The diet of the swordfish 



Xiphias gladius Linnaeus, 



1 758, \n the central east Atlantic, 



with emphasis on the role of 



cephalopods 



Vicente Hernandez-Garcia 



Dpto. de Biologia, Facultad de Ciencias del Mar 

 Univ. de Las Palmas de G. Canaria 

 C.R 35017, Canary Islands, Spain 



tained from a longline vessel between 

 1 and 15th March 1991 by using 

 mackerel, Scomber spp., as bait. 



Zone C Gulf of Guinea (3°09- 

 0°36'N and 26 27'-4°17'W). Fifteen 

 swordfish (standard length [SL] 

 between 140-209 cm) were selected 

 on the basis of the presence of 

 cephalopods in their stomachs. 

 These were taken from catches 

 made by a longliner between May 

 and July of 1991 by using mackerel 

 and squid, lllex sp., as bait. 



The swordfish Xiphias gladius 

 Linnaeus, 1758, is a mesopelagic 

 teleost with a cosmopolitan distri- 

 bution between 45°N and 45°S lati- 

 tude. It is an opportunistic preda- 

 tor feeding mainly on pelagic ver- 

 tebrates and invertebrates (Palko 

 et al., 1981). The diet of the sword- 

 fish has been studied mainly in the 

 western Atlantic Ocean (Tibbo et 

 al., 1961; Scott and Tibbo, 1968, 

 1974; Toll and Hess, 1981; Stillwell 

 and Kohler, 1985). Earlier reports 

 reflected the importance of fish in 

 the swordfish diet, but recently 

 Stillwell and Kohler (1985) cited 

 squid as the predominant compo- 

 nent in the diet of swordfish. 

 Azevedo ( 1989) and Moreira ( 1990), 

 reporting from off the Portuguese 

 coast, mentioned fish (principally 

 Micromesistius poutssou ) and cepha- 

 lopods as the main prey groups of 

 swordfish. This finding was cor- 

 roborated by Guerra et al. (1993) 

 from the Northeastern Atlantic 

 where cephalopods were found to 

 be the most important component 

 of the diet. Maksimov (1969) stud- 

 ied the diet of swordfish in the east- 

 ern tropical Atlantic Ocean. Cepha- 

 lopods were a major component of 

 the diet in all areas sampled and 

 although no cephalopod species 

 were identified, the genus Omma- 

 strephes was present among the five 

 genera found in stomach contents. 



Stomach content analysis is an 

 important tool in ecological and 

 fisheries biology studies. Oceanic 

 vertebrates are often more efficient 

 collectors of cephalopods than any 

 available sampling gear (Bouxin 

 and Legendre, 1936; Clarke, 1966). 

 The purpose of this study was to 

 expand knowledge of the diet of the 

 swordfish from the central east At- 

 lantic Ocean, with special emphasis 

 on the role of cephalopod species. 



Material and methods 



Sampling areas and capture 

 methods 



The stomach contents of 75 sword- 

 fish, Xiphias gladius, were ana- 

 lyzed. Specimens were caught at 

 night in three different areas of the 

 central east Atlantic Ocean (Fig. 1) 

 from the commercial landings of the 

 Spanish fleet: 



Zone A Strait of Gibraltar 

 (35 53'-35 42'N and 6°35'-6 o 30'W). 

 Thirty five swordfish were caught 

 with drift nets (2.5 miles long x 20 

 m deep) between 10 and 22 Septem- 

 ber 1990. 



Zone B South of the Canary Is- 

 lands (23°-26°N and 17°-22°W). 

 Twenty-five swordfish were ob- 



Stomach preservation and 

 analysis 



In zones A and B, fish were stored 

 in ice. After landing, fish were mea- 

 sured from the tip of the lower jaw 

 to the fork of the tail (SL). During 

 commercial operations the internal 

 organs were removed before the 

 fish were weighed. The stomach 

 contents of each fish, including all 

 hard parts found in the stomach 

 wall folds (otoliths, very small 

 beaks, and lenses), were weighed 

 (in grams) and preserved in 70% 

 ethyl alcohol. Otoliths were also 

 removed from the fish prey. Nema- 

 todes were found in some stomachs 

 in small quantities, but these were 

 assumed to be parasites and were 

 not considered prey. In general, 

 stomach contents showed an ad- 

 vanced level of digestion. A stomach 

 fullness index (SF) was calculated as 

 SF = (wet weight of the stomach con- 

 tent / wet weight of the swordfish 

 without internal organs) x 100. 



In zone C only cephalopods were 

 preserved frozen. Fish were frozen 

 immediately after capture without 

 internal organs. This material was 

 not included in comparative analy- 

 ses because the main objective was 

 to record the relative proportions of 

 cephalopod species that were preyed 

 upon in this zone. 



Scomber spp. and lllex sp. were 

 not considered prey from zones B 



Manuscript accepted 17 November 1994. 

 Fishery Bulletin 93:403-411 (1995). 



403 



