600 



Fishery Bulletin 101(3) 



There have been few data indicating dietary differences 

 between thornback rays collected at different depths. 

 Smale and Cowley ( 1992) reported that bottom type used 

 by rays varies with depth and predicted that the prey spec- 

 trum would thus also vary, but no depth-related analyses 

 of diet composition were preformed in their study. Despite 

 similarities in size (i.e. no differences in the mean size by 

 depth strata; Menezes-*), we found that rays inhabiting dif- 

 ferent depths prey upon different resources. The decreasing 

 consumption of Liocarcinus spp., "other reptants," and T. 

 picturatus, and the increasing consumption of penaeids, 

 seabreams, and myctophids with depth of capture of rays, 

 appears to be in general agreement with the relative abun- 

 dance of prey with depth. Therefore, such depth-related 

 variations in diet may simply reflect differences in prey 

 availability. It is not clear, however, why Scyllarus arctus, 

 a species with a known depth distribution of 4 to 50 meters 

 (e.g. Alvarez, 1968; Castellon and Abello, 1983), appears in 

 stomachs of thornback rays caught between 201 and 350 

 meters (see Table 2). There is no evidence of vertical mi- 

 grations of thornback ray associated with feeding activity; 

 therefore this prey was likely eaten at deep water. Thus, the 

 depth distribution range of S. arctus in the Azores may be 

 significantly greater than what was previously known. The 

 only study that could corroborate this hypothesis (Fransen, 

 1991) reported one S. arctus caught between 420 and 700 

 meters depth in the Canary Islands. 



Our comparisons between areas (coastal and offshore 

 banks) were unable to clearly separate the influence of 

 depth because nearly all coastal samples were obtained 

 from shallow waters, and offshore bank samples were 

 collected from much deeper waters. Hence, we were 

 incapable of determining whether the high level of poly- 

 chaetes, penaeids, cephalopods, mysids, seabreams, and 

 myctophids in the diet of rays caught at offshore banks 

 reflects the availability of these prey in these areas, or in 

 deeper waters, or both. Nevertheless, our findings indicate 

 that coastal rays have different diets from rays taken in 

 offshore banks. 



Tope sharks preyed almost exclusively upon teleosts, 

 along with very few crustaceans and cephalopods. Previ- 

 ous observations on the feeding behavior of this species 

 suggested that fish and cephalopods are the main prey 

 categories ( Elhs et al., 1996; Olsen, 1954). The diet of tope 

 shark in the Azores consists of fewer species (mainly small 

 shoaling fish, mainly boarfish and snipefish) compared to 

 the diet of tope shark documented in previous studies. 

 These two fish were also important diet components of 

 other piscivorous species around the Azores between 1993 

 and 1997, namely cephalopods (Pierce et al., 1994), elas- 

 mobranchs (Clarke et al., 1996), fishes (Clarke et al., 1995; 

 Morato et al., 1999, 2000, 2001) and seabirds (Granadeiro 

 et al, 1998; Ramos et al., 1998a, 1998b). The role of these 

 two small shoaling fish in the marine food web of the Azores 

 is not yet fully understood. The fact that these prey may 

 exhibit strong variation in abundance, raises the question 



^Menezes, G. 1995-97. Unpubl. data. Department of Ocean- 

 ography and Fishcrie.s, University of the Azores. Cais de Santa 

 Cruz, PT9901-862 Horta, Portugal. 



of how well predators can adapt to extensive changes in 

 their availability. 



Stomach-content data offer a good snapshot of the feed- 

 ing habits of fish species, but diets may vary substantially 

 with food availability, depth, location, and season. Caution 

 is, therefore, required when drawing conclusions about 

 the trophic ecology of marine predators. The trophic role 

 of thornback rays and tope sharks in the Azores could be 

 further clarified by year round sampling and by an analysis 

 of stable isotopes (Gu et al., 1996; Jennings et al., 1997; Pin- 

 negar and Polunin, 2000), which could provide a less biased 

 average estimate of predator trophic level. 



Acknowledgments 



This work is part of a more comprehensive study supported 

 by the European Union (Design optimization and imple- 

 mentation of demersal cruise survey in the Macaronesian 

 Archipelagos (study contract DG XIV/94/034 and DG XIV/ 

 95/095). We thank Joao Gongalves, Ricardo Serrao Santos, 

 Filipe Porteiro for help with identification of stomach con- 

 tents, and Helena Krug for help with the identification of 

 otoliths. Special thanks are due to the scientific staff and 

 to the crew of the KV Arquipelago for working overtime at 

 sea. We are also grateful to Malcolm J. Smale, Pedro Afonso, 

 Joel Carlin, and Natacha Carvalho for reviewing the manu- 

 script. Comments and suggestions of anonymous reviewers 

 greatly improved the quality of the manuscript. 



Literature cited 



Ajayi, T. O. 



1982. Food and feeding habits of Raja species (Batoidei) in 

 Carmarthen Bay, Bristol Channel. J. Mar Biol. Assoc. U.K. 

 62:215-223. 



Alvarez, R. Z. 



1968. Crustaceos decapodos Ibericos. Investigacion Pes- 

 quera, tomo 32, 510 p. Imprenta Juvenil, Barcelona. 

 Castellon, A., and P. Abello. 



1983. Bathymetric distribution of some Reptantia Decapoda 

 in the Catalan area (Spain). Rapp. Comm. Int. Mer Medit. 

 28(3):291-294. 



Clarke, M. R., D. C. Clarke, H. R. Martins, and H. M. Silva. 



1995. The diet of swordfish {Xiphias gladius) in Azorian 

 waters. Arquipelago (Life Mar. Sci.) 13A:53-69. 



1996. The diet of blue shark (Pnonace glauca. L.) in Azorean 

 waters. Arquipelago (Life Mar Sci.) 14A:41-56. 



Clarke, K. R., and R. M. Warwick. 



1994. Change in marine communities: an approach to statis- 

 tical analysis and interpretation, 144 p. Natural Environ- 

 ment Research Council, UK. 



Compagno, L. J. V. 



1984. FAO Species catalogue. Vol. 4, Sharks of the world: 

 an annotated and illustrated catalogue of sharks species 

 known to date. Part 2 Carcharhiniformes. FAO (Food 

 and Agriculture Organization) Fish. Synop. 125(4) Part 2: 

 251-655. 



Cortds, E. 



1997. A critical review of methods of studying fish feeding 

 based on analysis of stomach contents: application to elas- 

 mobranch fishes. Can. J. Fish. Aquat. Sci. 54:726-738. 



