Environmental Biology of Fishes Vol. 20, No. 1, pp. 3347.1987 0 Dr W. Junk Publishers, Dordrecht.

Age and growth of Pacific herring larvae based on length-frequency analysis and otolith ring number

Michael D. McGurk Envirocon Pacific Ltd., 205-2250 Boundary Road, Burnaby, British Columbia V5M 323, Canada

Keywords: Clupea harengus pallasi, Length-weight relations, Fish cohort

Synopsis

Age and growth in length and dry weight of cohorts of wild Pacific herring larvae, Clupea harengus pallasi, were measured using successive modes in the length-frequencies of the catches and the number of rings in the otoliths. Average linear rates of growth in length ranged from 0.36-0.41 mm d-l, and average exponential rates of growth in dry weight ranged from 0.063-0.084 d-r. Length-date and dry weight-age curves were best described using one-cycle and two-cycle Gompertz functions, respectively. Weight-length relationship were, therefore, curvilinear on double logarithmic plots and were best described by a non-linear allometric function. Average rates of otolith ring deposition were 0.90,1.09 and 0.73-1.26d-‘. Rings were deposited daily from the day of complete yolk absorption in the first two cohorts, but interrupted ring deposition was observed over the first 27 d of the third cohort. Relatively low water temperatures, <9” C, coincided with the interruption and may have caused it.

Introduction

Growth in length of wild Pacific herring, Clupea harengus pallasi, and Atlantic herring, Clupea harengus harengus, larvae has, until recently, been measured by following successive modes in the length-frequencies of the catches (Marshall et al. 1937, Bowers 1952, Stevenson 1962, Iizuka 1966. Das 1968, Sameota 1972, Bolar et al. 1973, Schnack 1974, Henderson et al. 1984, Lambert 1984). The discovery of daily rings in the otoliths of fishes has offered an independent method of constructing growth curves [see Campana & Nielson (1985) for a review of the microstructure of fish otoliths]. Enclosure rearing experiments (Brothers et al. 1976, Geffen 1982, Laroche et al. 1982, Lough et al. 1982, McGurk 1984a) support the assumption that rings are deposited in the otoliths of healthy, grow- ing fish at rates of about 1 d-l, beginning at or soon after hatching. However, there is evidence from field and laboratory studies that ring deposition is

not always daily in fish living under stress. Towns- end & Graham (1981) reported that Atlantic her- ring larvae of the Sheepscot River estuary. Maine, exhibited interrupted growth in length and inter- rupted ring deposition during a 2-3 wk period in late January and early February 1979. Food densi- ties’ are lowest and water temperatures approach the lower lethal limit at this time of year. Lough et al. (1982) reported that autumn-spawned Atlantic herring larvae of the Gulf of Maine-Georges Bank region appear to deposit rings at a rate much less than daily during the first 25 d after hatching. This may have been due to an inability of first-feeding larvae to find enough food in the sea to meet their metabolic requirements.

These observations were supported by enclosure rearing experiments by Geffen (1982) and McGurk (1984a) that showed that the average rate of otolith ring deposition varies directly with the rate of growth in length for Atlantic and Pacific herring larvae and for turbot, Scophthalmus maximus lar-

34

vae. This raises the possibility that the number of rings in the otoliths of fish larvae is not a reliable index of age or, alternatively, that the ring struc- ture, i.e. the number of rings or their width, is a record of growth history. Both possibilities have implications for ecological studies of the early life history of herring. Clearly, it is important to: (1) measure the rate at which rings are deposited in the otoliths of wild fish, (2) compare growth curves obtained from length-frequency analysis and oto- lith analysis, and (3) learn which factors: food, temperature or both, are correlated with the natu- ral rates of ring deposition.

Herring larvae hatch in discrete batches called cohorts that usually contain only 2-4 day-classes [see Lambert (1984) for a review of cohort structure in herring]. Cohorts appear as long-lived modes in the length-frequency plots. In addition, Pacific her- ring deposit their eggs in masses in the intertidal zone of their spawning grounds. Therefore, the release of larvae into the plankton can be antici- pated (2 wk incubation at 6-8°C) and the mean date of hatching can be measured accurately. These two factors make Pacific herring a con- venient species with which to test the daily ring assumption.

This paper reports: (1) the growth in length and dry weight of four cohorts of wild Pacific herring larvae based on length-frequency analysis, (2) the rates of otolith ring deposition for three of these cohorts, and (3) the growth in length of these three cohorts based on otolith ring age. The only pre- vious estimates of growth in length for wild Pacific herring larvae were reported by Stevenson (1962) for Barkley Sound, Vancouver Island, cohorts and Iizuka (1966) for Akkeshi Bay, Hokkaido Island, Japan, cohorts. No reports of growth in dry weight for wild herring larvae are available in the litera- ture. This is the first report of otolith-ageing of wild Pacific herring larvae.

Study site

Bamfield Inlet is a 3.8 km long inlet on the south- ern coast of Barkley Sound, Vancouver Island, British Columbia (Fig. 1). Surface water tempera-

ture ranges from 9-17” C. A thermocline occurs at 4-6 m in the summer, below which the water ranges from 8-10” C. Surface salinity ranges from 13-31”/00. There is a shallow halocline in the top 4m and below 10 m the water is a constant 31-32%,. Pacific herring spawn almost every year between February and April on the seagrass of the intertidal zone at or near the head of the Inlet. Larvae disperse down the Inlet out into Trevor Channel in l-2wk (McGurk 1986).

Materials and methods

Samples were taken from March to May in 1981 and 1982 using two types of gear: twin bongo plankton- nets and a night-light. Two stations were sampled in the Inlet: upper Bamfield (A), and off the Bam- field Marine Station (B). Other plankton-net sam- ples were taken at a station in Grappler Inlet (C), and at six stations in Trevor Channel: off Bamfield Inlet (D), Kelp Bay (E), Whittlestone Point (G), Cape Beale (H), Ohiat Rock (I), and Nanat Island (J). Plankton-nets were 1.5m long with a mouth diameter of 40 cm and a mesh diameter of 471 pm. Nets were towed at l-2 m see-’ at a depth of about 2 m in Bamfield Inlet (upper Bamfield Inlet is only 4 m deep) and at some stations outside the Inlet. In 1982 some of the tows outside the Inlet were oblique tows to 20-50m depths. Each tow took 5-15min to complete after which the contents of the codends were preserved either in 2% for- maldehyde and 30%0 seawater for length-frequency analysis or in 37% isopropyl alcohol and freshwater for otolith analysis.

The night-light floated at the sea surface aLthe dock of the Bamfield Marine Station, opposite sta- tion B. It was set out after civil twilight and plank- ters were allowed to accumulate about it for several hours. A dipnet was used to capture any fish larvae within a 0.5 m radius of the light. Larvae were preserved in either 2% formaldehyde and 30%, seawater or in 37% isopropyl alcohol and freshwa- ter.

Larvae were stored at 20” C for at least 30d before measuring in order to allow the dimensions time to stabilize. At least 100 larvae were randomly

0 -xt _ /Kelp Bay

J scale (km)

Fig. I. Map of the Bamfield Inlet-Trevor Channel area showing the plankton sampling stations.

chosen fro:m each sample and measured for stan- dard length (notochord length) with the vernier scale of a compound microscope. Dry weight was measured for formalin-preserved larvae by rinsing a larva in freshwater, drying it at 60” C for 24 h, and weighing the residue with an electrobalance to the nearest pg. Standard lengths and dry weights of the plankton-net samples were adjusted upwards with a Gompertz model (McGurk 1985: Fig. 1) in order to correct f,or the shrinkage in length andloss in dry weight that accompanies net-capture. Lengths of alcohol-preserved plankton-net and night-light samples were then reduced by 5% in order to com- pare them with the lengths of formalin-preserved larvae. Preservation in formalin causes an average

5% loss in standard length of herring larvae (Schnack & Rosenthal 1978, Hay 1982), but preser- vation in alcohol does not affect length (McGurk 1984a).

The two sagittal o<oliths of an alcohol-preserved larva were removed, photographed, and the rings in each otolith were counted according to the pro- tocol described by McGurk (1984a). In herring otoliths the first ring is deposited only after com- plete yolksac absorption, which takes 4-6d at 6-10” c.

Length-frequency distributions were plotted for the pooled samples at date and the cohorts were identified and separated by eye. When the distribu- tions of two neighbouring cohorts overlapped they

36

were separated with a frequency analysis program (MacDonald & Pitcher 1979). The program as- sumed that the length-frequencies were mixtures of normal distributions.

Mean hatching dates of each cohort were calcu- lated from the rates at which yolk-sac larvae disap- peared from the catches. Ratios of yolk-sac to non- yolk-sac numbers, P, calculated from the pooled catches for each day, were normalized with the arcsin fltransformation and then regressed on the date of capture. Each transformed ratio was weighted by its sample size because the accuracy of a statistic varies directly with sample size (Gilbert 1973). Surface water temperature at Bamfield Inlet was 8-10” C and since the yolksac is exhausted 4-6 d after hatching at 6-10°C the mean hatching dates were 5 d before the date at which 50% of the larvae were still with yolk. The age of a cohort was the number of days between the date of capture and the date of mean hatching. Mean length at hatch- ing, 7.7mm, and dry weight at hatching, 16Opg, were based on hatching experiments on Bamfield Inlet herring eggs reported in McGurk (1986).

All linear regressions were of the predictive rather than the functional type (Ricker 1973) be- cause both variables were measured with relatively little error. All non-linear functions were fit with the BMDP:AR non-linear least-squares regression program (Dixon 1983). When the data consisted of mean ring numbers-at-date, lengths-at-date or weights-at-age with associated variances, then each mean was weighted by its sample size divided by its variance (Gilbert 1973).

Results

A total of 197 plankton-net samples and 36 night- light samples were taken in 1981-1982, of which 176 and 36, respectively, contained at least one herring larva. Seven cohorts were identified: cohorts one, two and three in 1981 (Fig. 2), and cohorts four, five, six and seven in 1982 (Fig. 3). Cohorts one, four and seven were each represented by only a few larvae and so they were excluded from the analysis. The eggs from which cohort six hatched were abun- dant and visible on the seagrass beds at the head of

Bamfield Inlet 2 wk before the hatching date. Egg beds for cohorts two, three and five were not ob- served, probably because they were relatively small spawnings laid below the low-tide line. It is reasonable to assume that these three cohorts also hatched from the seagrass beds in Bamfield Inlet because the highest densities of larvae in each co- hort were found in the Inlet or close to it. Herring have been known to spawn in Grappler Inlet and in other protected embayments on the southern coast of Barkley Sound (Hourston 1958,1959, Stevenson 1962) but there was no *evidence in the form of larval density gradients or egg-bed sightings that would support assigning these cohorts to other lo- cations.

Mean hatching dates for cohorts two, four, five and six were 26 March 1981, 14 March 1982, 25 March 1982 and 13 April 1982, respectively (Fig. 4). The mean date of hatching for cohort three was estimated to have been 23 April 1981 by assuming that the average linear rate of growth in length over the first 10d was the same as that of cohort two. Times between the mean hatching dates were 28,ll and 19 d for cohorts two-three, four-five and five- six, respectively. Mean separation time was 19.3 (SD = 8.5) d, which was close to the mean separa- tion time of 17.5 (SD = 6.5) d reported for cohorts of Atlantic herring larvae by Lambe.rt (1984).

Age and growth from length-frequencies

Average linear rates of growth in length for cohorts two, three, five and six were 0.39, 0.40, 0.41 and 0.36mmd-I, respectively (Fig. 5). However, the slight curvilinearity of the growth trajectory meant that the mean lengths-at-date were best fit with a one-cycle Gompertz function (Zweifel & Lasker 1976))

L = L, [2 (1- exp[-aq)],

where L, = length at hatching (= 7.7 mm), A, = growth rate at hatching (d-l), a = rate of exponen- tial decay of A, (d-l) and T= date (d from mean hatching date). Age of individual larvae was there- fore estimated from length using the rearranged

Fig. 2. Length-frequency distributions of herring larvae captured in the Bamfield Inlet-Trevor Channel area in 1981 with plankton-nets and a night-light. Circled numbers indicate the cohorts and broken lines connect the mean standard lengths-at-date of the cohorts.

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ip

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Standard length (mm)

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Fig. 3. Length-frequency distributions of herring larvae captured in the Bamfield Inlet-Trevor Channel area in 1982 with plankton-nets

and a night-light. Circled numbers indicate the cohorts and broken lines connect the mean standard lengths-date of the cohorts.

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2 April

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Standard length (mm)

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Y =74.0-6.4X Y z85.2-3.3X Yr 66.2-4.2X Yz 108.6-8.0X r z0.92 rz 0.58 rz 0.85 co.96

origin z26 Mar origin= 13 Mar origin: 25 Mar origin= 10 Apr

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die :: :$\ A 2;: “T ‘, 216 .I

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March April March April

1982:d 5 6

Fig. 4. Linear predictive regression of arcsin fl(P = fraction of yolk-sac larvae in a sample) on sampling date for cohorts two, four, five and six. Numbers are total sample sizes. Mean hatching dates were calculated as 5 d before the date at which 50% of the cohort had still yolk.

equation (1) after t (absolute age in d) was sub- stituted for T,

t= +og, 1-+3ge + . [ 0 ( )I 0

(2)

Exponential rates of growth in dry weight for co- horts two, three, five and six were 0.083, 0.074, 0.084 and 0.063d-1, respectively (Fig. 6). How- ever, the growth curves have two distinct cycles: an initial pericld of no increase in weight or of a de- crease in weight during which the yolk is absorbed and the larvae learn how to feed and a second period of steady increase as the larvae feed suc- cessfully. Therefore, the mean dry weights-at-age were best fit with a two-cycle Gompertz function (Zweifel & Lasker 1976),

W = W, exp [ 1 (1 - exp [-a min (t, t”)])

+ 3 (l- exp[-p max(t- t*, O)])], (3)

where W,, = weight at hatching (= 16Opg), A,, = growth rate at hatching (d-l), a = rate of exponen- tial decay of A, (d-r), t” = age at the end of the first cycle of growth and the beginning of the second cycle (d), B,, = growth rate at the start of the second cycle (d-r), /3 = rate of exponential decay of B,, (d-l) and min and max are functions that choose the minimum and maximum of their two argu- ments. Times of the start of the second cycle ranged from 4.7-8.7 d, which compares well with the 4-6 d period of yolk absorption at 6-10” C.

Weight-length relations

Zweifel & Lasker (1976) and Theilacker (1980) argued that when the growth of two body dimen- sions is best described by Gompertz functions, then the two dimensions are curvilinearly related on a double logarithmic plot. The argument was tested by plotting dry weight on standard length for each

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Days from hatch Fig. 5. Mean (+l SD) standard length-at-date of cohorts two, three, five and six and the fitted one-cycle Gompertz growth curves. S, and S,, arc the asymptotic standard deviations of the Gompertz parameters A,, and a.

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50.0

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Age (d) Fig. 6. Mean (+l SD) dry weight-at-age of cohorts two, three, five and six and the fitted two-cycle Gompertz curves.

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! a

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0.050 - - log.W= 37.41-39.12(3.25-lo&L)

- * * ’ ’ n - * *****‘*a**h

10 20 30 10 20

Standard length (mm)

Fig. 7. Dry weight-standard length curves for cohorts two, three, five and six. Every fourth case was plotted. The non-linear allometric relationship [equation (4)] is shown.

of the four cohorts separately. The log-log plots were all distinctly curvilinear (Fig. 7). This was not an artifact of correcting the body dimensions for net capture because the uncorrected dimensions were also distinctly non-linear. The non-linear re- lationship,

log, W = b, - b, (b, - 10gJ)~4, (4)

was fit to the data for each cohort. Equation (4) was derived by Theilacker (1980) by eliminating time from two one-cycle Gompertz equations: b, is the natural logarithm of the asymptotic dry weight, b, is the natural logarithm of the asymptotic stan- dard length, b, is the ratio of the decay parameters, a, for one-cycle length and weight Gompertz curves and b, has no obvious biological interpreta- tion.

Rate of otolith ring deposition

Ninety-four of the 96 fish from 1981 that were ex- amined for otolith ring number belonged to cohort two, and two belonged to cohort one (Fig. 8). The latter fish had such high ring numbers at date that they could not have belonged to cohort two. The mean ring number of the 7 April cohort two sample was 8.4 (SD = 3.0, n =25), indicating that rings were deposited at a daily rate for at least the first week after the date of complete absorption of the yolksac, 30 March. The slope of the regression of mean ring number on date was 0.90 (SE = 0.04) d-l, which was significantly (P~0.01) less than Id-‘.

The first two samples of 1982 alcohol-preserved

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1981: 2 Y = -2.79+0.90x Y = -2.79+0.90x sb= 0.04 sb= 0.04 r=0.92 r=0.92 n=94 n=94

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1982:s - Y= -20.81 +1.28X s,=0.01 r = 0.99 n=63

6 m-m

ring rate = 1.09 day’

1. II 1 4

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March April May March April May

Fig. 8. Frequency distributions of otolith ring numbers with date of capture for larvae of cohorts one (circled numbers), two, five and six. Horizontal bars mark the mean ring number and boxes mark one standard deviation of the mean for each sample. Solid lines are the regressions of ring number on time since the mean hatching dates for cohorts two and five. The broken line is the average rate of ring deposition for cohort six.

fish, 26 April and 6 May, were mixtures of cohorts five and six because the range of ring numbers was 20 or greater. The third sample, 16 May, was com- posed only of fish from cohort five because the range of rink numbers was only 10d. Two means were identified in the 26 April sample with Mac- Donald & Pitcher’s (1979) frequency analysis pro- gram: 8.5 (SD =2.8, n=68) rings and 19.7 (SD = 3.2, n = 24) rings, corresponding to cohorts six and five, respectively. The mean ring number for cohort six indicates that rings were deposited at an average rate of Id-’ during the 8-9d after ab- sorption of the yolk on 18 April. The mean for cohort five indicates that rings were deposited at an average rate of 0.73 d-l for the 27 d from the date of yolk absorption, 30 March, to the date of capture, 26 April. The sample captured on 6 May was clearly a mixture of cohorts five and six because it

spanned a range of 27 rings, too large for a single cohort. The frequency analysis program was not able to identify two unique groups in the sample so the mean ring number for cohort five was fixed at the most likely mode of 32 rings and the mean ring number for cohort six was then estimated by the program to be 19.3 (SD = 4.3, n = 65) rings. This indicates an average ring deposition rate of 1.09 d-’ for cohort six. The slope of the predictive regres- sion of mean ring number on time for cohort five over the period from 26 April to 16 May was 1.26 (SE = 0.01) d-l, which was significantly greater (KO.001) than 1 d-l.

These results offer partial support to the assump- tion that the average rate of ring deposition is about Id-‘, beginning after exhaustion of the yolk. Ad- ditional support for this assumption comes from the fact that the average linear rates of growth in

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%I = 0.002 sa zo.004

. I I 4 I I I I

0 053(1-e”-048t) L-7 7eOo48 - .

S, ~0.006 / / /

/ /

/

a I 1 1 t I

0 20 40 0 20 40 0 20 40

Otolith age (d)

Fig. 9. Plots of standard length on otolith age (= otolith ring number plus 5 d) for larvae of cohorts two, five and six. The solid lines are the fitted Gompertz growth curves. sAo and s, are the asymptotic standard deviations of the Gompertz parameters, A0 and a, respectively. Broken lines are the growth curves calculated from length-at-date data.

length for cohorts two, five and six were equal to or greater than 0.36mm d-i, the rate at which ring deposition was daily in enclosure rearing experi- ments (Geffen 1982, McGurk 1984a). However, these results also indicate that the rate can be less than daily over some time periods.

Length with otolith age

If the average rate of ring deposition was daily, then growth curves based on otolith age (= number of otolith rings + 5 d) should closely resemble the growth curves calculated from modes in the length- frequency plots. This was tested by calculating single one-cycle Gompertz growth curves (Fig. 9). The curves of cohorts two and six followed closely the length-date curves over the range of the data. Above the upper limit of the age range, 30-40d, these curves diverged rapidly from the length-date

curves, predicting lengths at 60d that were 8.3 and 18.6% lower. Otolith ages for cohort five included fish older than 40 d and so the divergence in length between the otolith-age and length-date curves was lower, a maximum of 6.3% at age 30 d. The otolith- age curves, particularly that of cohort five, are the most accurate growth curves over the age range of the data because their calculation does not incor- porate assumptions about the mean date of hatch- ing. However, for cohorts two and six the rear- ranged length-date curves are the most appropriate for estimating age from length because the range of the length data is greater than the range of the otolith-age data.

Discussion

The linear rates of growth in length for the four cohorts are the highest recorded for wild popula-

45

tions of Pacific herring larvae and they are among the highest recorded for Atlantic herring larvae [see review by McGurk (1984b)]. They are higher than almost all growth rates recorded from enclosure-reared Pacific and Atlantic herring lar- vae.

The curvilinearity of the length-date curves oc- curs mainly in older (30-60 d) larvae so it was prob- ably caused in whole or in part by net evasion. The curvilinearity of the weight-age curves occurs in both young (O-20 d) and older larvae so it was due to other factors as well as net evasion: (1) larvae lose weight as their yolk is absorbed and (2) young post-yolk-sac larvae do not gain weight as rapidly as older larvae because they must learn to feed successfully and it takes several days for them to ascend the learning curve.

The curvilinearity of the length-date and weight- age curves implies that the weight-length relation- ships of larval Pacific herring are also curvilinear on a double logarithmic plot. The data presented here is the clearest example of a non-linear allometric relationship between weight and length in fish lar- vae. All previous examples of weight-length curves for herring larvae have assumed a linear allometry (Marshall et al. 1937, Sameoto 1972, Laurence 1979, Gamble et al. 1981).

The result of this study confirm the assumption that rings are deposited in the otoliths of wild Pa- cific herring larvae at an average rate of ld-I. It also confirms that ring deposition can be inter- rupted or slowed below Id-l for short periods of time. However, the practical effects of interrupted ring deposition on the ageing of herring larvae and on the shape of the growth curve were shown to be relatively minor in the cohorts described in this study because the growth rates were high enough that the interruption in ring deposition in cohort five did not amount to more than about 5 d. This is less than the average variance of the otolith age- at-length (Fig. 9) The major source of variance in the length-at-,otolith age was the natural hetero- geneity of growth rates that occurs in every popula- tion of herring larvae.

Interrupted ring deposition was probably caused by low water temperatures. The water tempera- tures of the EJamfield Inlet-Cape Beale area were

14 7

- 1981 . .

Llnr+J

. 12 - I I

:. . P

6 L L

1 10 20 30 10 20 30 10 20 30

March April May

Fig. 10. Water temperatures of the Bamfield Inlet area for March-May of 1981-1982. Solid lines indicate daily temperatures at 2 m depth, one hour before daylight high tides, at Cape Beale, 8.5 km from Bamfield Inlet (collected by the staff of the Cape Beale lighthouse). Broken lines interpolate between days in which no measurements were taken. Dotted horizontal lines are the mean monthly surface water temperatures for Bamfield Inlet taken from Wheeler & Srivastava (1984).

lower by about 1°C in late March 1982 than they were in late March 1981 (Fig; 10). This suggests that the slow rate of ring deposition in cohort five dur- ing the first 27d of post-yolk-sac existence may have been due to low temperature. An average monthly March temperature of 8.2” C is far above the lower lethal limit of about 4” C for Pacific her- ring larvae (Alderdice & Velsen 1971, Alderdice & Hourston 1985). However, McGurk (1984a) found, in a stepwise multiple regression analysis, that the rate of ring deposition in enclosure-reared cohorts of larval Pacific herring was directly and signifi- cantly correlated with water temperature as well as growth rate. At a constant growth rate of 0.36mm d-l the rate of ring deposition was 1.22 d-l at 10” C, 1.07 d-l at 9” C, and 0.76 d-l at 8°C. The evidence is consistent with the hypothesis that daily rings are deposited only in Pacific herring larvae that are growing at a rate of 0.36mm d-l in water with a temperature of 9” C. These conditions were met for cohorts two and six but not for the first 10-15 d of cohort five.

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Lough, R.G., M. Pennington, G.R. Bolz & A.A. Rosenberg. 1982. Age and growth of larval Atlantic herring, Clupea harengus L., in the Gulf of Maine-Georges Bank region based on otolith growth increments. U.S. Fish. Bull. 80: 187- 199.

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I gratefully acknowledge the staff of the Bamfield Marine Station: Ronald Foreman, Sabina Leader, Ann Bergey, Tom Bedford and Sigurd Tveit, for assisting in the collection of field samples. This research was supported by two University of Brit- ish Columbia Summer Research Scholarships, a Graduate Research, Engineering and Technology Award from the British Columbia Science Council, and by National Sciences and Engineering Re- search Council grants to N.J. Wilimovsky.

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Received 3.9.1986 Accepted 20.12.1986