280 



Fishery Bulletin 100(2) 



mon dolphinfish, and their prey in both 

 coastal and oceanic areas of the EPO and to 

 provide prehminary estimates of their daily 

 rates of food consumption. 



Materials and methods 



30 N 



20 



10 







10 



20 



O Floating-object sets 



a Sctiooifisti sets 



X Dolphin sets 



North 



140 



The dolphinfish were caught by tuna purse- 

 seine vessels of Colombian, Mexican, Pan- 

 amanian, and Venezuelan registry from 

 December 1992 through September 1994. 

 The fish were caught as bycatch of the 

 purse-seine fishery for tunas associated 

 with dolphins, with floating objects, and as 

 unassociated schools ("schoolfish"). In dol- 

 phin sets, the net is deployed around aggre- 

 gations of primarily yellowfin tuna and 

 spotted or spinner dolphins {Stenella atten- 

 uata or S. longii-ost/'ig) (or both dolphin spe- 

 cies) after a high-speed chase by speedboats. 

 Floating-object sets are made by encircling 

 flotsam, commonly tree parts and artificial 

 fish-aggi-egating devices (FADs), and associ- 

 ated fauna with the purse seine, usually in 

 the early morning. Schoolfish are detected 

 by seabird activity and disturbance of the water surface 

 caused by the fish swimming just below. The species com- 

 position and size and age distribution of the fauna are 

 distinctly different for the three aggregation types and 

 fishing strategies (Hall, 1998). 



Stomach samples 



Common dolphinfish stomach samples were taken at sea 

 by observers of the Inter-American Tropical Tuna Com- 

 mission (lATTC). The purse-seine sets yielding the dol- 

 phinfish samples were distributed across the geographical 

 range of the EPO tuna fishery at that time (Fig. 1). 

 We obtained samples from 74 purse-seine sets over a 

 22-month period: 61 sets (82'7f) were made on floating 

 objects; 4 sets were made on dolphins; and 9 sets were 

 made on unassociated tuna. On board the vessels, the 

 observers measured the fork length (mm) of each dol- 

 phinfish, determined the sex if possible, and excised and 

 immediately froze the stomachs. In the laboratory, we 

 thawed the stomachs and visually estimated the stomach 

 fullness as a percentage of the stomach capacity. Then, we 

 identified the stomach contents to the lowest taxon pos- 

 sible, weighed them to the nearest gram, and enumerated 

 them when individuals were recognizable. The counts 

 of paired structures, such as cephalopod mandibles and 

 fish otoliths, were divided by two to estimate numbers 

 of prey. We categorized the digestion state of the prey: 

 1 = intact or nearly intact; 2 = soft parts partially 

 digested; 3 = whole or nearly whole skeletons without 

 flesh (or comparable state for nonfish taxa); and 4 = only 

 hard parts remaining (primarily fish otoliths and ceph- 

 alopod mandibles). We measured the length, or maxi- 

 mum dimension of individual prey to the nearest mm. 



West 



130 



120 



Souttiwest 



80 W 



Figure 1 



Locations where common dolphinfi.sh samples were caught by three types of 

 purse-seine sets. We stratified the data into the five areas shown. 



if sufficiently intact. For cephalopods, we recorded the 

 mantle length excluding tentacles. 



Identifying the prey depended on the digestion state of 

 the remains. We used the following keys to identify fish 

 prey in digestion state 1: Jordan and Evermann (1896), 

 Meek and Hildebrand ( 1923), Parin ( 1961 ), Miller and Lea 

 (1972), Thomson et al. ( 1979), Allen and Robertson ( 1994), 

 and Fischer et al. ( 199.5b and 1995c). When the fishes were 

 digested to state 2 or 3 we used taxonomic keys of ver- 

 tebral characteristics (e.g. Clothier, 1950; Monod, 1968; 

 Miller and Jorgenson, 1973) and compared skeletons of 

 whole fishes collected in the EPO. We identified the crus- 

 tacean prey from exoskeleton remains using the keys of 

 Garth and Stephenson (1966), Brusca (1980), and Fischer 

 et al. (1995a). We identified cephalopod prey from man- 

 dible remains (Clarke, 1962; Iverson and Pinkas, 1971; 

 Wolff 1982; Clarke, 1986). The fish collections at Scripps 

 Institution of Oceanography and the Natural History Mu- 

 seum of Los Angeles County, and the cephalopod collection 

 at the Santa Barbara Museum of Natural History were 

 used to compare and validate prey identifications. 



Data analysis 



We analyzed the diet data by calculating three diet indices 

 for each prey taxon. We calculated gravimetric impor- 

 tance of the prey {9fW) as percentages of the total prey 

 weights, numerical importance (9(N) as percentages of 

 total counts, and frequency of occurrence as the number 

 of dolphinfish stomachs that contained a particular prey. 

 We calculated percent occurrence {"tO) as a percentage of 

 all the dolphinfish sampled, regardless of whether their 

 stomachs contained food. We present these three indices 

 by prey taxon in detailed tables, summarized at several 



