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The Role of Cephalopods in the Diet of Swordfish (Xiphias Gladius Linnaeus, 1758) in the Aegean Sea (Eastern Mediterranean)

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The Role of Cephalopods in the Diet of Swordfish (Xiphias Gladius Linnaeus, 1758) in the Aegean Sea (Eastern Mediterranean) BULLETIN OF MARINE SCIENCE, 74(1): 21–29, 2004 THE ROLE OF CEPHALOPODS IN THE DIET OF SWORDFISH (XIPHIAS GLADIUS LINNAEUS, 1758) IN THE AEGEAN SEA (EASTERN MEDITERRANEAN) Alp Salman ABSTRACT In this study, stomach contents of 108 swordfish specimens, Xiphias gladius Lin- naeus, 1758 were examined from the southern Aegean Sea. The taxonomic distribution of major prey categories in stomachs was 81.5% teleosts, 17.8% cephalopods, and 0.7% crustaceans. Teleosts (97%) were the most important prey regarding the frequency of oc- currence values, followed by cephalopods (59.2%), and crustaceans (5.8%). In total, 1230 individuals (1001 teleosts, 220 cephalopods, nine crustaceans) belonging to 34 species (15 teleosts, 18 cephalopods, one crustacean) were identified. The swordfish, Xiphias gladius Linnaeus, 1758 is an opportunistic predator with a cosmopolitan distribution (Toll and Hess, 1981). It feeds mainly on pelagic cephalopods and fishes. The importance of cephalopods in marine food webs was discussed in detail by Amaratunga (1983), and swordfishes are significant among the numerous predators on cephalopods. Opportunistic species play an important role in predator/prey relations in marine ecosystems. Because of the foraging habits of such species, otherwise unrecorded prey species for a given locality can be encountered in their stomachs. Similar situations were reported by Bello (1992, 1993, 1994) from the Adriatic and Mediterranean Seas. Previous studies on stomach contents of swordfish were carried out in the Florida straits (Toll and Hess, 1981), northeastern Atlantic Ocean (Guerra et al., 1993), central-eastern Atlantic waters (Hernandez-Garcia, 1995) and the Mediterranean Sea (Bello, 1991), all of which included a list of prey items. The marine fish (Mater et al., 1989) and cephalopod fauna (D’Onghia et al., 1992; Salman et al., 1997; Salman et al., 2002) of the Aegean Sea and Turkish coasts have been documented in detail; however, observing the relevant fauna from stomach contents of predators provides additional data. In this study, the stomach contents of swordfish from the Aegean Sea were examined with an emphasis on the role of cephalopods in the diet of the species. MATERIAL AND METHODS The stomach contents of 108 swordfish were analyzed. Stomachs examined had gross weights ranging from 3.5–6 kg and were obtained at the wholesale fish market from local fishermen of the southern Aegean Sea. The specimens were caught at night during autumn (1999), winter (1999– 2000), and spring (2000), but none in the summer period due to fishery regulations. The stomach contents were analyzed while fresh and were sorted into three major categories, i.e., cephalopods, teleosts and crustaceans. All fish species were identified according to Whitehead et al. (1984–1986) and Bauchot (1987). The cephalopod species were identified according to Nesis (1982), Mangold and Boletzky (1987), Guerra (1992) and Sweeney and Roper (1998), and the higher classification according to Boletzky (1999). Identification of cephalopod lower beaks obtained from stomachs was based on Mangold and Fioroni (1966) and Clarke (1986) and compared with assembled beaks from the personal collection of the author. For a quantitative description of the diet, two measures, percentage frequency of occurrence (FO%), and percentage composition by number (N%) were Bulletin of Marine Science 21 © 2004 Rosenstiel School of Marine and Atmospheric Science of the University of Miami 22 BULLETIN OF MARINE SCIENCE, VOL. 74, NO. 1, 2004 used. Composition by number represents the proportion of the number of individuals in a particular prey category to the total number of individuals in all prey categories (Windel and Bowen, 1978; Hyslop, 1980). RESULTS Five of the 108 stomachs examined were empty (~ 4.6%). In total, 1230 individuals were identified from the remaining 103 stomachs, comprising 1001 teleosts, 220 cephalopods and nine crustacean specimens. The proportion of teleosts in the swordfish stomachs was 81.5%, followed by cephalopods (17.8%) and crustaceans (0.7%). The frequency of oc- currence of prey items in stomach contents revealed that teleosts were the most numerous group (97%), followed by cephalopods (59.2%) and crustaceans (5.8%; Table 1). A total of 34 species was identified including 15 teleost, 18 cephalopod, and one crustacean species (Table 2); some prey items were not identified to the species level. In addition, some algae and plastic remains (i.e., white pvc and jar parts) were present in stomach contents. In terms of frequency of occurrence, the majority of the prey items were pelagic and nectobentic species, i.e., Sardina pilchardus (49.51%), Engraulis encrasicholus (39.81%), Sepietta oweniana (17.48%) and Todarodes sagittatus (16.50%). However, a few benthic species (Citharus linguatula, Upeneus moluccensis, Octopus macropus, Eledone moschata) also were encountered. Pelagic species were dominant in the diet in all sampling seasons. Sardina pilchardus was the most important prey item in all seasons (Table 2). The cephalopods within the stomach contents were examined separately and the pro- portions of prey were grouped as follows: Sepiida (2.7%), Sepiolida (29.2%), Teuthida (39.8%) and Octopoda (28.3%). Sepietta oweniana was the most abundant species by number; however, T. sagittatus had the highest frequency of occurrence values in all sam- pling seasons. The high number of individuals of Argonauta argo during the spring can be explained by its aggregative reproductive behavior (Table 3). A total of 42 individuals of A. argo encountered in a single swordfish stomach during the late winter is noteworthy. The beak measurements of the identified cephalopod species are given in Table 4. The most important cephalopod beak frequencies are given in Figures 1, 2 and 3. Table 1. Seasonal frequencies of the general food groups in stomach contents of Xiphias gladius (NS: Number of occurrence in stomach contents, % FO: Percentage frequency occurrence). Fall Winter Spring Total NS %SFO N %SFO N %SFO N %FO Teleosts 17 10 0 65 9 7 1 8 94. 7 10 0 9 7 Cephalopods 8 47 4 6 68. 6 7 36.8 6 1 59. 2 Crustaceans 1 5.8 5 7.4 0 0 6 5.8 Empty stomachs 1 2 2 5 Total stomachs 18 69 21 108 SALMAN: CEPHALOPODS IN SWORDFISH DIET 23 , l a i 9 9 8 8 9 8 6 9 9 . d 0 0 1 2 0 1 6 0 0 u % v i d n i f 2 2 5 5 2 2 0 2 2 F o . g 0 0 1 5 4 5 4 55 4 55 9 4 4 r %N e n 11 9 3 2 0 i b r p m S u N NO 2 6 1 6 1 6 1 6 1 6 23 3 3 71 5 6 : I N , h c a 1 1 1 NI m o t s f o 5 3 1 2 1 4 8 1 1 6 3 5 1 2 . r 5 1 2 1 10 2 2 40 4 1 0 0 0 0 0 0 0 0 e %S b m u N 4 8 4 4 9 4 4 9 9 9 4 4 9 4 F : . r 714 8 24 0 9 2 9 2 3 9 4 9 9 424 7 9 9 3 0 7 S %N e 2 5 1 8 1 5 t N ( n i s u W i 1 9 92 5 9 4 8 6 6 32 8 9 2 9 23 9 8 5 7 d NO 1 1 5 8 2 8 4 8 a l g s a i h 5 3 2 2 3 2 3 p NI i 1 7 1 5 X f o 2 8 s t 0 0 0 0 0 0 4 0 0 0 0 . n 1 0 2 11 6 1 112 3 7 2 21 3 1 e t %S 1 2 n o c h c F 8 8 8 8 8 8 6 7 8 8 8 . a 7 1 55 4 5 4 55 4 5 4 8 5 4 5 4 5 8 4 %N l m 1 1 l o a t s F n i 11 8 0 112 8 8 1 8 2 8 12 6 8 8 1 8 0 NO 1 5 d n u o f s NI m . e ) t e i c d n o 3 6 e r o 4 6 1 2 1 2 8 3 1 0 3 7 1 1 1 3 1 2 2 r . f u 0 400 1 0 1 200 1 0 1 0 1 20 1 0 3 0 2 0 1 4 1 00 0 1 l c %S a t c o o t y e c h 8 4 9 9 9 8 7 9 9 9 5 8 9 9 9 8 9 9 9 n t . 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