Fne(dlander and Ziemann: Impact of hatchery releases on recruitment of Polydactylus sexftlts 



41 



impact of a relatively small number of released fish on 

 the recreational fishery shows that hatchery releases of 

 limited numbers of fish have the potential to impact both 

 the number of fish taken in the fishery and the rate at 

 which the fishery can recover. The differences in contribu- 

 tion rates for different release years suggest that natural 

 factors affecting the survival of juveniles, as well as early 

 larval stages, vary between years. 



Hatchery-reared and released fish collected in the rec- 

 reational fishery showed growth rates, condition factors, 

 and gonadosomal indices similar to those of wild fish, 

 suggesting that hatchery-reared fish are able to adapt 

 to the natural environment and integrate into the wild 

 population. Our data (unpubl.) for wild and hatchery fish 

 collected in nursery habitats showed no significant differ- 

 ences in growth rates. The mean size of hatchery-reared 

 fish collected in 1998 was smaller than in 1999 (over 95% 

 of the fish collected in 1998 and 1999 came from the same 

 releases in 1997). Mean size for hatchery-reared fish in 

 1999 was not different from the mean size of wild fish 

 for both years, which suggests that size of hatchery fish 

 in 1999 represents the approximate size of 2-3 year-old 

 threadfin and mean age of fish in the recreational fish- 

 ery is also 2-3 years. The size-frequency distributions of 

 hatchery and wild fish in 1998 and 1999 suggest that a 

 significant portion of the wild fish in the fishery is younger 

 than two years. 



Small hatchery fish at release made a higher relative 

 contribution to the recreational fishery than did the larger 

 size group (but not significantly so, except for fish taken 

 in Kailua Bay in 1999), and the nursery habitat sampling 

 conducted after the 1997 releases showed the same (Leber 

 et al., 1998; Ziemann et al.'-). This pattern is in contrast 

 to that observed for mullet in Hawaii (Leber, 19951 and 

 Pacific threadfin for other years (Ziemann et al.'^) 



Hatchery fish disperse slowly from the point of release 

 along the windward coast of Oahu. In nursery habitats 

 three months after release (Ziemann et al.-i, hatchery 

 fish represented in excess of 70'7( of the threadfin and 

 they decreased within nine months to 10% or less. Some 

 decrease is due to predation, but some is due to dispersal 

 because in 1998, after 1 year at large, fish were caught in 

 the recreational fishery a mean distance of 11.2 km from 



the release point, and after two years, mean distance had 

 increased to 15.2 km. Dispersal from the two release sites 

 differed: after one year mean distance for Kahana Bay 

 releases was 14.6 km, whereas mean distance for fish 

 releases in Kailua Bay was 9.6 km. 



The 1999 reward sample contained 16% females, 44% 

 males and hermaphrodites, and 40% immature fish. The 

 life cycle of Pacific threadfin (protandric hermaphrodites) 

 makes this skewed sex ratio even more problematic be- 

 cause individuals do not become functional females until 

 about 30 cm FL and these larger fish are selectively re- 

 moved from the population by fishing. For protogynous 

 species, size-selective fishing mortality may result in 

 differential loss of larger males (Sadovy, 1996; Beets and 

 Friedlander, 1999). The percentage of juveniles in the 

 catch was high. Mean size of Pacific threadfin in all sexual 

 categories was significantly smaller than that reported by 

 Kanayama* in 1962-68 (Table 7, Fig. 6); further, females 

 constituted 23.5% of the catch in the 1960s, but only 16.2% 

 of the catch in 1999. We demonstrated that cultured Pa- 

 cific threadfin juveniles released in known nursery habi- 

 tats survive and recruit successfully into the recreational 

 fishery 1-2 years later. Our Pacific threadfin data indicate 

 that recruitment of young fish to the population may be 

 jeopardized because there are few mature females left 

 in the population (recruitment overfishing), even with 

 supplementation of hatchery-reared fish. 



The underlying problem of the threadfin fishery on Oa- 

 hu and the other Hawaiian Islands is primarily an intense 

 local harvest by subsistence and recreational fishermen, 

 as well as habitat loss from coastal and upland develop- 

 ment. Current state regulations, as well as unregulated 

 removal of larger individuals from the population, contrib- 

 ute to the male-biased sex ratios observed in our study. 

 Stock recovery based on natural reproduction will be a 

 long-term process. Implementation of an enhancement 

 progi-am for Pacific threadfin focused on juveniles and 

 perhaps larger females could speed the rate of recovery of 

 the local population. 



Kanayama, R. 1973. Life history aspects of the moi Po/vrfac- 

 tylus sexfilix in Hawaii, 50 p. State of Hawaii, Department of 

 Lands and Natural Resources, Honolulu, Hawaii. 



