Olson and Galvan Magana: Food habits and consumption rates of Coryphaena hippurus 



295 



The marked differences in food habits with predator size 

 are noteworthy. For all areas combined, the general trend 

 was for increased predation on cephalopods and decreased 

 predation on flyingfishes as the dolphinfish grew larger. 

 The exception was the smallest size group (417-650 mm). 

 These dolphinfish ate 50% cephalopods and 40% flying- 

 fishes by weight. Zavala-Camin ( 1986) also found greater 

 predation on cephalopods by large dolphinfish (>850 mm) 

 than by smaller specimens off Brazil. 



Piscivores are known to feed selectively according to prey 

 body size (Tonn et al., 1992). Maximum prey size is deter- 

 mined by the mouth gape of the predator (Magnuson and 

 Heitz, 1971; Hambright, 1991 ), and minimum prey size was 

 correlated with the gap width between the gill rakers for a 

 variety of tunas, mackerels, and dolphinfishes (Magnuson 

 and Heitz. 1971). Preference for the largest prey a preda- 

 tor can ingest is supported on theoretical grounds (Ivlev, 

 1961; Harper and Blake, 1988), but a sui-vey of studies ex- 

 amining prey-size selectivities of piscivorous fishes showed 

 a consistent pattern of selection for small prey ( Juanes, 

 1994). In our study, the common dolphinfish of all sizes in- 

 gested small prey (Fig. 6A). Ratios of prey size to predator 

 size for piscivores tend to average 0.2-0.3 (Juanes, 1994). 

 In our study, the dolphinfish ingested prey that averaged 

 slightly smaller, 17% of their length. A few dolphinfish ate 

 prey that were greater than the maximum reported for pi- 

 scivores, 50% of their length (Juanes, 1994). 



Consumption rates 



The method w-e employed for estimating daily rates of food 

 consumption was judged by Cortes (1997) to be among 

 the two most appropriate methods for top predators. How- 

 ever, applying gastric evacuation rates derived for yellow- 

 fin tuna to estimate daily rations of common dolphinfish 

 requires justification. Suzuki's (1992) obser\'ations of gut- 

 evacuation times of small dolphinfish do not unequivo- 

 cally justify our assumption that gastric evacuation rates 

 are comparable to those of yellowfin tuna, which were 

 based on food passage through the stomach alone. How- 

 ever, energetics requirements suggest that gastric evac- 

 uation times for dolphinfish, at the same temperature, 

 would be on the order of those for yellowfin. Our hypoth- 

 esis is supported by the similar standard metabolic rates 

 iif common dolphinfish and yellowfin (Benetti et al., 1995; 

 Brill, 1996 ). Brill ( 1996 ) argued that high rates of digestion 

 are consistent for high-performance fishes like tunas, bill- 

 fishes, and dolphinfishes and demonstrated that dolphin- 

 fishes share several characteristics of high-performance 

 physiology with tunas and billfishes. Other, similar-size 

 teleost fishes require about five times as long as yellowfin 

 and skipjack to evacuate a meal (Magnuson, 1969; Olson 

 and Boggs, 1986). Until gastric evacuation rates of dol- 

 phinfish are measured, we are confident that our first- 

 order estimates are adequate approximations of daily 

 rations of common dolphinfish in nature. 



Our ration estimates for common dolphinfish are great- 

 er than those for yellowfin tuna of comparable size, esti- 

 mated by the same method (Olson and Boggs, 1986; Olson 

 and Mullen, 1986). Our estimates are consistent with the 



observation that, although standard metabolic rates and lo- 

 comotion costs are comparable for these species, common 

 dolphinfish have greater growth rates than yellowfin (Uchi- 

 yama et al., 1986; Wild, 1986) and may require more energy 

 for growth. 



In summary, dolphinfish are an important component of 

 the pelagic food web in the EPO, and as such, their feeding 

 ecology provides clues to the underlying ecosystem struc- 

 ture. Clearly, C. hippurus imparts predation pressure on 

 cephalopods, flyingfishes, and other prey that are shared 

 by a suite of predators ( Juhl, 1955; King and Ikehara, 1956; 

 Blunt, 1960; Perrin et al., 1973; Nakamura, 1985; Olson 

 and Boggs, 1986; Robertson and Chivers, 1997; Markaida 

 and Sosa-Nishizaki, 1998). This predation pressure lends 

 support to the hypothesis that cephalopods and flyingfish- 

 es are abundant or have high ratios of production to bio- 

 mass (P/Bi (or both! in the EPO. This hypothesis is based 

 on 1) high consumption rates on these prey, indicated by 

 the stomach contents in our present study, 2) high P/B of 

 dolphinfish (Oxenford, 1999), 3) high consumption rates 

 of cephalopods and flyingfishes by other predators (cited 

 above), and 4) high P/B of those predators (Boggs, 1989; 

 lATTC, 1999). This analysis illustrates the importance of 

 diet studies for providing ecological insights. 



Our study provides key data for implementing ecosys- 

 tem analyses based on food-web models (Christensen and 

 Pauly, 1992; Walters et al., 1997). For example, Ecopath 

 with Ecosim (www.ecopath.org) requires data on both the 

 diet compositions and consumption rates of predators. Ac- 

 cordingly, we summarized the prey data by several levels of 

 taxonomic resolution and functional groups for dolphinfish 

 sampled at multiple spatial scales and size classes. These 

 data help lay the groundwork for a community- and ecosys- 

 tem-level approach to fisheries management in the EPO. 



Acknowledgments 



We thank Richard Rosenblatt and H. J. Walker for access to 

 the fish collection of the Scripps Institution of Oceanogra- 

 phy, Robert Lavenberg and Jeff Siegel for access to the fish 

 and otolith collections of the Natural History Museum of 

 Los Angeles County, and F. G. (Eric) Hochberg of the Santa 

 Barbara Museum of Natural History for reviewing iden- 

 tifications of cephalopod beaks. Robert Pitman (NMFS, 

 Southwest Fisheries Science Center) provided information 

 on flyingfishes, and Edward Brinton (Scripps Institution of 

 Oceanography) identified some crustaceans. George Wat- 

 ters. Inter- American Tropical Tuna Commission (LATTC), 

 helped us with the regression-tree analysis and the graph- 

 ics. We thank the personnel and observers of the LATTC 

 in Colombia, Mexico, Panama, and Venezuela for collecting 

 stomach samples. Julio Martinez (lATTC, Cumana, Ven- 

 ezuela) identified some of the prey. This manuscript was 

 improved by reviews of William Bayliff (LATTC), George 

 Walters (LATTC), and three anonymous reviewers. The 

 second author (FGM) thanks the lATTC and the Institute 

 Politecnico Nacional, Comision de Operacion y Fomento de 

 Actividades Academicas for fellowships to participate in 

 this project. 



