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).