LATITUDINAL DIFFERENCES IN FISH COMMUNITY TROPHIC STRUCTURE, AND THE ROLE OF FISH HERBIVORY IN A COSTA RICAN STREAM

We experimentally manipulated fish grazing pressure to determine whether fish herbivory played an important role in the structure of a Costa Rican stream. Non-planktonic plant matter represented a significant percentage (greater-than-or-equal-to 25%) of the diet of 77% of the 17 fish species in the community. We prevented fish grazing on macrophytes, tree leaves, and periphyton using fish exclusion cages. Fish grazed Panicum sp., used as a generalized aquatic macrophyte, to the stems after 6 days in control areas, and consumed all or much of Ficus insipida and Monstera sp. leaves when placed in the stream after 48 hours. Plants and leaves experimentally protected by cages remained intact. In periphyton studies, fifty percent more ash free dry weight occurred on 25 x 25 cm floor tiles protected from fish grazing by cages than on tiles in roofless controls exposed to fish grazing for 19 days, suggesting a reduction in periphyton biomass. These results demonstrate that fish herbivory affects macrophyte abundance, and impacts the amount of leaf litter in the stream. Fish herbivory may also have an important effect on overall periphyton biomass. Herbivorous fish species generally represent a larger proportion of the total fish community in tropical compared to temperate streams; thus fish grazing is more likely to have an important influence on plant and animal abundances and distributions in tropical streams.