Xiphias Gladius Stomach Content Analysis Is an in Zones a and B, Fish Were Stored Linnaeus, 1758, Is a Mesopelagic Important Tool in Ecological and in Ice
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The diet of the swordfish tained from a longline vessel between 1 and 15th March 1991 by using Xiphias gbdius Linnaeus, mackerel, Scomber spp., as bait. 1 758, in the central east Atlantic, Zone C Gulf of Guinea (3O09'- 0°36'N and 26O27'-4O17'W). Fifteen with emphasis on the role of swordfish (standard length [SL] between 140-209 cm) were selected cephalopods on the basis of the presence of cephalopods in their stomachs. Vicente Hernández-García These were taken from catches Dpto de Biología, Facultad de Ciencias del Mar made by a longliner between May Univ. de Las Palmas de G. Canaria and July of 1991 by using mackerel C P 350 1 7, Canary Islands, Spain and squid, Illex sp., as bait. Stomach preservation and analysis The swordfish Xiphias gladius Stomach content analysis is an In zones A and B, fish were stored Linnaeus, 1758, is a mesopelagic important tool in ecological and in ice. After landing, fish were mea- teleost with a cosmopolitan distri- fisheries biology studies. Oceanic sured from the tip of the lower jaw bution between 45"N anci 45"s iati- vertebrates are often more efficient to the fork of the taii (S¿). During tude. It is an opportunistic preda- collectors of cephalopods than any commercial operations the internal tor feeding mainly on pelagic ver- available sampling gear (Bouxin organs were removed before the tebrates and invertebrates (Palko and Legendre, 1936; Clarke, 1966). fish were weighed. The stomach et al., 1981). The diet. of the sword- The p'irpose nf thir st11dy was to rnntents nf earh fish, incli~dingal1 fish has been studied mainly in the expand knowledge of the diet of the hard parts found in the stomach western Atlantic Ocean (Tibbo et swordfish from the central east At- wall folds (otoliths, very small al., 1961; Scott and Tibbo, 1968, lantic Ocean, with special emphasis beaks, and lenses), were weighed 1974; Toll and Hess, 1981; Stillwell on the role of cephalopod species. (in grams) and preserved in 70% and Kohler, 1985). Earlier reports ethyl alcohol. Otoliths were also reflected the importance of fish in removed from the fish prey. Nema- the swordfish diet, but recently Material and methods todes were found in some stomachs Stillwell and Kohler (1985) cited in small quantities, but these were squid as the predominant compo- Sampling areas and capture assumed to be parasites and were nent in the diet of swordfish. methods not considered prey. In general, Azevedo (1989) and Moreira (1990), stomach contents showed an ad- reporting from off the Portuguese The stomach contents of 75 sword- vanced leve1 of digestion. A stomach coast, mentioned fish (principally fish, Xiphias gladius, were ana- fullness index (SF)was calculated as M~icromesistiuspoutssou)and cepha- lyzed. Specimens were caught at SF = (wet weight of'the stomach con- lopods as the main prey groups of night in three different areas of the tent / wet weight of the swordfish swordfish. This finding was cor- central east Atlantic Ocean (Fig. 1) without internal organs) x 100. roborated by Guerra et al. (1993) from the commercial landings of the In zone C only cephalopods were frnm the N~rtheasternAtlantic Ypanirh fleet: preserved fr~zefi.Fish WPE frnzm where cephalopods were found to immediately after capture without be the most important component Zone A Strait of Gibraltar internal organs. This material was of the diet. Maksimov (1969) stud- (35O53'-35O42'N and 6O35'-6"30'W). not included in comparative analy- ied the diet of swordfish in the east- Thirty five swordfish were caught ses because the main objective was ern tropical Atlantic Ocean. Cepha- with drift nets (2.5 miles long x 20 to record the relative proportions of lopods were a major component of m deep) between 10 and 22 Septem- cephalopod species that were preyed the diet in al1 areas sampled and ber 1990. upon in this zone. although no cephalopod species Scomber spp. and Illex sp. were were identified, the genus Omma- Zone B South of the Canary Is- not considered prey from zones B strephes was present among the five lands (23O-26ON and 17O-22OW). Manuscript accepted 17 November 1994 genera found in stomach contents. Twenty-five swordfish were ob- Fishery Bulletin 93:403-411 (1995). 404 Fisherv Bulletin 93121. 1995 the hood length (LHL)(defined in Clarke, 1962,1980) were measured by digital caliper or steroscopic mi- croscope and eyepiece micrometer. Mantle lengths (ML in mm) and weights (W in g) of the cephalopods from which lower beaks came were then estimated from LRL's or LHL's by using relationships published elsewere (Clarke 1962,1980,1986a;Pérez-Gándaras, 1983;Wolff, 1982 (cited by Clarke, 1986a),Wolff and Wormuth, 1979). Despite the advanced state of digestion of stom- ach contents, an attempt was made to examine the importance of each prey item. Two methods of stom- ach content analysis were used: percent numerical abundance and percent occurrence (Hyslop, 1980).A coefficient of prey numerical frequency, % No. = (Ni1 Ntj x 100, and a coefficient of prey frequency, % oc- currence = (NsilNsfl x 100 were calculated; where Ni is the number of prey of each group i, Nt is the total number of prey, Nsi is the number of stomachs containing each group i, and Nsf is the total number of stomachs with food. An index of prey numerical importance (Castro, 1993) was also obtained as 9% a -N importance = (% No. x 92 oc~urrence)~'~x 100. = Om - - Comparative analysis of degradation of hard m0 structures E Free hard structures (otoliths, beaks, and eye lenses) Zone C -7 - %*e%&+>* % were often found in the stomach contents. To esti- mate the importance of each prey item from the re- fractor~or hard structures, it is important to know Figure 1 the rate of degradation by stomach acids. Locations of the swordfishXiphias gladius fishing grounds Otoliths (sagittae) from four chub mackerel, in the central east Atlantic Ocean where samples were Scomber japonicus (18 to 20 cm SL), similar in size obtained in 1990-91. to those found in the swordfish stomachs, and five otoliths of blue whiting, Micromesistius poutassou, from head-specimens found in swordfish stomachs and C respectively because they were used as bait. were used to determine the rate of otolith degrada- pley items weiee ideiliifled to the Iowesi ~axoiioiiiic A: -.- ~L-I:LL 1 -- LL - L-I--- -.-LL - i -.----L -..:,. ...-.-- LIUII.w LUIILII 1e11gLIIS, Lanell u11 LII~IUII~~S L axis, ~el-e category possible. Fish were identified from otoliths between 3.70 and 4.10 mm for Scomber and between and bones (otolith guides of Harkonen (1986), Hecht 10.81and 13.53 mm for Micromesistius. Beaks from and Hecht (1979), and author's collection). Crusta- eleven Zllex coindetii and two Todarodes sagittatus ceans were classified from externa1 parts of the skel- were used, with LRL's between 3.90 and 6.19 mm eton (Zariquiey-Alvarez, 1968). Lower beaks (LB) and 7.53 and 8.41 mm, respectively. Degradation of were used as the primary means for classification of six eye lenses, three belonging to T sagittatus and cephalopods, and beak identity was established by three belonging to fish species, were also analyzed. methods described by Clarke (1962,1980,1986a) and Hard structures were placed in a hydrochloric acid supplemented by a collection of cephalopod beaks (in- (HC1) solution (pH=l.l;value lower than the range cluding beaks removed from locally caught cephalo- reported by Jobling and Breiby 119861 for fish in pods); upper beaks, other morphological characters which the digestive process had begun), and the tem- (Hess and Toll, 19811, and distributional knowledge perature was maintained between 18 and 20°C. The (Nesis, 1987)were used in some cases as well. Nearly experiment was carried out for 48 hours. Old acid solu- al1 the beaks collected were fresh. Al1 lower beaks tions were replaced with fresh solutions after 24 hours. were identified to the lowest taxon possible and the Cephalopod beak digestion (measured as decreas- lower rostral length (LRL) or, in the order Sepiida, ing upper rostral length, lower rostral length, and NOTE HernAndez-Garcia. Diet of Xph~asglad~us 405 lower wing length) and eye lens digestion (measured nal column (as long as 78.5 cm). It was estimated as decreasing lens diameter) were measured every that otoliths and spines belonged to a total of 476 12 hours. Otolith digestion (measured as decrease of fish. Blue whiting, Micromesistiuspoutassou, was the the longest axis length) was measured at intervals most important prey species by number (37.81%, of 12 hours for blue whiting, although the third in- Table 3). terval was divided into two intervals (measuring each Cephalopods were detected in 17.14%of the stom- 6 hours). Otolith digestion was measured at inter- achs sampled, and the number of specimens was es- vals of 2.5 or 1 hour for the chub mackerel. timated as 26 (Table 2). They were always present in those fish with the highest SF. The Omma- strephidae was the most important family by num- Results ber (35.6%)and Todarodes sagittatus was the pre- dominant species (23.4%). The Histioteuthidae Stomach fullness analysís ranked second in importance (21.45%)and Histio- teuthis bonnellii was the dominant species (11.4%, Data on fish length (SL), weight (W), and stomach Table 4 ). fullness (SF) from zones A and B are given in Table Decapods of the order Natantia were found in the 1. Values of SF in zone A were higher than those in stomach contents of two individuals (Table 2), and a zone B. In the latter area four stomachs were empty large specimen was identified as Acanthephyra whereas in zone A al1 stomachs had contents.