem l3J+4A-DNA ratios during the estimated stmation window (Calendar Days 94-105) are significantly greater @ < 0.05, Student's t-test) than the critical ratio md the percentage of l m a e below the critic81 ratio (Table 1) during this period is small and declines quickly. W e n the mean IXNA-DNA ratios from the field are compared with the critical ratio, one can see that there is a relatively rapid increase from values near the critical ratio on Cdenda Day 90 to approximately 5 by day 100. It appears, then, that

early yolk sac l m (around ~ day 98) grow very slowly or lose weight, as the mem WA-DNA ratios are so close to the critical ratio. This chmcteristic may dso be due perhaps to the production of bioche~calcompounds other thm protein. It is possible that the increase in the ratio is ~s%rerestimated, as slower gowing l m a e may experience higher mortality rates. The WA-DNA ratios hat we found in the field are higher than those reported by Fukuda et d. (1986) for Pacific herring l a n (1987) for Atlantic herring larvae, and Wright 5) for l m d striped bass fed in the laboratmy, but the differences may be due t s population differences or the fact that fish larvae are notoriously difficult to raise normally in captivity. Although there a e very few studies on WA-DNA ratios in h e field, the values obtained in this study me comparable with other studies of fish l m a e on Atlantic cod md haddock (Buckley a d Lough 1987). The early skewd-ight NA-DNA ratio distributions a d wide rmge (Fig. 5) suggest, due to the extend& hatching sf the cohort of l m d herring, that the observed protein growth rates may partly be caused by l m a e of slightly different ages. It may dso be a function of a few l m a e learning to feed a d supplementing the energy from their yolk sac reserve. As the mode moves right from Cdendar Day 90to 124, the percentage in the low MA-DNA ratio categories reduce. From Cdendar Day 94 on, there is a suggestion of bimodality in the distributions which could indicate that a group has learned to feed well md has a head stat on the rest of the cohort. More detailed samples me needed to determine whether or not this apparent feature is real. As the mode progresses to the right over time, the WA-DNA ratio distribution becomes more truncated and is similar to hat found for field-caughtpopulations of sand l a c e md haddock (BucMey 1984; Buckley and Lough 1987). The distribution on Calendar Day 224 126 shows that there are no l m a e below the critical ratio of a b u t 2, which means t h a they have either been lost (due to predation or s their feeding abilities have increased s s they can easily meet their basic nuef-itiond requirements. The truncated dis~bution above m RNA-DNA ratio of 8 may reflect net evasion by the lager, more actively gowing animals, or it may be that the Imae have reached a maximal protein growth rate. We feel that net evasion is a more likely explanation. To the best of our howledge, this study is the fmf to look at the WA-DNA ratios of individual fish larvae smaller than 800 pag dry weight. This increased sensitivity was gained by using a fluorome~ctechnique for analysis of the nucleic acids. We feel that the sped, simplicity9md versatility of' this technique for measuring growth and condition in larval fish emphasize h e usefulness of incorporating the technique into sampling designs for other l m d fish studies.

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We gratefully achswledge the use of the labratory facilities provided by BE.P. W* Hochacka and T. P. M o m s e n for the mdysis of the I3P-U-DNA ratios. H. Dovey, L. Pomeroy, a d H. Wu helped with the laboratory aidysis. During the collection of larvae in the field, the hospitality of the Hedey family was very much appreciated. Dr. J. Marliave of the Vmconver Aquarium aided in some of the preliminary work s f this project. Dr. T.R. Parsom, Dr. D. D ' h o u r s , md W. D. Webb kindly read an earlier version of &is mmusc~pt. The mmusckpe knefited from ~e comments by two monymsaas reviewers. Funding for this project was supplied by the Depmwaent of Fisheries and Oceans, Pacific Biological Station, to 9;. Rsbinsm and Dr- T.R. Pmons. Carpii. J Fish. A q u t . Sci., $101. 45, 1988

References J . H . S . , A ~ K . F ~ im. 1974.Changes in behavior during starvation of h e ~ n ga d plaice larvae, p. 575-588.In J. H. S. Blauckr [d.] The early fife history of fish. Springer-Verlag, New York, Heidelberg, and Berlin. B m , J. H. S., AND G . HEMEEL.1963.The iduence of egg size on herring 1mae (Clupea h e n g u s L.). J. Cons. 28:21 1-240. BUCXLEY,L. J. 1979. klationships between RNA-DNA ratio, prey density, md growth rate in Atlantic cod (Gadus m o r b ) larvae. J. Fish. Res. Bsard

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