Abstract. -The large jack spe- 

 cies Caranx igyiobilis and Caranx 

 melampygus were collected from the 

 nearly pristine shallow waters of the 

 atolls, reefs, and shoals of the North- 

 western Hawaiian Islands. Both spe- 

 cies were aged by counting daily 

 otolith increments and validating fre- 

 quency of otolith deposition by mark- 

 ing captive fish with tetracycline. 

 Growth for each species was well 

 described by a von Bertalanffy rela- 

 tionship. Measured growth of captive 

 C. melampygus was also in agree- 

 ment. Gravid fish of both species 

 were found only in April-November; 

 peak spawning season for both was 

 about May-August. Reproduction 

 began at ~350mm SL (~2 years old) 

 in C. melampygus and at ~600mm 

 SL (~3V2 years old) in C. ignobilis. 

 Fecundity of female C. melampygus 

 varied approximately as the 1.7 

 power of body weight. Both species 

 were >90% piscivorous, as judged 

 from combined volume and number 

 of prey and incidence of predation. 

 Crustaceans and cephalopods com- 

 prised several percent of the diet of 

 C. ignobilis; both groups were pres- 

 ent but less important in C. melam- 

 pygus. Both jack species ate gastro- 

 pods in trace amounts. The overall 

 diet of the two species showed mod- 

 erate overlap: "Pianka's" index of 

 overlap, A yz = 0.42. The rate of food 

 consumption for captive C. melam- 

 pygus was used to estimate the 

 respiratory metabolic coefficient, 

 which in turn was used to estimate 

 respiratory demands for all size- 

 classes of both species. The von Ber- 

 talanffy model predicted growth 

 energy, and reproductive energy was 

 estimated from values of the gonado- 

 somatic index. These energy terms 

 were combined to calculate the food 

 consumption required to sustain all 

 size-classes appearing in local catch 

 data. This distribution can be repre- 

 sented by a composite individual for 

 each species consuming a little less 

 than 50 kg/year (C. melampygus) and 

 about 150 kg/year (C. ignobilis). Rough 

 population estimates indicated that 

 at one well-studied atoll, the two 

 species populations combined may 

 eat over 30,000 metric tons of prey 

 per year. These results suggest a 

 quantitatively important trophic role 

 for these top-level carangid predators. 



Manuscript accepted 8 April 1991. 

 Fishery Bulletin, U.S. 89:493-513(1991). 



Life History and Ecology of Large 

 Jacks in Undisturbed, Shallow, 

 Oceanic Communities* 



Anthony E. Sudekum 



Department of Surgery, Stanford University Hospital 

 300 Pasteur Drive, Stanford, California 94305 



James D. Parrish 



Hawaii Cooperative Fishery Research Unit. U.S. Fish and Wildlife Service 

 2538 The Mall. University of Hawaii, Honolulu, Hawaii 96822 



Richard L. Radtke 



Biological Oceanography, School of Ocean and Earth Science and Technology 

 University of Hawaii, Honolulu, Hawaii 96822 



Stephen Ralston 



Tiburon Laboratory, Southwest Fisheries Science Center 



National Marine Fisheries Service, NOAA 



3 1 50 Paradise Drive. Tiburon, California 94920 



Large jacks (family Carangidae) are 

 widely distributed in tropical and sub- 

 tropical waters and are highly prized 

 in local fisheries wherever they oc- 

 cur. Where their populations have 

 not been depleted by fishing, these 

 large, active predators are often 

 abundant, and they are probably im- 

 portant in the ecology of nearshore 

 communities. In particular, Caranx 

 ignobilis (Forskal) and C. melampy- 

 gus (Cuvier and Valenciennes) are 

 large carnivores that occur common- 

 ly throughout much of the tropical 

 Indo-Pacific. These fast, wide-rang- 

 ing species occupy the entire water 

 column, but their habitats appear to 

 be mostly demersal. They seem to 

 maintain important ties with the 

 demersal fish and benthic fauna of 

 shallow coastal areas, islands, atolls, 

 and isolated shoals. 



In Hawaii, these two species are 

 commonly the most abundant of the 

 large, shallow-water, demersal jacks. 



* Sea Grant Publication UNIHI-SEAGRANT- 

 JC-91-27. Ocean Resources Branch Contri- 

 bution 95. SOEST Contribution 2614. 



Large jacks, and these Caranx spe- 

 cies in particular, have long been im- 

 portant food and market fishes in 

 Hawaii (Cobb 1905, Hamamoto 1928, 

 Titcomb 1972). Heavy commercial 

 and recreational fishing pressure 

 has developed, especially in the last 

 several decades (Gosline and Brock 

 1960, Ralston and Polovina 1982, 

 Ralston 1984, Ralston and Kawa- 

 moto 1988, Hawaii DLNR 1989), and 

 stocks in the main, inhabited islands 

 are certainly considerably depressed 

 (Shomura 1987). 



Despite their widespread occur- 

 rence and importance in fisheries, the 

 species biology and ecology of most 

 jacks have been inadequately studied, 

 and little information is available to 

 guide decisions for fishery manage- 

 ment. For species such as C. ignobi- 

 lis and C. melampygus, which com- 

 bine large size, high natural abun- 

 dance, and aggressive feeding habits, 

 the predatory effects on the demer- 

 sal fauna might be expected to be 

 significant. Therefore, study of the 

 species biology and ecology of these 

 jacks is important both for manage- 



493 



