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



453 



ties provide the technologies required to quantify and 

 evaluate marine stock enhancement ( Jefferts et al., 

 1963; Buckley and Blankenship, 1990; and see 

 Honma, 1993, for examples of growth in marine 

 aquaculture). There is an emerging optimism that 

 carefully implemented marine stock enhancement 

 might be a useful fisheries management tool (e.g. 

 Grimes, 1995). Use of cultured marine organisms to 

 help replenish depleted coastal stocks has finally 

 begun to receive empirical evaluation (Tsukamoto et 

 al. 1989; Svasand and Kristiansen, 1990; Svasand 

 et al., 1990, 1991; Bannister and Howard, 1991; 

 Barlow and Gregg, 1991; Kitada et al., 1992, 1994; 

 Iglesias and Rodriguez-Ojea, 1994; Jorstad et al., 

 1994, a and b; Nordeide et al., 1994; Smedstad et al., 

 1994; Stoner, 1994; St0ttrup et al., 1994; Kent et al., 

 1995; Leber, 1995; Leber et al., 1995; Willis et al., 

 1995). 



The hypothesis that hatchery releases can help 

 increase marine fish populations has at least two 

 corollaries that need to be tested. One is that cul- 

 tured fishes released into coastal waters actually 

 survive, grow, and contribute to recruitment. The 

 other corollary is that cultured fish do indeed increase 

 abundance rather than displace wild stocks. These 

 two postulates are basic assumptions of stock-en- 

 hancement theory, yet both remain largely untested 

 in coastal ecosystems (i.e. with organisms that re- 

 produce in marine environments). The former corol- 

 lary is the focus of this study, the latter is considered 

 elsewhere (Leber et al., 1995). 



A series of workshops was held to select species 

 for immediate stock-enhancement research in Ha- 

 waii. The species given priority was Pacific thread- 

 fin, Polydactylus sexfilis, an inshore carnivore. An 

 inshore herbivore, striped mullet, Mugil cephalus, 

 ranked second in this semi-quantitative selection 

 process ( Leber 1 ). We chose to begin field experiments 

 with striped mullet because it ranked high in the 

 selection process and culture techniques were avail- 

 able. We expected to apply lessons learned about 

 stock enhancement to Pacific threadfin once culture 

 techniques became available. 



To design a rigorous test, data would be needed 

 from pilot releases to define effective release strate- 

 gies. Fish size-at-release (SAR) and the timing of 

 releases were important choices that needed to be 

 made, as Hager and Noble (1976) and Bilton et al. 

 ( 1982 ) had already shown with coho salmon released 

 into streams in the Pacific northwest. Releasing fish 

 into coastal environments would also require care- 



ful consideration of release habitat. If any of these 

 three variables affected survival of released fish, they 

 would also affect the power of any test of stock-en- 

 hancement potential. In Hawaii, a series of pilot re- 

 lease experiments were conducted to identify effects 

 of release magnitude, SAR, release habitat, and re- 

 lease season on survival and contribution of cultured 

 striped mullet to wild stock abundance (Leber, 1995; 

 Oceanic Institute 2 ; Leber etal. 3 ). The results of those 

 pilot releases were used to design the present study, 

 which is to test the first corollary — that cultured fish 

 make a substantial contribution to a marine fish 

 population in Hawaii. 



As pilot release-recapture experiments began in 

 Hawaii, Tsukamoto et al. (1989) published results 

 that indicated that SAR affected survival of red sea 

 bream, Pagrus major, juveniles released into News 

 Bay, Japan. In 1990, Svasand and Kristiansen ( 1990) 

 showed similar results with cod, Gadus morhua, re- 

 leased into Norwegian fjords. In 1990, a similar pat- 

 tern was observed in Hawaii following summer re- 

 leases of about 40,000 tagged striped mullet into each 

 of two embayments on Oahu, Maunalua Bay and 

 Kaneohe Bay (Leber, 1995). Work with striped mul- 

 let revealed that recapture rates approached zero 

 when cultured fish smaller than 60-mm total length 

 (TL) were released in summer or fall months. These 

 results ruled out the alternative of stocking newly 

 hatched fry or postlarvae in an experimental test of 

 the stock-enhancement concept in Hawaii. Pilot re- 

 leases in Hawaii also revealed that survival of cul- 

 tured mullet was strongly affected by release habi- 

 tat and release season (Leber, 1995; Leber and Arce, 

 in press; Leber et al. 3 ). Hatchery release studies with 

 marine fishes in Norway, Florida, and California 

 (Svasand and Kristiansen, 1990; Drawbridge et al., 

 1995; Willis et al., 1995) and with cultured conch 

 released in the Caribbean (Stoner, 1994) have shown 

 substantial effects of release strategies on survival 

 in coastal environments. 



Based on the results of pilot hatchery releases in 

 Kaneohe Bay, an experiment was designed to incor- 

 porate improved release strategies and to evaluate 

 the potential of using hatchery releases to increase 

 significantly juvenile striped mullet recruitment in 

 Kaneohe Bay, the largest estuary in Hawaii. Crite- 

 ria for success were the following: 1) cultured fish 

 released in this study represented a substantial pro- 



1 Leber, K. M. 1994. Prioritizing marine fishes for stock en- 

 hancement in Hawaii. The Oceanic Institute. Honolulu, HI, 

 46 p. 



2 Oceanic Institute. 1990. Stock enhancement of marine fish 

 in the State of Hawaii (SEMFISH, Phase II). Annual report 

 to NMFS, February 1989^Iune 1990, Waimanalo, Hawaii, 106 p. 



3 Leber, K. M., S. M. Arce, H. L. Blankenship. and N. P. 

 Brennan. 1996. Influence of release season on size-depen- 

 dent survival of cultured striped mullet, Mugil cephalus, in a 

 Hawaiian estuary. In review. 



