Wide food availability favours intraspecific trophic segregation in predators: the case of a water snake in a Mediterranean river

In complex aquatic ecosystems, intraspecific competition for food can evolve into dietary differences related to body size, gender, capture ability, and habitat use. The diet of the viperine snake, Natrix maura, an aquatic predator which forages on fish and amphibians, has been studied in parallel with food availability in a small locality of the Matarranya River (NE Spain). Fish abundance values in this Mediterranean river, which hosts one of the highest rates of fish diversity in Europe, were estimated through electrofishing. Natrix maura fed on four of the ten fish species available and captured the only amphibian detected. Three of these fishes were the most abundant species in the river. The fourth fish was the river blenny, a small and rare benthonic species of which the males protect eggs laid under stones at the bottom of the river during reproductive periods. This behaviour makes it easy prey for N. maura. However, the river blenny was caught mostly by small- and medium-sized snakes, while larger individuals, particularly females, fed on larger and more mobile prey that were more abundant and energetically more profitable. Similar diet differences according to size and gender have been described in other water snakes. We discuss whether this coincidence may be related to the great tropic availability in aquatic environments for fish-eating snakes.