86 



Fishery Bulletin 102(1) 



functional response shifted from a type 3 to a type 2, releas- 

 ing 100,000 HR organisms prior to Julian day 146 resulted 

 in markedly decreased survival (and therefore increased 

 CPS ) compared to results obtained from releases after day 

 146 (e.g. releasing on Julian day 92 resulted in a decrease 

 in number of survivors and an increase in CPS of 22.8% 

 and 29.7%, respectively) (Fig. 9B). Thus, date-of-release 

 had a significant effect on the results (and therefore in 

 determining optimal release strategies) when the relation- 

 ship between density and mortality changed temporally, 

 suggesting that the presence of a temporal shift in the func- 



500 



£ 400 



300 



200 



E 

 z 



100 



220 



OaV' 



Size at re/ease 



Figure 7 



Response surfaces of (A) number offish (summer flounder) survivors and 

 (B) cost-per-survivor (CPS) as a function of date of release and size at 

 release at number released (NR) = 500 (postrelease density=0.005) under 

 a strong type-3 functional response. CPS values greater than $10 were set 

 equal to $10 for ease of presentation. 



tional response of the predator guild would have consider- 

 able effects on the number of survivors and CPS for stock 

 enhancement efforts with juvenile summer flounder. 



Correspondence between predicted and 

 observed temporal abundance patterns 



Under the assumption of a type-2 functional response, 

 predicted declines in juvenile summer flounder density 

 over time were rapid when initial density was relatively 

 low (i.e. 0.1 fish/m 2 ) (Fig. 10, A and B). These predictions 

 contrast with those observed in the field, 

 in which declines at relatively low initial 

 densities were gradual (compare Fig. 10A 

 and 10B to Fig. 10F). Under the assumption 

 of a type-3 functional response, predicted 

 declines were rapid when initial density was 

 relatively high (i.e. 0.5 fish/m 2 ) I Fig. 10, C 

 and D). These results generally contrast with 

 those observed in the field, in which declines 

 at relatively high densities were much less 

 rapid than those predicted under a strong 

 type-3 functional response, and somewhat 

 less rapid than those predicted under a weak 

 type-3 functional response (Figs. 10F and 11). 

 Under density-independent mortality, there 

 was little difference in predicted declines in 

 juvenile summer flounder density over time 

 between the three initial density levels (0.1, 

 0.3, and 0.5 fish/m 2 ); in each case there was 

 a gradual decrease in density over time (Fig. 

 10E). These results were similar to those 

 observed in the field, although declines at rel- 

 atively high densities in the field were some- 

 what more rapid than those predicted under 

 density-independent mortality ( compare Figs. 

 10E and 10F). Thus, a density-mortality rela- 

 tionship lying between that generated under 

 density-independence and that generated 

 under the weak type-3 functional response 

 in the model would most closely predict the 

 temporal declines observed in the field. 



Discussion 



Implications for stock enhancement of 

 summer flounder 



Regardless of the relationship between den- 

 sity and mortality, size-at-release was the 

 most important variable in the model affect- 

 ing survival and costs associated with stock 

 enhancement of summer flounder. The model 

 predicts that under all release scenarios, 1) 

 survival will be maximized and 2) costs asso- 

 ciated with stock enhancement (i.e. cost per 

 survivor) will be minimized when HR fish are 

 released at the largest size possible. From a 

 survival standpoint, these results are not 



