CONICET | Buscador de Institutos y Recursos Humanos

Competition for food has been hypothesized to be a major force driving changes in community structure of skates (Rajidae) subjected to fishing exploitation. Under this hypothesis, small skates are released from competition with larger skates, after fishing depleted large species. Here, we compare the patterns of abundance of two sympatric skates with similar niches but different life histories, Bathyraja albomaculata (larger and slow-reproducing) and Bathyraja macloviana (smaller and faster-reproducing), before (1971,1978) and after (1998-2004) a 108%-increase in industrial bottom trawling on the southeastern South American shelf, in order to test the prediction that B. macloviana should competitively exclude B. albomaculata after the increase in fishing mortality. In 1971 and 1978, there was no relationship between the abundance of both species, indicating that they coexisted over large scales. In 1998-2004, the relationship between the abundance of these skates was bell-shaped, indicating that both species increased in abundance at low densities, reached a peak, and then B. albomaculata decreased when B. macloviana became abundant, consistent with resource competition. We tested if food may be a potential limiting resource by comparing the diet of both species. Diet was almost identical with polychaetes dominating in both skates and differed only in the consumption of polychaetes of the family Nephthyidae which was much higher for B. macloviana. Bathyraja macloviana could replace B. albomaculata at high densities, when food resources may become scarce. These results support the hypothesis that competition release is an important factor explaining the changes in skate communities in overexploited areas.

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