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Role of Cephalopods in the Diet of the Swordfish, X/Ph/As Glad/Us, from the Eastern Mediterranean Sea

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Role of Cephalopods in the Diet of the Swordfish, X/Ph/As Glad/Us, from the Eastern Mediterranean Sea BULLETIN OF MARINE SCIENCE, 49(1-2): 312-324, 1991 ROLE OF CEPHALOPODS IN THE DIET OF THE SWORDFISH, X/PH/AS GLAD/US, FROM THE EASTERN MEDITERRANEAN SEA Giambattista Bello ABSTRACT The gastric contents of 38 swordfish, Xiphias gladius, from the eastern Mediterranean Sea were examined to study the cephalopod component. Cephalopods were the most abundant prey items (their coefficient of prey frequency = 85.7%). Remains of 20 I cephalopod spec- imens were found, including loose hard parts, e.g., beaks, gladii, and lenses. Cephalopods belonged to eleven species, all pelagic. Todarodes sagittatus was the predominant food item. Next most abundant cephalopods were Ancistroteuthis /ichtensteinii and Heteroteuthis dispar. All other cephalopod species were ingested occasionally by swordfish. Based on the analyses of the loose beaks mantle length estimations were made and histograms oflower rostral length distribution were generated for T. sagittatus and A. /ichtensteinii. Swordfish are efficient collectors of otherwise rare pelagic animals. Based on the analysis of gastric contents, four cephalopods were recorded for the first time from the area: H. dispar, Onychoteuthis banksii, Histioteuthis bonnellii, Ancistrocheirus lesueurii. Some reflections were made about the local swordfish fishery and the exploitability of pelagic cephalopod stocks. Swordfish, Xiphic.rs gladius Linnaeus, 1758 (Osteichthyes: Xiphiidae), a cos- mopolitan teleost, is an opportunistic predator which mainly feeds on pelagic species of both vertebrates and invertebrates; the taxon mostly preyed upon is Cephalopoda (see historical resume and results in Toll and Hess, 1981). As noted by Toll and Hess (1981), however, most authors fail to give a complete list of teuthological prey items found in the gastric content of swordfish. Although much work has been done on several aspects of the natural history and fishery biology of X. gladius, a commercially important fish, only one brief note reports a detailed list offood items ingested by swordfish from the eastern Mediterranean Sea (Bello, 1985). This, however, was limited to the examination of prey specimens with soft tissue remains and neglected loose hard parts (gladii, lenses, and beaks). In the present work examination was conducted on the gastric contents of 38 swordfish collected in the Apulian seas (Italy, eastern Mediterranean); they include 16 specimens whose partial analysis was previously reported in Bello (1985). Attention was focused on cephalopods, which represent the most abundant and diverse prey. All types of cephalopod remains were taken into account, including hard parts. The study of swordfish gastric content proved useful in gathering information on the ecology and fishery biology of both swordfish and cephalopods. Additions were made to the knowledge of pelagic cephalopod distribution in the eastern Mediterranean Sea, which is still poorly known (Mangold and Boletzky, 1988). MATERIALS AND METHODS The gastric content analysis was conducted on 38 specimens of Xiphias gladius collected by drifting longiine in the south Adriatic Sea, north Ionian Sea, and the Gulf of Taranto (inlet of the Ionian) (Fig. I), during the 1984 to 1986 swordfish fishery seasons. Swordfish ranged in length from 0.80 to 1.75 m (lower jaw to fork). Table I summarizes the swordfish capture data. The fishing season extends from late Spring (May-June) to early Autumn (October-November), with exact dates dependent on swordfish migration and prevailing meteorological conditions. The bottom depth in the fishing zone usually ranges from 500 m to the maximum depth in the Adriatic 312 BELLO: CEPHALOPODS IN THE DIET OF MEDITERRANEAN SWORDFISH 313 .. •'0 N t 't::i Figure I. South Adriatic and north Ionian Sea. The fishing area is shaded. (1,223 m) and over 1,700 m in the Ionian Sea and the Gulf of Taranto. Fishing occurs at night; the longline is paid off at evening and retrieved in the morning. Hooks are usually baited with mackerel. Whole swordfish stomachs were preserved at sea in plastic jars containing 10% formalin. In the laboratory the stomachs were cut open and their contents sorted according to taxon. As noted by Hess and Toll(l98l) and Tolland Hess(l98l), cephalopods found in predator stomachs easily lose those taxonomic characteristics usually adopted for identification purposes. Because of this it became necessary to look for other characters for identification, e.g., radula and mandibles. Indeed several cephalopod specimens were represented solely by buccal masses and loose beaks. Beak iden- 314 BULLETIN OF MARINE SCIENCE, VOL. 49, NO. 1-2, 1991 Table I. Swordfish collection data (an asterisk marks specimens with empty stomachs) Fishing zone Harbor Swordfish no. Dale landed South Adriatic Mola di Bari Al to A3 24 July 1984 South Adriatic Savelletri A4 and A5 29 July 1985 South Adriatic Monopoli A6 25 Sept. 1986 North Ionian Leuca 11 31 July 1984 North Ionian Leuca 12 7 August 1984 North Ionian Leuca I3 and 14 28 August 1984 North Ionian Leuca 15 to 18 4 Sept. 1984 North Ionian Leuca 19(*) 7 August 1984 Gulf of Taranto Porto Cesareo GI to GI5 Ist half of September 1984 Gulf of Taranto Porto Cesareo GI6 to GI8 26 May 1985 Gulf of Taranto Porto Cesareo GI9 and G20 28 May 1985 Gulf of Taranto Porto Cesareo G21 14 August 1985 Gulf of Taranto Porto Cesareo G22* and G23* 26 May 1985 tification was performed using available guides including Naef (1923), Clarke (1962, 1986), Mangold and Fioroni {I966), and Voss (\ 969) and by comparisons with examples extracted from whole spec- imens of known identity. Only Cranchiid beaks were not checked against identified material. Cephalopod remains were measured. Dorsal mantle length (ML) or ventral mantle length (VML) was taken whenever possible, otherwise beak dimensions were measured as suggested by Clarke (1962, 1986). These measurements included lower or upper rostral length (respectively LRL and URL) in teuthoid, lower hood length (LHL) in octopod beaks. Size frequency histograms were made for the lower rostral lengths of Todarodes sagittatus and Ancistroteuthis Iichtensteinii. Dorsal mantle lengths were estimated (EML) from beaks by reference to regression equations reported by Clarke (1986). In taxa lacking such equations a rough estimate was made by simple comparisons with beaks extracted from specimens of known mantle length. RESULTS Most swordfish stomachs contained some food remains; only three of them (7.9%) were empty. The majority of prey remains consisted primarily of cepha- lopods and secondarily ofteleosts. In addition, one decapod crustacean chela and some small gelatinous organisms Gellyfish?) were also found. The coefficients of prey frequency are reported in Table 2. The preponderance of cephalopodan prey in the swordfish stomachs is even more obvious when the prey dimension is taken into account. Most fish were small, about 90% of them ranging from 3 to 8 cm in length. The longest teleost was 35 cm long, whereas several ommastrephid squids were longer than that (see results below); the estimated TL for the largest cephalopod, a specimen of Om- mastrephes bartramii, was 60 cm. Teleostan prey volume exceeded that of ceph- alopods in only five stomachs. Cephalopods remains were attributed to 11 species, all pelagic. The taxonomic list is reported in Table 3. Table 4 summarizes the distribution of prey in swordfish stomachs. The coef- ficient of prey-cephalopod frequency and the coefficient of cephalopod numerical frequency are reported in Table 5. Heteroteuthis dispar. -- This is the only sepioid and the smallest cephalopod found. Its remains consisted of hard parts only, beaks and possibly lenses, with the exception of a single buccal mass. Ommastrephes bartramii.-According to Bello (1986) this is the only species of Ommastrephes in the Mediterranean Sea. Remains of two specimens of the species were found. First specimen in swordfish no. G6. Slightly digested mantle (ML ~ 10.5 + cm) BELLO: CEPHALOPODS IN THE DIET OF MEDITERRANEAN SWORDFISH 315 Table 2. Coefficients of prey frequency in swordfish stomach, Fp. Fp = nlNp x 100, where N = number of stomachs containing prey of each taxonomic group; Np = number of stomachs containing prey All specimens Adriatic Sea Ionian Sea Gulf of Taranto Cephalopods 85.7 83.3 87.5 85.7 Teleosts 62.9 33.3 37.5 81.0 Crustaceans 2.9 4.8 Unidentified gelatinous organisms 2.9 4.8 Np 35 6 8 21 with its internal organ and buccal mass. LRL = 2.8 mm, to which an EML of 13.0 cm corresponds. The ratio ilj = 1.12 is lower than the one for T. sagittatus juv. (Clarke, 1986; personal observations). Identification was confirmed by the examination of the radula (cf. Naef, 1923). Second specimen in swordfish no. G9. Pair of large loose beaks. LRL = 12.3 mm; EML = 39.2 cm; ilj = 1.15. Todarodes sagittatus. -As reported by Bello (1985), this is the predominant food item in the diet of swordfish from the eastern Mediterranean Sea (tables 4, 5). Both adult and juvenile squid remains were recovered from the swordfish stom- achs. Several loose ommastrephid beaks were tentatively referred to this species because of the darkening pattern of lower beak wings (Clarke, 1986), the shape of the upper rostrum (Mangold and Fioroni, 1966), and the ratio i/j > 1 (Clarke, 1986; personal observations). LRLs ranged from 1.8 to 8.4 mm, to which EMLs ranging from 6.3 to 33.6 cm correspond. Figure 2 gives the lower beak size distribution, which has three modes. The graphs in Figure 3 were generated by splitting the beaks according to their wing darkening stages. At stage B in particular, i.e., isolated spot on the wing, there are two completely separate beak groups (smaller beaks: mean LRL = 4.54 mm, standard deviation = 0.33, EML = 17.7 em; larger beaks: mean LRL = 6.13 mm, standard deviation = 0.62, EML = 24.2 cm). Clarke (1962) shows that the extension of pigmentation to lower beak wing is related to sexual maturity and that wings darken at a smaller size in males than in females.
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