Abstract
The present study investigated the trophic ecology of two species living in an alluvial lake ecosystem in terms of season, length groups, sex, and mouth morphology. The fish were caught in a coastal lake area between December 2014 and March 2016 using a seine. In general, C. aquaedulcis was the most important prey for S. abaster, accounting for 10.99% of all prey, while amphipods accounted for 54.04% of the S. acus diet. Amphipods were the most frequently consumed prey in autumn, while C. aquaedulcis was the most important prey in spring. This result indicates that the species preferred similar prey groups and generated food competition depending on the season. The short-length group contained samples of S. abaster with empty guts, whereas S. acus in the same group consumed C. aquaedulcis. Length group II and III of individuals indicated that both species consumed amphipods and juvenile Syngnathus sp. Thus, the two species had statistically different preferences in terms of prey length. The mouth width of S. abaster was larger than that of S. acus. In conclusion, zooplankton constituted the main prey for both species in the lake and the morphological differences between the mouth apparatus of the species were due to the differences in feeding habits.