Kellison and Eggleston: Modeling release scenarios for Paralichthys dentatus 



83 



make ontogenetic habitat shifts are 

 inhabited by relatively high densities of 

 potential predators (e.g. blue crabs, age 1+ 

 flounders, red drum [Sciaenops ocellatus], 

 searobin [Prionotus sp.], and lizardfish 

 [Synodus sp.] ), which may be considerably 

 less abundant in shallow-water habitats. 

 These relatively large and abundant 

 predators would presumably expose small 

 migrating fish to high rates of predation 

 (see, for example, Elis and Gibson, 1995; 

 Furuta, 1999; Manderson et al„ 1999). This 

 assumption is supported by research with 

 the congener Japanese flounder (Paralich- 

 thys olivaceus). Although a range of sizes 

 of hatchery-reared Japanese flounder may 

 survive within relatively shallow nursery 

 habitats, fishes less than 90 mm TL moving 

 into relatively deep waters are poorly rep- 

 resented in subsequent age classes, most 

 likely due to predation-induced mortality 

 (Yamashita et al., 1994; Furuta, 1999). 

 There is no relationship between length 

 of rearing period (time spent in the 

 hatchery environment) and probability of 

 postrelease mortality related to behavioral 

 deficits (Olla et al., 1998). Hatchery-specific 

 selection pressures may result in HR fish 

 that are behaviorally selected to survive in 

 the hatchery and not in the wild (see Olla 

 et al., 1998; Kellison et al., 2000; for discus- 

 sion). We assume that behavioral deficits 

 are not exacerbated with time spent in the 

 hatchery (i.e. behavioral deficits are equal 

 for all sizes-at-release). 



Results 



The most important factor affecting the 

 number of survivors (and therefore percent 

 survival) was size-at-release because the 

 greatest numbers and percentages of survi- 

 vors were always produced by releasing the 

 largest fish possible (80 mm TL in the model). 

 Number of survivors decreased with decreas- 

 ing size-at-release and with increasing Julian 

 day of release (Fig. 3A). The cost-per-survivor 

 ( CPS ) was also most affected by size-at-release, 

 such that CPS decreased with increasing size- 

 at-release (Fig. 3B). CPS generally increased 

 with increasing Julian day of release (Fig. 3B), although 

 this effect was less important than the effect of size-at- 

 release. Because mortality was originally assumed to be 

 density-independent, the optimal cost-per-survivor did 

 not vary with the number offish released (Fig. 4), and the 

 relationship between number offish released and number 

 of survivors was linear (Fig. 4), such that the maximum 

 number of survivors were generated from the greatest 

 number offish released (NFR=400,000). 



220 



20 80 



90 220 



Figure 3 



Response surfaces of iAi number offish survivors (summer flounder I 

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

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

 under density-independent mortality. CPS values greater than $10 were 

 set equal to $10 for ease of presentation. 



Sensitivity of model predictions to violations of 

 density-independent mortality assumption 



Model results varied considerably under the various den- 

 sity-mortality relationships (Fig. 5, A and B), indicating 

 the importance of knowledge of the relationship between 

 numbers of fish released (density) and mortality in the 

 wild to predicting optimal release scenarios. Variation in 

 model output was dependent on the type and strength of 



