Leber et al Marine stock-enhancement potential in nursery habitats of Mugil cephalus 



465 



and survival of cultured fish in the wild, each focused 

 on different life stages: 



Level 1 Initial recapture rate: A key issue is 

 whether a release-recapture design is ad- 

 equate to evaluate stocking effectiveness. 

 Initial sampling of released fish after a pi- 

 lot release (e.g. during the first couple of 

 months) will establish whether monitoring 

 is feasible under the chosen release strat- 

 egies (e.g. release habitat, fish size at re- 

 lease) and with the condition of released 

 fish and sampling design. An early look at 

 recapture rates reveals a maximum ex- 

 pected recapture rate for a particular re- 

 lease (which will likely decrease over time 

 because of mortality and dispersal ) and can 

 help determine whether release strategy or 

 sampling strategy needs to be redesigned. 



Level 2 Growth, survival and impact on abun- 

 dance through the nursery stage of the 

 life cycle: Assessing comparative effective- 

 ness of release strategies in increasing ju- 

 venile recruitment can provide an early 

 indication of enhancement potential. Two 

 corollaries of the enhancement concept 

 should be considered here: the first — cul- 

 tured fish survive, grow, and contribute to 

 population size — cannot be evaluated 

 meaningfully without information on how 

 chosen release strategies affect postrelease 

 survival; the second corollary — cultured 

 fish do not displace wild stocks — can be 

 evaluated experimentally, once dispersal 

 patterns are understood (Leber et al., 

 1995). 



Level 3 Growth, survival, and release impact 

 through asymptotic growth: involves as- 

 sessment of release impact on adult popu- 

 lation size and fishery landings. Recapture 

 rates and growth can also be modeled to 

 evaluate enhancement potential (Polovina, 

 1990, 1991). Results gained at level 2 on 

 release-strategy impacts on survival may 

 need to be confirmed at this level (e.g. a 

 collecting gear bias favoring smaller fish 

 can mask a size-at-release impact on sur- 

 vival at level 2; Leber et al., in press; Leber 

 and Arce, in press). 



Level 4 Impact on reproduction and recruitment 

 in subsequent generations: With genetic 

 markers to track hatchery impact across 

 generations, an assessment can be made 

 of hatchery impact on production of the 

 next generation (J0rstad et al., 1994a). 



Criteria for success need to be specified as test- 

 able hypotheses in enhancement programs (Larkin, 

 1979; Peterman, 1991; Blankenship and Leber, 1995). 

 Percent increase in fish population size needed for 

 success will depend on fish species and enhancement 

 objectives. Cost-benefit evaluations can help deter- 

 mine yields required to break even. But value can be 

 subjective and difficult to quantify when the objec- 

 tive is to enhance a recreational fishery or a threat- 

 ened or endangered species. We evaluated success 

 in terms of impact on recruitment and improvement 

 in recapture rate in this study compared with our 

 earlier studies. Our data reveal how information from 

 pilot studies can be used to identify effective release 

 strategies. Break-even costs for striped mullet enhance- 

 ment are considered elsewhere (Leber and Cantrell 5 ). 



Enhancement concept with striped mullet In this 

 paper, we address level 2 above and reveal a sub- 

 stantial hatchery contribution in nursery habitats 

 following releases of cultured mullet. Results of this 

 study corroborate the first corollary of the marine 

 stock enhancement concept, that released fish can 

 survive, grow, and contribute to recruitment. Re- 

 leased juveniles integrated with wild mullet at pri- 

 mary nursery habitats in Kaneohe Bay. Cultured 

 mullet were abundant in samples on every collec- 

 tion date over the 11 months. Cultured fish showed 

 linear growth; those released in May 1992 doubled 

 in size within 48 weeks, with growth rates similar to 

 wild striped mullet (Leber et al., in press). The sec- 

 ond corollary, that cultured mullet are not displac- 

 ing wild mullet at the Kahaluu Stream release site, 

 was experimentally evaluated and corroborated in a 

 follow-up field experiment (Leber et al., 1995). 



Hatchery effect on abundances in nursery habi- 

 tats was remarkable after adjusting release strat- 

 egy to incorporate findings from pilot releases in 

 Kaneohe Bay. Except for anadromous fishes, there 

 are very few examples where hatchery releases have 

 revealed the potential to double juvenile recruitment 

 success with a marine organism (e.g. Kristiansen and 

 Svasand, 1990, for cod; Kitada et al., 1992, for floun- 

 der; Honma, 1993, for scallops). Cultured fish re- 

 leased in this study increased recruitment of juve- 

 nile striped mullet at the release site in 1992 by at 

 least 100%. This large effect was partly a function of 

 a poor recruitment year for wild fish and was partly 

 due to higher survival following summer releases, 

 compared with survival in earlier studies (Leber, 

 1995; Leber et al. 3 ). Release impact on abundance 



5 Leber, K. M., and R. N. Cantrell. 1996. Effect offish size-at- 

 release on the relative cost to enhance striped mullet in 

 Hawaii. In review. 



