Feeding ecology and abundance of two sympatric skates, the shortfin sand skate Psammobatis normani McEachran, and the smallthorn sand skate P. rudis Günther (Chondrichthyes, Rajidae), in the southwest Atlantic

Abstract

Abstract Mabragaña, E., and Giberto, D. A. 2007. Feeding ecology and abundance of two sympatric skates, the shortfin sand skate Psammobatis normani McEachran, and the smallthorn sand skate P. rudis Günther (Chondrichthyes, Rajidae), in the southwest Atlantic. – ICES Journal of Marine Science, 64: 1017–1027. The diet, feeding strategy, and abundance of Psammobatis normani and P. rudis in the southwest Atlantic was investigated to determine whether the species are segregated by habitat or dietary preference. The two coexist along the Argentine continental shelf, but there are differences in abundance. The most important prey for P. normani were crustaceans (mainly crabs and isopods) and, to a lesser extent, polychaetes, whereas P. rudis fed almost exclusively on crustaceans (mainly isopods, crabs, and gammarids), and fish and polychaetes contributed less to the diet. This suggests that P. normani and P. rudis are secondary consumers (trophic level <4). The two species fed on similar taxa, but in slightly different proportions according to region. However, an ANOSIM test failed to reveal significant differences in their diets. Circumstantial evidence of food competition is suggested, because the two species attain similar adult size and there are no notable morphological differences between them. Interspecific competition may be reduced by the use of distinct feeding behaviour and by the abundance of prey especially along shelf-break fronts. The use of standard ecological indices of similarity and multivariate techniques to calculate dietary overlap is evaluated.