Kellison and Eggleston: Modeling release scenarios for Paralichthys dentatus 



85 



B 



a type-2 predator functional response when 

 postrelease densities were relatively low. 



Type-3 mortality As in all other simulations, 

 the most important factor affecting number 

 of survivors under type-3 mortality was size- 

 at-release, such that the greatest numbers of 

 survivors were always produced by releasing 

 the largest fish possible (Fig. 7A). Number of 

 survivors decreased with increasing Julian 

 day of release (Fig. 7A). Percent survival 

 was considerably lower (>25% in some cases) 

 under type-3 mortality than under density- 

 independent mortality at moderate to high 

 numbers released (NFR=10, 000-400, 000) 

 (Fig. 5 A). 



In nearly all simulations, the lowest CPS 

 values were produced by releasing the larg- 

 est fish possible (Fig. 7B). The exceptions to 

 the "large size = optimal CPS" rule occurred 

 when postrelease densities were small (cor- 

 responding to numbers released of 100, 500, 

 and 1000) and the mortality curve was type 3 

 (weak or strong). In these instances, mortality 

 was sufficiently low at low release densities 

 ( Fig. 7B ) so that the difference in overall sur- 

 vival between small- and large-released fish 

 was small enough to be overridden by the in- 

 creased cost of the larger fish, and the mini- 

 mum CPS was obtained when small (42-44 

 mm TL) fish were released (e.g. Fig. 7B). 



At low numbers released (NFR=100-1000), 

 optimal cost-per-survivor was considerably 

 lower (>45% in some cases) under type-3 

 mortality than under density-independent 

 mortality (Fig. 5A). As NFR increased, CPS 

 under type-3 mortality became greater ( -40^ 

 in some cases) than that achieved under den- 

 sity-independent mortality (Fig. 5B). 



Temporal shift in functional response from 

 type 2 to type 3, and from type 3 to type 2 

 The optimal numbers of survivors under 

 varying numbers released were identical, and 

 optimal CPS values nearly identical, when 

 the form of the functional response changed 

 from a type 2 to a type 3, and from a type 3 to 

 a type 2, midway through the juvenile nurs- 

 ery season (Fig. 8, A and B). The differences 

 at low postrelease densities between optimal 

 CPS values under shifting type 2 to type 3 and type 3 to 

 type 2 scenarios (Fig. 8A) occurred because initial mortality 

 under the type-3 functional response was sufficiently low 

 that the difference in overall survival between small- and 

 large-released fish was small enough to be overridden by 

 the increased cost of the larger fish (Fig. 8A). The minimum 

 CPS was obtained when small (42-44 mm TL) fish were 

 released (in all other cases, optimal results were obtained 

 when size-at-release was maximized) (Fig. 8A). The major 

 difference between the two shifting scenarios is that the 



re/ease 



Figure 6 



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

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

 release at number released (NR) = 5000 (postrelease density=0.05l under 

 a strong type-2 functional response. CPS values greater than $10 were set 

 equal to $10 for ease of presentation. 



release dates producing optimal results for a given number 

 of fish released varied depending on the direction of the 

 shifting functional response. For example, when the func- 

 tional response shifted from a type 2 to a type 3, a release 

 of 100,000 HR organisms achieved optimal results when 

 release occurred early in the season (Julian day <145) 

 (Fig. 9A). When the functional response shifted from a 

 type 3 to a type 2, a release of 100,000 HR summer floun- 

 der achieved optimal results only when releases occurred 

 later in the season (Julian day >145) (Fig. 9B). When the 



