Fishery Bulletin 102(1) 



the density-mortality relationship. For example, at postre- 

 lease densities of 0.5 fish/m 2 (NFR=50,000), survival of 

 released flounder under density-independent mortality 

 was ~28% higher than that predicted under strong type-3 

 mortality, but only -2% higher than that predicted under 

 weak type-2 mortality (Fig. 5A). At postrelease densities 

 of 0.001 fish/m 2 (NFR=100), survival of released flounder 

 under density-independent mortality was ~41% higher 



12 3 4 5 



Density (number of fish/m 2 ) 



Figure 5 



l A i Optimal percent survival and iBi optimal cost-per-survival (US$) as a func- 

 tion of postrelease density undci density-independent and varying density- 

 dependent, mortality relationships for summer flounder. 



than that predicted under strong type-2 mortality, but -2% 

 less than that predicted under strong type-3 mortality ( Fig. 

 5A). In contrast, when postrelease densities were relatively 

 high, there was less of an impact of density-mortality rela- 

 tionship on postrelease survival and costs associated with 

 stock enhancement. For example, at postrelease densities 

 of three fish/m 2 (NFR=300,000), survival of released floun- 

 der differed by less than 4% between density-independent, 

 weak or strong type-2, and weak type-3 mor- 

 tality, although survival under strong type-3 

 mortality was ~99c less than that predicted 

 under density-independent mortality and 

 -11% less than that predicted under strong 

 type-2 mortality (Fig. 5A). Thus, the model 

 results were most sensitive to violations of the 

 assumption of density-independent mortality 

 at low densities offish released in the field. 



Type-2 mortality As with density-indepen- 

 dent mortality, the most important factor 

 affecting number of survivors and cost per 

 survivor under type-2 mortality was size-at- 

 release (Fig. 6, A and B). In all simulations, 

 the greatest number of survivors was pro- 

 duced by releasing the largest fish possible. 

 Number of survivors decreased with increas- 

 ing Julian day of release (Fig. 6A). There was 

 a considerable interaction between size- 

 at-release and number of fish released, 

 such that low postrelease densities were 

 subjected to relatively high proportional 

 mortality. Thus, when fish were released 

 in low numbers and at small sizes, the 

 fish were subjected to relatively high 

 proportional mortality rates for long 

 periods of time (while they grew towards 

 the 80-mm-TL ontogenetic shift size) and 

 consequently produced few or no survi- 

 vors (Fig. 6A). Optimal release scenarios 

 under strong type-2 mortality produced 

 substantially lower (>40% in some 

 cases) percent survival (and therefore 

 substantially higher cost-per-survivor) 

 estimates at low to moderate numbers 

 released (NFR= 100-50,000; postrelease 

 density=0.001-0.5 fish/m 2 ) than under 

 density-independent mortality (Fig. 5, A 

 and B). Differences in percent survival 

 estimates (and thus cost-per-survivor 

 estimates) between density-indepen- 

 dent survival and weak or strong type-2 

 mortality declined to less than 5 r i when 

 the numbers released increased to 

 25,000 (postrelease density=0.25 fish/m 2 ) 

 under weak type-2 mortality and 75.000 

 (postrelease density=0.75 fish/m 2 ) under 

 strong type-2 mortality (Fig. 5A). Thus, 

 model predictions under density-inde- 

 pendent mortality differed most from 

 predictions under mortality governed by 



- density-independent 

 -type 2 - weak 



- type 2 - strong 

 -type 3 - weak 

 ■type 3 - strong 



density-independent 

 type 2 - weak 

 type 2 - strong 

 type 3 - weak 

 type 3 - strong 



