Feeding habits of the narrow nose smooth hound, Mustelus schmitti (Springer 1939) (Carcharhiniformes:Triakidae) in the Argentine Sea: Analysis of historical data 1986-1994

BELLEGGIA MAURO; SÁNCHEZ FELISA; MARI NOEMI; FIGUEROA DANIEL E.; BREMEC CLAUDIA

Fortaleza

Congreso; VI Reuniao da Sociedade Brasileira para o Estudo de Elasmobranquios; 2008

SBELL

Mustelus schmitti is one of only three species of Triakidae known from the SouthwestAtlantic Ocean which is distributed from southern Brazil (22°S) to Ría Puerto Deseado(47º45´S, Argentina). It occurs from shallow waters to 120m and constitutes a fishingresource of commercial value. Food habits of M. schmitti were studied based on analysisof stomach contents of specimens collected from fifteen research cruises carried out byInstituto Nacional de Investigación y Desarrollo Pesquero during 1986-1994 on theArgentinean continental shelf (34ºS - 43ºS). Prey items were counted, weighted andidentified to the lowest possible taxonomic level. Diet composition was assessed using theIndex of Relative Importance expressed as percentage (%IRI) and trophic level wasestimated. Dietary overlap between sexes, regions (Río de la Plata estuary-adjacentmarine waters and Rincón coastal system) and size classes (<600mm, 600-700mm,>700mm) were compared by Schöener Index, whereas similarity in the composition of thediet was determined by the ANOSIM using the R software. Of the 1009 stomachsanalyzed, 902 (89.39%) contained food. Fifty-eight prey taxa were identified in thestomach contents. M. schmitti fed mostly on Decapods (73.82%), followed by Polychaets(12.11%) and Fishes (11.09%). The fraction of unidentified species of crabs was ratherhigh. The relative importance of Amphipods, Isopods, and Molusks was comparativelylow. Trophic level was 3.62 suggesting that the narrow nose smooth hound is a secondaryconsumer. High degrees of dietary overlap between sexes, regions and size classes werefound. However, for each region analyzed, ANOSIM showed that there were significantdifferences in dietary composition among size classes (Río de la Plata: P< 0.001, global Rstatistic 0.063; Rincón: P< 0.001, global R statistic 0.032). Smaller M. schmitti consumedDecapods (48.07%) followed by Fishes (23.9%) and Polychaets (21%), whereas largerindividuals fed almost exclusively on crabs (84.62%). All size classes of M. schmitti forageon similar prey groups, but in different proportions. In spite of there was some overlap inprey species among coexisting M. schmitti, ANOSIM found dietary changes associatedwith the size of the predator. This suggests that there is some degree of resourcepartitioning. A shift in the dietary composition of elasmobranches during their ontogeny,from carcinophagous to ictiophagus, has been currently reported. Our results show a surprising inverse pattern, very likely associated with the paviment-like dentition of M.schmitti.