Feeding strategy of silverside species in eutrophic Lake Pátzcuaro, Mexico

The trophic guild, niche breadth, and trophic position of two different ontogenetic phases were measured according to the size of Chirostoma spp. captured in different zones of Lake Pátzcuaro, Mexico. Analyses were carried out at the genus level due the difficulties to find diagnostic characters to separate the three small silverside species accurately, since even the molecular analyses of mitochondrial DNA for a subsample of the most morphologically differentiated specimens were conducted without success. Fish were captured with a seine net during the wet (September and November 2009) and dry (February and June 2010) seasons at four environmentally different sites. Gut contents of 480 Chirostoma specimens were divided into size-class intervals of 38–51 mm and 66–71 mm for analyses. The contribution of each food item was quantified using the frequency of occurrence and area percentage. Fish were categorized by size, and diets were compared between fish sizes and sites. Dorsal muscle tissue and zooplankton were obtained for nitrogen isotope signature (δ¹⁵N) analyses. The trophic guild was determined using a modified quadrant method for gut content analysis; the importance of each prey item was estimated according to a modified index of relative importance. Niche breadth was calculated by the Levins’ Index. The trophic position was assessed using the TrophLab Program and stable isotope analysis. The index of relative importance among study sites and fish size classes was determined by a multivariate approach. Silversides represent a secondary and tertiary consumer in Lake Pátzcuaro that feed mainly on evasive prey, such as copepods (55%) and on non-evasive prey, such as cladocerans (25%) at sites with the lowest transparency in the water column (from 350 mm to 150 mm). Fish scales (11%) and insects (6%) were secondary food items. Ingestion of juvenile silversides was rare (1%). Chirostoma spp. prey on insects on the lake bottom and on water hyacinth roots. Based on δ¹⁵N analysis, the trophic position of silversides changes between wet and dry seasons. Their opportunistic and specialized predatory feeding strategy indicates the capability of silversides to resist anthropic changes in the lake.