196 



Fishery Bulletin 88(1 



1990 



A. 



1°" 



c 04 

 0) 



E 



0} 



u 03 



20 30 40 50 



Days since hatching 



Q ODserved Mean Lei^gih 

 — Model Wean Length 



4f4Hr 



' I " ' ' I ' '  M ' ' ' ' I " I I I I " ' I ' 



' I " " I " " I' 



1 6 11 16 21 26 31 36 41 46 51 58 



Days since hatching 



E 



&. 10 



V 1 



Observed Mean Weight 

 Model Mesn weight 



21 26 31 36 41 



Days since hatching 



Figure 6 



(A) Growth increments in lengtli (nini/ii) and weight (nig/d) for Pacitlc 

 herring from Bristol Bay, Alaska, in a 2000-m-' basin at Fl0devigen 

 Biological Station, Norway. (B) Observed length-at-age and 95% con- 

 fidence interval, and length-at-age estimated by a Gompertz model 

 (solid line), of Pacific herring in the basin. (C) Observed weight-at- 

 age (mg) and 95% confidence interval, with a Gompertz model growth 

 estimate, of Pacific herring from Bristol Bay, Alaska, in the 

 2000-nr basin at Fl0devigen Biological Station. Norway. 



Observed density 

 Predicted Constant Z 

 Predicted Variable Z 



20 30 40 50 



Days post-hatching 



Figure 7 



Estimated survival of Pacific herring from Bristol Bay, Alaska, in 

 the 2000-in" basin at Fl0devigen Biological Station, Norway, from 

 yolksac stage to termination on day-63, based on plankton net sam- 

 ples fit to constant and variable mortality (Z) models. 



The great variability in weight made it difficult to 

 fit a growth model by nonlinear least-squares. A mocii- 

 fied Gompertz model (Zweifel and Lasker 1976) was 

 examined iteratively by fixing one parameter and vary- 

 ing the others. The Gompertz model and observed 

 weights are shown in Figure 6c. It can be seen that 

 the model describes growth well to day-25, but then 

 observed growth deviates from predicted growth. The 

 point of deviation is at the time metamorphosis com- 

 mences, when the more robust fish may have begun 

 to avoid the sampling net. Ware (1975) points out 

 that net avoidance often results in underestimation of 

 growth rate. 



Survival 



Of the 24,840 larvae released into the basin, 4891 were 

 recovered when the basin was drained on day-63. The 

 overall survival rate in the basin was 19.69%, total in- 

 stantaneous mortality (Z) = 1.625. The daily mortal- 

 ity rate was 2.7%. 



Comparison of the estimated density (fish/m''), 

 based on decay of the initial population by the daily 

 mortality rate, with the density of larvae estimated 

 from plankton net samples shows that net samples 

 underestimated larval aliundance in the basin if the rate 

 of decline was constant (Fig. 7). The net samples sug- 

 gest that mortality was initially high following release 

 and then declined. A two-part mortality curve is shown 

 in Figure 7 which estimates daily mortality to be 

 15.08% during day-3 to day-12 followed by a rate of 

 0.05% for the remainder of the experiment. 



