Kellison and Eggleston: Modeling release scenarios for Paralichthys dentatus 



87 



surprising. Larger fish spend fewer days than smaller fish 

 in the wild nursery habitats before making an ontogenetic 

 habitat shift to deeper waters and thus are susceptible to 

 daily natural mortality for fewer numbers of days than are 

 smaller fish. Thus, total mortality of smaller fish is greater 

 than that of larger fish. Additionally, although we chose to 

 make mortality independent of size in the model, abundant 

 literature suggests that natural mortality (especially due 

 to predation ) may decrease with increasing size by mecha- 

 nisms such as enhanced resistance to starvation, decreased 

 vulnerability to predators, and better tolerance of environ- 

 mental extremes (Sogard, 1997; Hurst and Conover, 1998; 

 Lorenzen, 2000). Thus, the difference in predicted survival 

 between 1 ) relatively large and relatively small fish and 2 ) 

 fish released early versus late in the season in our model 

 would be even greater if larger summer flounder suffered 

 lower natural mortality than smaller fish. Furthermore, 

 the daily mortality estimate used in the density-inde- 

 pendent simulations and to parameterize the different 

 types of density-mortality relationships may have been 

 an underestimate of daily mortality (Kellison, 2000). If a 

 greater estimate of daily mortality had been used, the dif- 

 ference in predicted survival between relatively large and 

 relatively small fish in our model would have been further 

 exacerbated because smaller fish spend longer amounts of 

 time in the model growing to the 80-mm-TL ontogenetic 

 shift size. These conclusions are supported by empirical 

 research demonstrating that relatively large released HR 

 fish suffer lower mortality than relatively small HR fish 

 released in the field (e.g. Yamashita et al., 1994; Leber, 

 1995; Willis et al., 1995; Tominaga and Watanabe, 1998; 

 Svasandetal.,2000). 



Although the survival predictions of the model (total 

 mortality decreases with increasing size-at-release) are 

 not surprising, the economic (cost-per-survivor) predic- 

 tions were unexpected. The paradigm for stock enhance- 

 ment strategy is that the rearing of relatively large fish 

 for release is cost prohibitive, so that mass releases of 

 relatively small, inexpensive-to-rear fish are a better 

 strategy than the release of larger, expensive-to-rear fish 

 (Kellison, personal obs.). Thus, relatively small juveniles 

 are released in virtually all current stock enhancement 

 programs (e.g. Russell and Rimmer, 1997; Masuda and 

 Tsukamoto, 1998; McEachron et al., 1998; Svasand, 1998; 

 Serafy et al., 1999). Nevertheless, large-scale hatcheries 

 and grow-out facilities are using ever-increasing technol- 

 ogy to minimize the costs associated with the production 

 of relatively large fishes (Sproul and Tominaga, 1992). 

 Thus, for species for which 1) hatcheries are capable of 

 producing relatively large fish at relatively low costs (as 

 is likely for summer flounder), and 2) postrelease survival 

 rates increase with release size, release scenarios utilizing 

 the largest fish possible may maximize the potential (i.e. 

 produce maximum survival at minimum costs ) of stock en- 

 hancement efforts. In these cases, the "small fish maximize 

 stock enhancement potential" paradigm might be replaced 

 with a "large fish maximize potential" paradigm. As a ca- 

 veat, this "large fish" strategy may be limited by spatial 

 limitations of hatcheries in producing large numbers of 

 relatively large fish. Because reared fish generally must 



Type 2 to 3 

 Type 3 lo 2 



20-1— -! , 1 r 



Postrelease density 



Figure 8 



Optimal lA) economic cost-per-survivor and (B) per- 

 cent survival of released hatchery-reared summer 

 flounder under temporally shifting functional re- 

 sponses of type 2 to type 3 and type 3 to type 2. 



be kept below critical densities in hatchery environments 

 because of water quality and fish interaction issues (e.g. 

 cannibalism), larger fish necessarily require more space 

 than smaller fish for rearing. If the demand for space to 

 rear large quantities of large fish can be realized, then the 

 model simulations indicate that stock enhancement strat- 

 egies in which size-at-release is maximized will produce 

 the maximum number of survivors. 



Although not as important as size-at-release, Julian day 

 of release had a significant effect on survival and cost-per- 

 survivor in the model, such that enhancement efforts were 

 always more successful (more survivors, lower costs) when 

 fish were released at the earliest Julian day possible. These 

 results occurred because growth in the model decreased 

 with increasing Julian Day. Although the mechanisms un- 

 derlying this decrease in growth with increasing Julian day 

 are unknown, they may be related to decreased prey avail- 

 ability or metabolic efficiency as temperatures increase 

 with increasing Julian day (Malloy and Targett, 1994a, 

 1994b; Fujii and Noguchi, 1996; Howson, 2000). Thus, for 

 a given size-at-release, fish released earlier in the season 

 experienced greater growth rates than fish of the same 

 size-at-release released later in the season and therefore 

 reached the 80-mm-TL ontogenetic shift size faster (over a 

 period of fewer days) than fish released later in the season. 

 Thus, fish released earlier in the season were susceptible 

 to natural mortality for fewer days than fish released later 

 in the season and therefore suffered lower total mortality. 

 These results emphasize the importance of knowledge of 

 possible time-dependent growth in the field prior to stock 

 enhancement efforts. 



