Specimens were identified to species level
and their stomachs were extracted and fixed in 10%
formalin solution for later analysis. Preys found
were identified to the lowest possible taxon and data
were quantified using the numerical methods such as
percent by frequency of occurrence (%O), percent
by number (%N) and percent by weight (%W)
revised by Hyslop (1980). In the case of preys such
as plankton, items were counted by using a
microscope (Olympus CH-20, zoom 40X) and only
percent by number was calculated. Cumulative prey
curves were constructed for each species to
determine if an adequate number of stomachs had
been collected to accurately describe diets (Cortés
1997). When the curves reach a stable asymptote,
the number of stomachs analyzed is considered
sufficient for describing dietary habits.