ELSEVIER Fisheries Research 22 (1995) 265-277
Estimation of age and growth of sardine, Sardina pilchardus ( Walbaum, 1792 ), larvae by reading daily
Jakov DulEiC Institute of Oceanography and Fisheries, set. I, MeftroviCa 63, Split, Croatia
Accepted 15 June 1994
Surdina pilchardus (Walbaum, 1792) were sampled during the period February-Ma) 199 1 and in April 199 1 in the eastern part of the Adriatic and in the Bay of Biscay, respec- tively. Specimens were aged by means of growth rings in the sagittal otoliths. The Gom- pertz and Laird-Gompertz growth equations, commonly used in larval growth analysis, described the growth of this species in the length range sampled. The initial length of sar- dine larvae at time r=O for the Adriatic was 4.138 mm at a mean temperature, 7, of 13.1OC. The daily growth rate was found to be slightly higher in sardine larvae from the Bay of Biscay than in those from the Adriatic, but the difference was not statisticallq significant.
Keywords: !iardinapilchardus; Growth, fish; Age determination; Otoliths
The sardine, Sardina pilchardus ( Walbaum, 1792 ), represents the main pelagic species of the purse-seine catch in the Croatian Adriatic.
Larval fish growth is of particular importance to population dynamics, espe- cially to recruitment and biological models incorporating environmental param- eters (Methot, 198 1 a; Houde, 1986, 1987 ).
Microscopic markings in thin sections of otoliths of fishes, with evidence that these marks present a record of daily growth (Pannella, 197 1, 1974)) provide an alternative means of ageing tropical species for which seasonal and annual growth rings are often hard to interpret. These marks are produced in many fishes
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266 J. DulM /Fisheries Research 22 (I 995) 265-277
(Struhsaker and Uchiyama, 1976; Brothers et al., 1976; Neilson et al., 1985; Jones and Brothers, 1987; Palomera et al., 1988; Karakiri and Westernhagen, 1989; Moksness and Wespestad, 1989). Counting otolith microincrements has been useful in estimating the growth of juvenile and larval fishes (Barkman, 1978; Methot, 1981b; Geffen, 1986; Jones, 1986).
Age and growth information on larval sardine, especially in the natural envi- ronment, is limited. Gamulin and Hure ( 1955 ) first observed sardine egg devel- opment at 13 C and 18 C in a natural environment. Blaxter ( 1969) reared sar- dine larvae from La Manche, at 10, 15, 16, 17 and 17.3C, but did not provide any numerical data or mathematical estimates. From Blaxters growth graphs, Regner et al. ( 1987) mathematically determined the relationship between the length and age of larvae. RC ( 1983a) suggested the existence of daily growth in- crements in the otoliths of pilchard larvae and that these might be used to age field-capture specimens. More recently (RC, 1983b) the growth rates of sardine larvae and the thickness of daily units were found to vary in relation to the time of year. The daily nature of these microincrements in the otoliths of Sardina pil- chardus has been demonstrated by RC ( 1983a,b, 1984).
The objective of this study was to analyse and estimate the growth character- istics of sardine larvae from the Adriatic and the Bay of Biscay, from otolith rings. The significance of the difference in growth rate between sardine larvae from the Adriatic and those from the Bay of Biscay was tested as well as that of the differ- ence in the regression coefficients of linear growth equations. This should pro- vide the basis for future studies of sardine growth parameters and of the condi- tions that affect the renewal of this population in the Adriatic.
2. Materials and methods
2. I. Sampling
Sardine yolk-sac larvae and larvae were collected from the middle of the Ad- riatic and the Bay of Biscay, by the research vessels Bios and Cornide de Saa- vedra, respectively. Between February and May 199 1, specimens were collected from the following locations in the middle of the Adriatic: ( 1) Kagtela Bay, (2) BraT: Channel, (3) Pelegrin and Pakleni otoci archipelago and (4) StonEica and B%evo Island (Fig. 1). These four locations are permanent stations where the Institute of Oceanography and Fisheries has performed hydrographic and plank- ton community investigations for more than 40 years.
Four methods of collection were used: ( 1) oblique tows from a maximum depth of 200 m to the surface using a 40 and 72 cm Bongo net with 0.260 and 0.333 mm mesh, (2) nektonic larvae were caught using gear specially made for this purpose with frame sides of 1 m and 1 mm mesh size, (3) by plankton net, mesh size 0.250 mm, with a circular aperture of 98 cm diameter and aperture surface 0.75 cm , and (4) by Hensen plankton net, mesh size 0.250 mm, bottom to surface hauls. Nektonic larvae were collected under artificial light of 250 and 400 W,
J. D&it /Fisheries Research 22 (1995) 265-277 261
Fig. 1. Area of investigation, Adriatic. A, KaStela Bay; B, BraE Channel; C, Pelegrin and Pakleni otoci archipelago; D, StonEica and Bikvo Island.
11. IO * 9. 8 7 6 5
Fig. 2. Area of investigation, the Bay of Biscay.
fishing for about half an hour at Pantan in Kagtela Bay. Four gear types were used to obtain a wide length range of larvae (from the end of yolk-sac absorption to metamorphosis).
The total material from the Bay of Biscay was collected at Station 12, Transect 4, area Gijon-Ortega& during a SARP pilot study for sardine off north and north- west Spain in April/May 199 1 (Fig. 2 ). Specimen sampling was carried out by
268 J. DulcW /Fisheries Research 22 (1995) 265-277
double oblique Bongo net, mesh size 280 p and 200 pm in either a 40 cm or 72 cm diameter frame, respectively.
Hydrographic sampling was carried out by CTD casts at selected stations in both areas. The mean surface temperature over the surveyed areas was 13.1 C in the Adriatic and 11.8 C in the Bay of Biscay.
Planktonic material from the middle of the Adriatic was preserved for a 0.5 h in 2% buffered formalin (pH 7.8-8.6). Thereafter larvae were transferred to 90% ethanol (after first having established that formaldehyde destroys otoliths, in agreement with findings of Radtke and Dean ( 1989) ). The material from the Bay of Biscay was preserved in 4% form01 of pH 8.6.
2.2. Length determination
A total of 160 sardine larvae from the Adriatic and 120 from the Bay of Biscay were examined. Specimens from the Adriatic contained some sardines at the end of metamorphosis, as shown by the furrows on the operculum characteristic of juvenile and adult fish.
Standard length was measured from the top of the mouth to the base of the notochord using a micrometer with loupe ocular at 10 x magnification. Shrink- ing of larvae in the formaldehyde was neglected in the length measurement. Re- duction in larval length caused by preservation depends on the initial lengths of the specimens and duration of storage. Preservation in formalin causes an aver- age 5% loss in standard length of larvae but preservation in ethanol does not af- fect length (McGurk, 1984).
Before removal of otoliths, larvae were rinsed in distilled water. Otoliths were removed from the otocysts using dissecting needles and placed in a drop of water. The distilled water was removed using line paper, and the otoliths were then moistened with immersion oil. In some cases, otoliths were polished using aliza- rin dissolved in alcohol to improve clarity. Otoliths were left in the immersion oil for 24 h since it has been found that otoliths are more easily read after storage in the oil. Rings were read at 450 x ,600 x and 1000 x magnification with a Reich- ert light microscope. Two workers read the otoliths to achieve higher precision; the percentage reading difference was negligible. The sagittae as viewed by light microscope revealed a pattern of alternating light and dark concentric rings sur- rounding a nucleus deposited in the earliest stages of development. One light and one dark ring visible all round the otolith were counted as one complete growth increment. Dark rings on the otolith edge were regarded as increments that were still forming and hence were counted as half a growth increment.
Data from the Adriatic and the Bay of Biscay were mathematically and statis- tically analysed. Age at length of larvae was estimated by common non-linear growth equations (Kramer and Zweifel, 1970; Regner, 1980).
J. DulEic /Fisheries Research 22 (1995) 265-277 269
Sagittae in small sardine larvae were round, and in older larvae ellipsoidal. The nucleus was easily discernible in all sagittae. Two rings were visible in yolk-sac larvae at the initiation of yolk-sac absorption, and up to five rings in yolk-sac larvae at the end of yolk-sac absorption. These latter were taken as indicating larvae of zero age for the analysis of growth curves. Larval rings were counted from the check ring. Results of ring counts in the sagittae larvae from the Ad- riatic and the Bay of Biscay are presented in Tables 1 and 2, respectively.
3.1. Length at age in larvae
Two equations were used to describe length at age of sardine larvae. The first equation is after Gompertz ( 1825 ):
&=aexp( -h-*) (1)
where I, is the length of larvae at time t, a is the asymptote, and b and c are con- stants. The second equation is after Laird et al. ( 1965 ) :
&=Zoexp[ (A,/c) (1 -e-C)] (2)
where 1, is the length of larvae at time t = 0, A0 is the instantaneous growth rate at time t=O, and c is the same constant as in Eq. ( 1). Function parameters were calculated from the data on age obtained from otolith readings and on length of larvae, by an iterative procedure using the FIT program (S. Regner, unpublished data, 1987).
Values for the parameters in Eqs. ( 1) and (2) are given in Table 3 for the Adriatic and in Tables 4 and 5 for the Bay of Biscay. For the latter location, Table 4 depicts the values of both functions for all the data, and Table 5 only the data for 7-day-old larvae, since up to that day larvae were more numerous per unit age.
Both functions show high coefficients of correlation at the 99% significance level with the data obtained from otolith counts and length measurements in lar- vae from the Adriatic and the Bay of Biscay. The initial length of larvae at time t = 0 was 4.138 mm at a mean temperature T of 13.10 C in the Adriatic. The mean length of captured yolk-sac larvae at the transition to larvae was 4.87 mm. In terms of statistical significance, this means that the measured value of initial length is quite close to the actual initial length. The values of initial length of larvae at time t = 0 obtained for the Bay of Biscay were useless because no zero or 1 -day-old larvae were collected. Graphical representation of the data fitted by the Gompertz function is given in Fig. 3 for the Adriatic and in Fig. 4 for the Bay of Biscay.
The differentiation of values obtained by the Gompertz function by means of the expression
270 J. DulEic /Fisheries Research 22 (I 995) 265-277
Table 1 Number of daily rings and the length range of sardine larva-Adriatic
Number of rings (days)
No. of larvae Length range (mm)
Mean length (mm)
0 7 4.06-5.67 4.87 2 2 5.39-5.42 5.41 2.5 3 5.42-5.84 5.61 3 17 4.58-7.22 5.86 3.5 6 5.62-6.55 5.89 4 32 5.56-8.33 6.60 4.5 9 6.1 l-9.72 7.19 5 26 6.81-10.83 8.43 5.5 2 8.33-10.14 9.24 6 8 6.67-12.92 9.46 6.5 2 9.31-10.56 9.94 7 5 9.31-14.17 12.17 8 2 12.50-14.45 13.48 9 1 13.61 13.61
10 1 15.97 15.97 11 1 15.64 15.64 12 1 17.50 17.50 14.5 1 17.64 17.64 18 1 19.45 19.45 20 1 21.25 21.25 24 1 21.11 21.11 29 1 23.34 23.34 30 1 24.17 24.17 32 1 21.09 27.09 33 1 26.81 26.81 35 3 22.36-24.59 23.61 36 3 24.31-27.22 24.31 38 2 26.67-29.17 21.92 39 2 23.89-24.86 24.86 41 2 27.09-28.47 27.87 42 2 25.84-27.92 26.88 45 1 27.09 27.09 46.5 1 27.43 21.43 53 1 29.86 29.86 55 3 26.1 l-28.75 21.82 56 2 30.56-31.39 30.98 57 2 28.56-35.89 32.39 59 1 31.25 31.25 61 1 32.36 32.36 64 1 34.03 34.03
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Table 2 Number of daily rings and the length range of sardine larvae-Bay of Biscay
Number of rings (days)
No. of larvae Length range (mm)
Mean length (mm)
2 3 3.61-3.89 3.80 3 6 3.61-4.86 4.40 3.5 2 4.45-4.86 4.70 4 8 4.17-6.39 5.23 4.5 2 5.56-6.39 5.98 5 8 6.53-8.75 7.34 5.5 5 7.08-8.75 7.89 6 27 7.22-l 1.81 9.79 6.5 4 9.72-l 1.81 10.84 7 18 9.03- 14.03 10.97 7.5 3 9.72-11.53 10.41 8 5 9.85-l 3.61 12.17 8.5 4 10.97-12.92 11.74 9 6 10.83-15.97 12.99 9.5 1 15.97 15.97
10 3 12.78-13.89 14.31 10.5 1 13.89 13.89 11 2 11.81-14.17 12.99 12 2 14.17-16.39 15.28 15 2 14.03-17.36 15.70 17.5 1 17.22 17.22 18 2 15.28-18.06 16.67 20 2 15.28-17.78 16.53 20.5 1 20.00 20.00 22 1 19.42 19.42 26.5 1 23.06 23.06
Table 3 Estimated values of Gompertz and Laird-Gompertz functions-Adriatic
P kO8410072 b.982 i 0.00 1
Laird-Gompertz 10 Ao P 4.138228 0.1654595 kO8411218 b.982
272 J. D&W /Fisheries Research 22 (1995) 265-277
Table 5 Estimated values of Gompertz and Laird-Gompertz functions-Bay of Biscay (data on 7-day-old sardine larvae)
b 3.304057 :. 1759623 i.923
Laird-Gompertz Ll Ao P 1.086755 0.582053 :.I761917 L.923 CO.001
0.00 1,,,,,,,,~,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,~ 0 5 IO 15 20 25 30 35 LO I.5 50 55 60 65
Fig. 3. Growth of sardine larvae in the Adriatic observed (asterisks) and estimated, by Gompertz and Laird-Gompertz functions (solid line). Age was estimated from the number of daily growth rings in otoliths.
d_6y_Yi-Yi-1 -6X-Xi -Xi_ 1 (3)
where yi and Xi are ith values, and yi_, and Xi_, are the preceding values, allowed estimation of the growth rate of larvae, i.e. their daily growth increment. The daily growth rate was found to be higher in larvae from the Bay of Biscay than in those from the Adriatic. The greatest increments were recorded for 7-day-old ( 1.9 1 mm) larvae from the Bay of Biscay, and for 9-day-old (0.92 mm) larvae from the Adriatic. This is shown in Fig. 5.
The linear form of the Gompertz function was used to establish statistically significant differences in the growth rate of larvae by the method of linear regres- sion for higher y values per x value (Solcal and Rohlf, 1969). Only data for larvae up to 12 days old were considered, since the data for that period were relatively most homogeneous both for the Adriatic and for the Bay of Biscay. An F-test was performed to determine the difference between two regression coefficients. The
J. DulEiC /Fisheries Research 22 (1995) 265-2 77 273
. 7 g 2000 E
Fig. 4. Growth of sardine larvae in the Bay of Biscay observed (triangles) and estimated, by Gom- pertz and Land-Gompertz functions (solid line, values of both functions for all the data; dashed line, values of both functions only for data by the seventh day of age of sardine larvae). Age was estimated f...