METHOT and KRAMER GROWTH OF NORTHERN ANCHOVY LARVAE 



Standard deviation of log,, (size) at age. calcu- 

 lated from the set of combined data, ranged from 

 0.045 at 4 increments to 0.144 at 19 increments 

 with a mean of 0.0904. The standard error of re- 

 gression (standard deviation of residuals) of the 

 Gompertz curves fit to the data (Table 2) were 

 similar to these estimates of standard deviation of 

 size at age. Any difference in growth rate between 

 these samples, small scale environmental hetero- 

 geneity integrated by our nets, or random error in 

 the aging of the larvae causes the standard error of 

 regression to overestimate variability in the 

 growth process. Over the same age range in a 

 laboratory experiment (Hunter 1976), standard 

 deviation ranged from 0.064 to 0.15.3 with a mean 

 of 0.115. 



EVALUATION OF POTENTIAL BIASES 



The conditions under which larvae are reared 

 and growth is measured in the laboratory may 

 differ sufficiently from conditions in the sea to bias 

 the comparison of growth in the sea with growth in 

 the laboratory. In the laboratory, growth is esti- 

 mated from a time series of samples from a cohort. 

 The exact age of each larva is known; temperature 

 rarely varies >1°C: prey concentration is rather 

 constant; and mortality is low (about S'/r/day in 

 Hunter 1976) and except for cannibalistic species, 

 not influenced by size-selective predation. Growth 

 of sea-caught larvae is estimated from one sample 

 containing larvae of several ages. The age of each 

 is estimated from the number of daily growth in- 

 crements in its otoliths, the environmental condi- 

 tions are measured only when the sample is taken, 

 mortality is over lO'^f/day (Smith and Lasker 

 1978 ) and probably size selective, and large larvae 

 avoid the net disproportionately. 



Age Determination 



Anchovy larvae deprived of food at the normal 

 time of first feeding may delay deposition of daily 

 growth increments until food is provided and 

 growth resumes (Theilacker-^), but if larvae are 

 still about 4.2 mm when the first increment ap- 

 pears, estimates of the growth rate of larger larvae 

 will be unaffected. Taubert and Coble ( 1977 ) found 

 that increment formation in late larval and 

 juvenile centrarchids stopped if growth was 



^G. H. Theilacker, Southwest Fisheries Center La Jolla 

 Laboratory, NMFS, NOAA, P.O. Box 271. La Jolla, CA 920.38, 

 pers. commun. June 1978. 



slowed sufficiently by low temperature. We 

 examined the otoliths of some anchovy larvae 

 whose growth was drastically retarded by a reduc- 

 tion in rations (Methot unpubl. data) (F in Figure 

 3 ). The slowest growing of these larvae had fewer 

 increments than expected, so increment formation 

 can stop before the point of no return is reached. 

 However, if we use number of increments rather 

 than known age to estimate growth rate of these 

 laboratory-reared larvae, the resulting overesti- 

 mated growth rate is still slower than that ob- 

 served in any field samples used in this study. We 

 conclude that the sea-caught larvae were growing 

 fast enough to deposit a growth increment every 

 day. 



Temperature Determination 



Anchovy larvae usually remain above the 

 thermocline ( Ahlstrom 1959). so surface tempera- 

 ture probably accurately represents the tempera- 

 ture they experience. Growth was slower than ex- 

 pected in March 1976. but it is unlikely that the 

 surface temperature overestimated the tempera- 

 ture experienced by the larvae because these sam- 

 ples were all collected shallower than 10 m (Table 

 1). Positive deviations in growth rate, equivalent 

 to a temperature change of up to 3° C, were ob- 

 served in March 1977, but additional temperature 

 data collected at that time indicate that the mea- 

 sured surface temperature accurately represented 

 the temperature experienced by the larvae 

 throughout their lifetimes. The relative surface 

 temperature field determined from satellite ob- 

 servations of infrared radiation (Bernstein et al. 

 1977 ). taken just before and after this 2-wk cruise, 

 was entirely consistent with the temperature field 

 measured during the cruise; there was no cooling 

 trend. The resolution of the satellite observations 

 was insufficient to distinguish details of the eddy 

 structure in the region of samples CI and C2, but 

 here the greater intensity of surface temperature 

 determinations allowed contouring of isotherms 

 for the first 4 days of the cruise and the next 5 days 

 (Figure 4). Samples CI and C2 were taken within 

 persistent water masses of about 10 km width, not 

 in regions with steep horizontal gradients in tem- 

 perature. Because of the shallow thermocline in 

 this region ( ■- 5 m at some stations), we may have 

 overestimated the temperature experienced by 

 the larvae but this would cause a negative devia- 

 tion from the model, not the observed positive de- 

 viations. 



419 



