510 



Fishery Bulletin 89(3). 1991 



This is an order of magnitude lower than the visual 

 census estimates of full populations of 460/km 2 and 

 260/km 2 , respectively. There is no basis for estimating 

 accurately what fraction of the standing stock of either 

 species was caught in 1900. However, there is no evi- 

 dence that the fishery was immediately depleted, so it 

 seems likely that the standing stock was several times 

 the annual catch for each species. It seems reasonable 

 that the actual present stock sizes at FFS are not much 

 greater than the figures in Table 6, and they are prob- 

 ably not less than a third of those estimated figures. 



Based on our study, it is clear that populations of 

 these two species combined eat at least a few thousand 

 metric tons annually at this medium size atoll (about 

 500 km 2 in area to a depth of about 20 m). From an 

 ecological perspective, this must represent one of the 

 most important top-level trophic paths in this system 

 (and probably in many others). The estimated combined 

 predation pressure (Table 6) by these two jacks exceeds 

 the combined estimate for the three dominant shark 

 species in the same system (DeCrosta et al. 1984) by 

 a factor of about 40. This source of mortality should 

 be considered in any quantitative examination of the 

 population ecology of the prey groups, particularly fish, 

 large crustaceans, and cephalopods. The diet composi- 

 tion results in Tables 3 and 4 permit such analysis at 

 lower systematic levels as well. 



The information now available about these two im- 

 portant jacks provides an outline of their life history 

 and trophic ecology. Both species are moderately fast 

 growing, attain large size, and live at least several 

 years. Both mature at relatively large size (a major con- 

 sideration in fishery management), have apparent high 

 fecundity, and may reproduce for at least 4-6 years, 

 with a pronounced seasonal spawning cycle. These 

 jacks are highly piscivorous, but show considerable 

 quantitative separation in their specific diets. Caranx 

 melampygus is more dependent on fishes, whereas 

 C. ignobilis has a more varied diet that includes large 

 crustaceans and cephalopods. Both species have high 

 metabolic demands and correspondingly high rates 

 of food consumption. Large populations of these spe- 

 cies may impose considerable predation mortality on 

 benthic/demersal communities, particularly on fishes. 

 These jacks are potentially of interest in many shallow- 

 water tropical environments because of their role in 

 community ecology as well as in local fisheries. 



Acknowledgments 



This research was supported in part by the University 

 of Hawaii Sea Grant College Program as Project 

 NI/R-4 of Institutional Grant Numbers NA79AA-D- 

 00085 and NA81AA-D-00070 from NO A A Office of 



Sea Grant, Department of Commerce. The Ocean 

 Resources Branch, State of Hawaii, Department of 

 Business, Economic Development & Tourism contrib- 

 uted additional support. Major financial and logistic 

 support was provided by the Hawaii Cooperative Fish- 

 ery Research Unit, which is jointly supported by the 

 Hawaii Department of Land and Natural Resources, 

 the University of Hawaii, and the U.S. Fish and Wild- 

 life Service. A number of persons from the Unit helped 

 in the collection of specimens and provided assistance 

 and criticism. Mark A. DeCrosta did much of the ini- 

 tial collecting and dietary analysis of Caranx ignobilis. 

 The Kewalo Research Facility of the National Marine 

 Fisheries Service Honolulu Laboratory provided 

 facilities and expertise to help maintain fish in cap- 

 tivity. James H. Uchiyama helped with ultraviolet 

 photography of otoliths. Skip Naftel and the crew of 

 the RV Easy Rider were very helpful on a number of 

 chartered cruises. Housing and subsistence, support 

 of boat operations, and other logistics were provided 

 (1) at Midway Islands by the U.S. Navy and its con- 

 tractor, Base Services Inc., and (2) at French Frigate 

 Shoals by the U.S. Fish and Wildlife Service and a 

 helpful wildlife refuge staff. The Hawaii Division of 

 Aquatic Resources and the National Marine Fisheries 

 Service provided useful fish catch statistics and size 

 data. William K.H. Kwok performed most of the ex- 

 tensive computer calculations of energy budget terms. 

 James E. Norris, Robert E. Schroeder, Leighton 

 R. Taylor, and Sidney J. Townsley were especially 

 helpful in a variety of ways. Kim N. Holland and James 

 E. Norris reviewed the manuscript and made helpful 

 suggestions. 



Citations 



Agegian, C.R. 



1985 The biogeochemical ecology of Porolithon gardineri 

 (Foslie). Ph.D thesis, Univ. Hawaii, Honolulu, 178 p. 

 Atkinson, M.J., and R.W. Grigg 



1984 Model of a coral reef ecosystem. II. Gross and net ben- 

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 Cobb, J.N. 



1905 The aquatic resources of the Hawaiian Islands. Section 

 III— The commercial fisheries of the Hawaiian Islands. Bull. 

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 DeCrosta, M.A., L.R. Taylor Jr., and J.D. Parrish 



1984 Age determination, growth, and energetics of three 

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 Everhart, W.H., and W.D. Youngs 



1981 Principles of fishery science. Cornell Univ. Press, 

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