Abstract

This study examined diet composition, selection, and trophic morphology among three zooplanktivore silverside fishes (Chirostoma spp.) in an attempt to describe niche partitioning among closely related species in the tropical, shallow Lake Chapala, Mexico. To emphasize species interactions, sampling was carried out at the peak of the dry season (May) when a concentration effect was expected to take place as a consequence of lake volume reduction. Prey selection differed among species: Chirostoma consocium selected cyclopoid and calanoid copepods; C. jordani selected Bosmina and Diaphanosoma; C. labarcae selected Daphnia and Ceriodaphnia. Larger fishes occasionally consumed Hemipteran larvae. Although two Chirostoma species preyed upon the same zooplankton species, prey selection was consistent across size classes. From a total of 32 morphometric and meristic characteristics related to trophic ecology, 13 characteristics differentiated species, according to the discriminant analysis. Ordination including 11 morphometric characteristics and six zooplankton food items graphically separated the three species. The first axis separated species according to prey classes (copepods vs. cladocerans) and gross morphology (head and anal fin lengths). The second axis emphasized size-selective predation on cladocerans (Bosmina vs. Daphnia consumers) and fine trophic morphology (oral gape and the gill rakers' structure). Our findings suggest that the permanence of all three species in a fluctuating environment is the result of the circumvention of competition by niche partitioning, promoted by prey availability, selective predation, and few but critical differences in trophic morphology.