Contribution of each prey category to fish
diets was estimated according to the index of
relative importance IRI, since it combines the three
indexes indicated above (Hyslop 1980, Cortés
1997). To facilitate diet comparisons among species,
IRI was standardized to %IRI (Cortés 1997). For
plankton, %IRI was not calculated because weight
was not taken. To determine diet specialization of
each species, dietary breadth was calculated based
on %N for each prey according to standardized
Levins’ measure which ranges from 0 to 1.0, where
values close to 0 indicate specialization while values
close to 1.0 show generalization (Hurlbert 1978).