334 Abstract.-Identified prey of pan­ Prey occurrence in pantropical tropical spotted dolphins. Stenella attenuata. include 56 species offish and spotted dolphins, Stenella attenuata, 36 species ofcephalopods. Species iden­ tifications were made from fish otoliths from the eastern tropical Pacific and cephalopod beaks recovered from 428 stomachs collected throughout the eastern tropical Pacific between 1989 Kelly M. Robertson* and 1991. The most frequently found fish were lanternfish (family Mycto­ Susan J. Chivers phidaeJ at 40%. and the most frequently Southwest Fisheries Science Center found cephalopods were flying squids National Marine Fisheries Service, NOM (family OmmastrephidaeJ at 65%. The PO. Box 271, La Jolla, California 92038 dominance of these primarily mesope­ .E-mail address: [email protected] lagic prey species and a significantly higher stomach fullness index for stom­ achs collected during the morning hours (X2=112.99, df=6, P<0.0001) sug­ gest that pantropical spotted dolphins feed at night when many mesopelagic Previously published analyses of In this paper, we describe the species migrate toward the surface. Sig­ nificant differences in prey composition the prey ofpantropical spotted dol­ prey ofpantropica1spotted dolphins by season and geographic region indi­ phins in the eastern tropical Pacific collected throughout their range in cate that pantropical spotted dolphins <ETP) have reported that many spe­ the ETP. We calculate the percent are flexible in their diet and may be cies are consumed and that the spe­ number and percent frequency of opportunistic feeders. Comparison of cies composition and importance occurrence for each prey species the diets of pregnant and lactating fe­ male dolphins revealed that lactating varies. Three previous studies re­ identified to quantify the relative females increase both the proportion of ported that epipelagic species are importance ofprey species. Variabil­ squid in their diet and quantity offood the dominant prey and included ity in the diet due to geographic re­ consumed. species belonging to the families gion, oceanographic season, and, for Ommastrephidae (flying squid), females, due to reproductive condi­ Onychoteuthidae <hooked squid). tion are examined. We also present and Exocoetidae (flying fish> (Fitch the size distribution of prey con­ and Brownell, 1968: Perrin et aI., sumed for species for which data 1973; Bernard and Hohn, 1989). were available in order to convert However, mesopelagic species in the otolith and beak measurements to families Myctophidae (lantern fish) prey size. and Enoploteuthidae (enope squid) were also identified in high num­ bers (Fitch and Brownell. 1968; Methods Perrin et aI.. 19731. A more recent study by Roberts <1994) examined Stomachs were collected from 428 only cephalopod prey and found pantropical spotted dolphins in 103 that primarily mesopelagic squid net sets by biological technicians species in the family Ommastrephi­ placed aboard U.S. tuna purse-seine dae were dominant. Another study vessels fishing in the ETP between reported on the prey of a spotted 1989 and 1991 <Fig. 1; see Jefferson dolphin caught offHawaii. The prey etaI.. 1994, for collection procedures). were predominantly mesopelagic Only the contents in the esoph­ species belonging to the families ageal (fore) stomach were examined Myctophidae and Enoploteuthidae for our analyses because this stom­ (Shomura and Hida. 1965). All these ach compartment contains the most studies were based on either a small recent meal and thus the most number ofsamples or samples that identifiable remains <Harrison et were collected from a restricted area al., 1967). The forestomach of each ofthe Pacific Ocean and, therefore, specimen was weighed full and may be limited in their represention empty to the nearest 0.1 g with a Manuscript accepted 6 September 1996. of the prey of pantropical spotted Mettler PC4400 balance. The con­ Fishery Bulletin 95:334-348 (19971. dolphins. tents were sorted and recovered by Robertson and Chivers: Prey of Stenella attenuata 335 150· 140· 130· 120· 110· 100· 90· 80· 70· 30· 30· 20· 20· Western 11=198 .... 10· .. 10° \ ... ... .. .. ... ..- .. , .. ..,'. 0° , 0° .. Southern 11=72 10° 10° 20° 20° 150° 140· 130· 120° 110° 100° 90· 80· 70° Figure 1 Distribution ofnet-sets (n=103) from which 428 pantropical spotted dolphin stom­ achs were collected between 1989 and 1991. For analysis ofgeographic variabil­ ity in prey composition. the sample was divided into areas which correspond to recognized stock boundaries and different oceanographic regions: northeastern, southern. and western. rinsing them through a series of sieves with mesh beaks at the Santa Barbara Museum ofNatural His­ sizes of 12.5 mm, 1.4 mm, and 500 11. Fish otoliths torr and identification keys published byWolff4) and and other skeletal remains, cephalopod beaks, crus­ Clarke (1986a). The relative importance ofprey was taceans, gastropods, and parasitic nematodes were determined by calculating the percent number and collected and enumerated. percent frequency of occurrence (Hyslop, 1980) for Left and right fish otoliths for each species were each individual species and family. separated and counted. The highest count of either was used as the minimum number of fish present Prey size for that species. Fish species were identified to the lowest possible taxQn by using voucher collections of For analysis ofprey size, maximum length ofotoliths otoliths at the LosAngeles County Museum ofNatu­ (tip ofthe rostrum to the posterior margin) and lower ral History CLavenberg1), the Southwest Fisheries rostral length (LRL) of beaks were measured with Science CenterCSWFSC)(Pitman and Carretta2), and an ocular micrometer disc accurate to 0.111, only for otolith identification keys published by Fitch and those items that showed little sign of erosion. Re­ Brownell (1968), Fitch (1969), and Butler (1979). gression equations and ratios of standard length to Frigate mackerel (Auxis thazard) were identified otolith length were used to convert measurements from vertebral characteristics rather than from to prey lengths and weights (Butler, 1979; Clarke, otoliths (Clothier, 1950; Uchida, 1981). For cephalo­ 1986a; Hecht; 1987; Wolff,4 Pitman and Carretta5 ). pods, upper and lower mandibles were separated and Length measurements ofA. thazard were obtained counted for each species; the highest count ofeither by estimating total length from whole fish and skel­ represented the minimum number present in the etons recovered from the stomachs. We used the prey stomach. Species identifications to the lowest pos­ sible taxon were made with the voucher collection of 3 Hochberg. E. 1992. Santa Barbara Museum ofNatural His­ tory. 2559 Puesta del Sol Road. Santa Barbara. CA 93105. 4 Wolff, G. A. 1982, A study of feeding relationships in tuna 1 Lavenberg. R. 1993. Los Angeles County Mus. Natl. History. and porpoise through the application of cephalopod beak Ichthyology Dep., 900 Exposition Blvd.• Los Angeles, CA90007. analysis. Final Tech. Report for DAR-7924779. 231 p. 2 Pitman, R., and J. Carretta. 1993. Southwest Fish. Sci. Cen­ 5 Pitman, R., and J. Carretta. 1993. Southwest Fish. Sci. Cen­ ter. Natl. Mar. Fish. Serv., NOAA. P.O. Box 271, La Jolla, CA ter. Natl. Mar. Fish. Serv.. NOAA. P.O. Box 271, La Jolla, CA 92038. 92038. Unpubl. data. 336 Fishery Bulletin 95(2), J997 size data to test the hypothesis that dolphins of all Wi = initial weight of the forestomach size classes eat prey of the same size. To do this, we with contents (g); and fitted a linear regression to prey size versus the to­ we = weight ofthe forestomach contents tal body length of the dolphins (Norris, 1961). We (g). also tested the null hypothesis of no correlation be­ tween size class of prey consumed and number of Using a X2, we tested the hypothesis ofno difference items consumed by using a Pearson Correlation in the SFI during the course ofthe day (all stomachs Matrix (SYSTAT , 1992; unless otherwise noted, all were collected between 0600 and 1800 h). The data statistical tests are interpreted with a=0.05). were stratified by time-of-day collected: 0600-0900, 0901-1200,1201-1500, and 1501-1800 h and by SFI Geographic and seasonal variability categories: 0-30%, 31-60%, and 61-100% full. To test for variabilityin diet, we stratified the sample Reproductive condition by season and area. For season, we used the two oceanographic seasons characteristic of the ETP: The reproductive condition of female pantropical winter (January-June) and summer (July-Decem­ spotted dolphins was determined by microscopic ex­ ber; Reilly, 1990). For area, we stratified the sample amination ofthe ovaries and macroscopic examina­ by the two recognized management stocks: northeast­ tion ofthe uteri and mammary glands (Perrin et aI., ern and western-southern (Perrin et aI., 1994). How­ 1976; Akin et aI., 1993). Using the mean number of ever, we divided the western-southern stock into a fish and squid consumed by lactating (n=57) and western and a southern section at the equator (Fig. pregnant (n=37) females, we used Student's t-test to 1) because biological differences in pantropical spot­ test the null hypothesis that there was no difference ted dolphins have been noted between the western in consumption of fish and squid between the two and southern sections ofthe western-southern stock groups. We also compared the SFI of lactating and (Perrin et aI., 1976, 1979; Barlow, 1985; Hohn and pregnant females by time-of-day as described in the Hammond, 1985; Myrick,