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Fishery Bulletin 100(2) 



stomachs (or containing only hard parts) ranged from 

 51% to 66% for the southeast, southwest, east, and west 

 areas, respectively. 



Discussion 



Four previous studies of the food habits of common dol- 

 phinfish have been conducted in the EPO. Our results are 

 difficult to compare meaningfully with those because they 

 used different analytical techniques and had lower sample 

 sizes. Larger sample sizes typically demonstrate greater 

 trophic diversity (Manooch et al., 1983). Hida ( 1973) exam- 

 ined the stomachs of seven dolphinfish (two C. hippurus 

 and five C. equiselis) caught at about 4°N-119°W. The 

 largest component of the diet was flyingfishes (33%), fol- 

 lowed by cephalopods (22%). Campos et al. (1993) sam- 

 pled an unspecified number of C. hippurus caught off the 

 Pacific coast of Costa Rica by experimental longline. Our 

 results were similar to theirs in that flyingfishes was the 

 most important component of the diet, and in both stud- 

 ies snake mackerel (Gempylidae) were found in the stom- 

 achs. Our samples from the east area, which encompasses 

 the coastal waters of Costa Rica, contained more prey 

 diversity, including mesopelagic fishes, portunid crabs, and 

 penaeid shruiips. Campos et al. (1993) identified tuna of 

 unknown species in the diet, and we found small amounts 

 of predation on Thunnus spp. (yellowfin or bigeye tuna) 

 in the east and southeast areas. Aguilar-Palomino et al. 

 (1998) reported on the food habits of 500 C. hippurus 

 caught by sport hook-and-line fishing in a small area at 

 the tip of the Baja California peninsula, Mexico. They 

 determined that the cephalopod Dosidicus gigas was the 

 most important component of the diet, followed by the red 

 crab Pleuroncocles pla/iipes, a triggerfish, a flyingfish, and 

 Auxis spp. Their results, however, could not be quantita- 

 tively compared with ours because they presented only the 

 index of relative importance (IRI) (Pinkas et al., 1971), but 

 not its three components (% W, %A', and %0). For example, 

 it is impossible to determine if the apparent importance of 

 Dosidicus gigas in their study was due to the presence of 

 fresh biomass (i.e. high %W). the accumulation of numer- 

 ous, small mandibles from previous meals (i.e. high %A^ 

 and/or %0), or both. Likewise, the IRI indicated that small 

 red crabs were important in the diet, but this index can 

 be overly influenced by numerous small prey. We sampled 

 only four dolphinfish in the north area near the south end 

 of the Baja California peninsula and found some of the 

 same prey taxa reported by Aguilar-Palomino et al. ( 1998). 

 Lasso and Zapata ( 1999) analyzed the stomach contents of 

 228 C. hippurus using nonstandard methods. Their results 

 were presented only by large categories, fishes, mollusks, 

 and crustaceans. 



Feeding periodicity 



Our analysis of diel feeding periodicity suggests that, al- 

 though common dolphinfish may be primarily visual pred- 

 ators (Massuti et al., 1998), they also feed at nighttime. 

 Fish prey, such as flyingfishes, dolphinfishes, wahoo, and 



snake mackerel, would need to be ingested during night- 

 time hours to reach digestion states 3 or 4 before 09:00 

 hours the next morning, as we obsei^ved in our study, unless 

 gastric evacuation rates are much faster than expected. 

 Shcherbachev (1973) concluded from the presence of par- 

 tially digested flyingfishes, myctophid fishes, and squids 

 in the stomachs of C. hippurus. and Crustacea in C. equise- 

 lis that dolphinfishes feed around the clock in the Indian 

 Ocean. Rothschild ( 1964) described active feeding on flying- 

 fishes and myctophids at night by C. hippurus in the cen- 

 tral Pacific. Massuti ot al. (1998) found that almost half of 

 the stomachs of C hippurus sampled at sunrise contained 

 mesopelagic prey. Massuti et al. (1998) and Oxenford and 

 Hunte (1999) also concluded that common dolphinfish feed 

 at night, as well as during the day 



Our data analysis by area revealed an apparent rela- 

 tionship between the principal prey taxa in the diet and 

 feeding periodicity. In the southwest and southeast areas, 

 recently eaten cephalopods were in the stomachs through- 

 out the daytime sampling period, although no samples 

 were taken in the southeast in the late afternoon (Fig. 3). 

 In contrast, in the east area the dolphinfish preyed mostly 

 on fishes (flyingfishes, Ai/.v/s spp., and gempylids) that re- 

 quire more time than cephalopods for gastric evacuation 

 (Olson and Boggs, 1986). In the east area, recently eaten 

 prey were present only in the early morning and early af- 

 ternoon, and the food remains in the late morning and late 

 afternoon were in advanced stages of digestion. These re- 

 sults suggest that foraging activity may have been influ- 

 enced by the digestibility and energy content of the avail- 

 able prey. Cephalopods are typically low in energy content, 

 whereas fishes store lipids in the musculature and viscera 

 and have higher energy densities (Cummins and Wuy- 

 check, 1971). Grove et al. (1978), Flowerdew and Grove 

 (1979), and Jobling (1981) demonstrated that low-energy 

 foods are emptied from the stomach more rapidly than 

 foods of higher caloric content. Elevated lipid content in 

 natural organisms is thought to have a retarding effect 

 on gastric evacuation iFange and Grove, 1979). In yellow- 

 fin tuna, gastric evacuation rates were inversely correlat- 

 ed with total lipid content of four food organisms (Olson 

 and Boggs, 1986). Apparently, the dolphinfish in the south- 

 west and southeast areas spent more time foraging to ful- 

 fill their energy requirements than the dolphinfish in the 

 east area. 



Diet considerations 



Our study indicated that only two prey groups, flyingfishes 

 and epipelagic cephalopods, were dominant in the diet of 

 common dolphinfish in the EPO (Fig. 4). 



Diet differences we attributed to spatial stratification 

 also had a seasonal component. Of the stomach samples 

 obtained north of the equator in the north, west, and east 

 areas, most (85%) of those that contained fresh food, were 

 caught from May through November The trends described 

 for common dolphinfish in the southwest and southeast 

 areas may also have been attributable to seasonality of 

 the prey from December through April because all the dol- 

 phinfish in these areas were caught during these months. 



