494 



Fishery Bulletin 89(3). 1991 



179° 



175" 



170° 



165° 



160° 



154" 



29- 



Kure I. 



-x Midway I 



JrPeorl and Hermes Reel 



Laysan I 



NORTHWESTERN 

 HAWAIIAN ISLANDS 



PACIFIC OCEAN 



MAIN 

 HAWAIIAN ISLANDS 



25" 



20° 



18° 



Figure 1 



The Hawaiian archipelago. Nearly all specimens analyzed in this study were collected from the Northwestern Hawaiian Islands. 



ment of their fisheries and for understanding the 

 ecology of the communities in which they occur. 



The uninhabited Northwestern Hawaiian Islands 

 (NWHI) (Fig. 1) offered a particularly appropriate 

 study area because of their size, isolation, and almost 

 pristine biotic communities. Their ecology and natural 

 resources were the subject of a 5-year, multi-agency 

 study (Grigg and Pfund 1980, Grigg and Tanoue 1984), 

 which included major trophic work on shallow-water 

 communities by the Hawaii Cooperative Fishery 

 Research Unit (e.g., DeCrosta et al. 1984, Sorden 1984, 

 Parrish et al. 1985, Norris and Parrish 1988). The 

 present study was done in conjunction with this larger 

 program. Results from these relatively undisturbed 

 carangid populations in the NWHI are also relevant for 

 other populations of large jacks and for management 

 of exploited jack populations. 



Materials and methods 



Collection information 



Specimens of one or both species were collected from 

 French Frigate Shoals (FFS), Maro Reef, Midway 

 Islands, Pearl and Hermes Reef, Lisianski Island, and 

 Necker Island (Fig. 1). Samples were obtained by a 

 wide variety of methods, including handlining, trolling, 



casting, longlining, and spearing by divers using 

 SCUBA. All jacks came from water less than 30 m 

 deep. In atoll waters, they were taken both inside and 

 outside the barrier reefs. Collections were made irreg- 

 ularly between September 1978 and March 1983, in- 

 cluding all months of the year except February (C. 

 melampygus) and February and December (C. ignobi- 

 lis). A little over half of all specimens were taken in 

 fall (September-November). Night collections were 

 logistically difficult and largely unproductive. There- 

 fore, there is a daytime bias in the sampling that may 

 have influenced dietary results. The unpreserved fish 

 were weighed whole, and the standard (SL), fork (FL), 

 and total (TL) lengths were measured to the nearest 

 millimeter. Guts of all specimens were injected with 

 formalin in the field and/or frozen promptly. Data 

 were taken on a total of 154 C. melampygus and 120 

 C. ignobilis. 



Age and growth 



For a selected group of specimens covering a wide 

 range of sizes of both species, age was estimated by 

 counting short-period increments of carbonate deposi- 

 tion in otoliths (Radtke 1987). The fish were decapi- 

 tated along a boundary represented by the edge of the 

 operculum. A sagittal cut was then made through the 

 midline of the head and the cranial cavity exposed. The 



