Morato et al.: Diets of Raia davata and Galeorhinus galeus 



599 



stomachs for tope shark was 47.7% — much higher than the 

 4.3% obsen'ed by ElHs et al. ( 1996). The percentage of empty 

 thornback ray stomachs was high (37.1%) when compared 

 to values reported for the North Sea (9%, Daan et al.'; and 

 3.7%, Ellis et al., 1996), Carmarthen Bay, South Wales 

 (4.5%, Ajayi, 1982), west coast of Southern Africa (4.5%, 

 Ebert et al., 1991; and 2.6%, Smale and Cowley, 1992) and 

 the Portuguese mainland coast (2.5%, Cunha et al., 1986). 

 We attribute the high percentage of empty stomachs found 

 in our study to the use of longlines to catch the fish in the 

 Azores (trawls were used in the other studies ). Longlining is 

 a passive fishing method, which suggests that fish that feed 

 to satiation have a reduced response to bait odor ( Lokkeborg 

 et al., 1995), meaning that fish with full stomachs tend not 

 to eat the bait and be caught. Thus, only those fish with 

 empty stomachs or partial stomach fullness were caught. 



Thornback rays captured by longline in the Azores dur- 

 ing the spring of 1996 and 1997 fed upon a wide variety 

 of organisms. Fishes (81.6 %IRI) and reptants (17.4 %IRI) 

 dominated the diet, which also consisted of polychaetes, 

 mysids. natants, isopods, and cephalopods. In general, 

 thornback rays in the Azores preyed more heavily upon 

 fish in comparison with the predation patterns described 

 in other studies. Ajayi et al. (1982) reported a predomi- 

 nance of crustaceans (83%W) for all size classes and a low 

 importance offish (11.6%W) in the diet of thornback rays 

 in Carmarthen Bay, Bristol Channel. They also reported 

 amphipods, polychaetes, and some natants as food items. 

 Using the points method of Hyslop (1980), Ellis et al. 

 (1996) reported that thornback rays from the North Sea 

 fed primarily on crustaceans (78.9% ) compared to mollusks 

 (10.2%) and fish (7.3%). Several others have also reported 

 a dominance of crustaceans and low importance of fish in 

 the diet of thornback ray (Fitzmaurice, 1974; Marques and 

 Re, 1978; Quiniou and Andriamirado, 1979; Cunha et al., 

 1986; Gibson and Ezzi, 1987; Smale and Cowley, 1992; 

 Olaso and Rodriguez-Marin, 1995; Daan et al.'; Ebeling^). 

 Polychaetes (Holden and Tucker, 1974; Marques and Re, 

 1978), bivalves (Quiniou and Andriamirado, 1979), holo- 

 thurians (Ebeling-), and cephalopods (Holden and Tucker, 

 1974; Marques and Re, 1978; Smale and Cowley, 1992; 

 Olaso and Rodriguez-Marin, 1995) that were considered 

 important prey items in the other studies mentioned were 

 not recorded or were insignificant in our samples. 



Differences in diet composition of several predators may 

 reflect the geographic peculiarities in fauna composition 

 (e.g. Smale and Cowley 1992), but when comparing diets 

 based on higher taxonomic levels (such as fish, reptants, 

 and natants categories), such geographic differences 

 should not be so obvious. Our geographic analysis (see 

 Fig. 4) distinguished three major groups: I) the Azorean 

 individuals; II ) other large individuals; and III ) other small 

 individuals. Further, the estimated mean trophic levels for 

 these three major groups were significantly different: 4.14 

 (±0.09 SD) for the Azores; 3.46 (±0.84 SD) for other large 

 rays; and 3.35 (±0.21 SD) for smaller rays. The higher 



2 Ebeling, E. 1988. A brief survey of the feeding preferences 

 of Raja davata in Red Wharf Bay in the Irish Sea. ICES CM. 

 1988/G:58, 5 p. 



trophic level for the Azores is a result of a higher degree 

 of piscivory in this region and an increased consumption 

 of decapods and fish by larger rays, compared with small 

 rays. Notwithstanding the difference in sampling methods 

 (longline vs. trawl caught), it appears that the Azores can 

 be considered a separate group. In other studies, predator 

 size played the major role in controlling feeding patterns. 



The diet of the thornback ray in the Azores consists of 

 a greater proportion of fish than in any other area and 

 may reveal differences in the function of different environ- 

 ments, because seamounts and oceanic islands are the ma- 

 jor topographic feature of the Azores region and the other 

 studies were conducted on continental shelves. The general 

 function of oceanic seamount environments is still not 

 completely understood but they are characterized by sub- 

 stantial enhancement of primary production due to topo- 

 graphic effects on local hydrographic conditions (Genin and 

 Boehlert, 1985). However, evidence for enhanced primary 

 production leading to concentrations offish over seamounts 

 is sparse (Rogers, 1994). Additionally, the availability and 

 relative abundance of the two most important fish prey 

 items found in our work (the benthopelagic species C. aper 

 and M. scolopax ) vary considerably both seasonally (Grana- 

 deiro et al., 1998) and annually. Therefore, the high degree 

 of piscivory in the Azores may result from environmental 

 features and exceptional fish prey availability during the 

 sampled years or seasons. 



Thornback rays also fed on pelagic fish, as indicated by 

 the presence of chub mackerel and jack mackerel in stom- 

 achs — a finding that confirms previous suggestions (see 

 Daan et al. ' ; Ebeling- ) that thornback rays are active preda- 

 tors and able to feed semipelagically. The most important 

 reptants in the diet, Liocarcinus spp., were also reported as 

 the main prey item for thornback rays by Ellis et al. ( 1996). 

 The level of importance of isopods and amphipods, mysids, 

 cephalopods, and polychaetes in the diet of thornback rays 

 in the Azores was similar to values reported by other au- 

 thors (Ellis et al., 1996; Daan et al.'; Ebeling^). 



Differences in the dentition of females and males were 

 reported by Quiniou and Andriamirado ( 1979) but we and 

 Smale and Cowley (1992) observed no differences in the 

 major prey consumed between sexes. Therefore, sexual di- 

 morphism in dentition does not appear to be manifested 

 in dietary preferences between sexes, as was initially 

 expected. 



Several studies have demonstrated differences in preda- 

 tion patterns for rays of different size classes — primarily 

 a decrease in importance of crustaceans and an increase 

 offish with size (e.g. Smale and Cowley, 1992; Elhs et al., 

 1996; Daan et al.'; Ebeling^). Some authors attribute these 

 differences to the ability of large predators to prey upon 

 larger prey (Smale and Cowley, 1992); others suggest the 

 difference is due to a pronounced shift from a benthic to a 

 benthopelagic feeding behavior (Skjaeraasen and Bergstad, 

 2000; Ebeling^) or the reverse (Quiniou and Andriamirado, 

 1979). We found no significant size-related differences in 

 diet. Quiniou and Andriamirado (1979) reported shifts in 

 diet at a size of 30 to 40 cm TL but we could not verify these 

 conclusions because our sample included only rays larger 

 than 49 cm. 



