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



595 



Table 2 



Percentage of relative importance C/t IRI ) of food categories of Raja davata by sex, total length, depth strata, and areas (coastal and 

 offshore banks). Prey items occurring in less than five stomachs were grouped into higher taxonomic levels. The null hypothesis 

 of not feeding upon the same most important prey categories was tested by using the top-down correlation method (being r.^ the 

 top-down correlation coefficient) and the top-down concordance method (being Cj the top-down concordance coefficient). NS = non 

 significant, *P<0.01. 



Sex 



Total length (cm) 



Depth (m) 



Areas 



M 



49-60 61-70 71-80 81-93 0-100 101-200 201-350 Banks Coastal 



Cephalopoda 0.52 0.03 1.44 0.00 0.38 0.63 0.03 



Polychaeta 0.62 1.70 0.21 0.43 0.73 6.44 0.54 



Penacidea 0.34 0.62 0.72 1.32 0.18 0.00 0.19 



Other Natamia 0.10 015 0.52 O.I I 0.06 0.00 0.48 



Diogenidae 0.07 1.58 1.45 0.00 0.88 0.45 0.69 



Scyllarus anms 1.54 0.57 1.12 0.25 2.48 0.64 0.76 



Cahippa xraimlata 0.73 0.31 0.45 0.30 0.90 0.36 0.00 



Liocanimis -.pp. 8.12 0.60 1.64 3.30 9.49 0.19 10.44 



Other Reptantia 9.20 8.32 1.43 11.88 22.48 0.00 47.44 



Mysidacea 0.68 0.50 0.18 0.62 1.02 0.00 0.00 



Isopoda 0.53 0.00 0.00 0.24 0.30 0.35 0.02 



Cciproscipci- 41.20 24.16 36.26 38.11 23.39 53.34 20.06 



Mmnmimphosus scolopax 35.15 58.88 53.60 41.65 34.91 37.35 15.81 



Pagelhis^p. 0.46 1.07 0.00 1.08 1.24 0.00 0.56 



Myctophidae 0.04 0.12 0.00 0.14 0.09 0.00 0.03 



Tnulumis picnmiliis 0.14 0.57 0.34 0.00 0.87 0.26 1.25 



Other Pisces 0.58 0.82 0.64 0.56 0.61 0.00 1.71 



0.21 

 0.40 

 0.12 

 0.01 

 0.21 

 0.84 

 1.52 

 1.43 

 0.31 

 1 .00 

 0.47 

 38.26 

 54.84 

 0.19 

 0.04 

 0.01 

 0.15 



5.60 

 15.13 

 14.72 

 0.00 

 0.00 

 4.35 

 0.00 

 0.00 

 0.00 

 2.21 

 0.00 

 10.63 

 36.72 

 9.99 

 0.65 

 0.00 

 0.00 



Stomachs with contents (/I) 89 49 19 47 60 11 47 



Ct=0.51NS 

 78 



13 



3.48 

 4.23 

 1.41 

 0.08 

 0.00 

 0.91 

 0.10 

 0.00 

 0.52 

 16.79 

 0.00 

 35.56 

 33.50 

 2.27 

 1.16 

 0.00 

 0.00 



0.06 

 0.57 

 0.29 

 0.12 

 0.53 

 1.21 

 0.64 

 12.30 

 6.33 

 0.00 

 0.31 

 32.53 

 42.91 

 0.43 

 0.01 

 0.40 

 1.36 



24 



=0.44NS 



110 



prey were also recorded in the stomachs of thornback rays: 

 the chub mackerel, {Scomber japonicus [7fIRI=0.3]) and 

 the blue jack mackerel {Tyachuriis picturatus [%IRI=0.3]). 

 Some individuals also fed upon mesopelagic myctophids 

 (%IRI<0. 1 ) and upon shallow water benthic fish such as the 

 red striped mullet (Mullus surmuletus [%IRI<0.1] ) and the 

 Azorean chromis (Chrornis linibata [%IRI<0.1|). 



Reptants occurred in 47.1% of the stomachs examined 

 and represented 17.0% by weight and 31.9% by number 

 of the total prey found (Fig. 3A). Swimming crabs (Liocar- 

 cinus spp. 1%IRI=5.5]), which include both L. marmoreiis 

 (%IRI=2.8) and L. corrugatus (%IRI=0.8), were the most 

 important reptant prey item in the diet of thornback ray 

 (Table 1). Other important reptants included the lesser 

 locust lobster (Scyllarus arctus [%IRI=0.9]), the shame- 

 faced crab (Calappa granulata |%IRI=0.5]), as well as 

 some unidentified Diogenidae (%IRI=0.3) and brachyura 

 (%IRI=1.5). 



Polychaetes (%IRI=0.8) were the third most important 

 prey category and occurred in 9.4% of the stomachs with 

 food (Fig. 3A). Mysids (%IRI=0.5), natants (%IRI=0.3), 

 isopods (%IRI=0.2), and cephalopods (%IRI=0,1) also 

 occurred in stomachs of thornback rays sampled in the 

 Azores (Table 1). 



A comparison of thornback ray's diet in relation to sex, 

 length, depth and area of capture (Table 2) suggests that C. 



aper and M. scolopax were by far the most important prey 

 for all subgroups examined. The diets of both sexes were 

 significantly correlated (r.p=0.70, P<0.01), indicating a high 

 degree of similarity in the diets of males and females. Both 

 sexes fed primarily upon two benthopelagic fish species (M. 

 scolopax and C. aper) and reptants (Table 2). Schoener's 

 diet overlap index between males and females was 0.72, 

 also indicating a high level of similarity between diets. 



Significant concordance (C^=0.74,P<0.01) was displayed 

 among thornback rays of different size classes (49-60, 

 61-70, 71-80 and 81-93 cm TL). Prey categories had simi- 

 lar %IRI values for the different size classes (Table 2 ), with 

 the exception of reptants (both Liocarcinus spp. and "other 

 reptants"), which were more important in the diet of the 

 two middle size classes. Schoener's index also suggested 

 a high degree of overlap (>0.60) among all size classes 

 (Table 3). 



Examination of depth-related differences was lim- 

 ited by the small sample size of rays from deeper waters 

 (n2oi_350n,= 13). However, the top-down concordance coef- 

 ficient suggested that individuals captured at different 

 depths (0-100, 101-200, and 201-350 m) do not feed upon 

 the same most important prey categories (Ct.=0. 52, P>0.05). 

 Reptants (both Liocarcinus spp. and "other reptants") and 

 the fish species T. picturatus were more important in the 

 diet of rays captured in shallow waters (0-100 m); whereas 



