The impact of predation by perch on the size-structure of Chironomus larvae - The role of vertical distribution of the prey in the bottom sediments, and habitat complexity

Abstract

The study was performed in June-August 1993, in eighteen enclosures (1 x 2 m) built in a fertile, small (area 2.5 ha) and shallow (max. depth 1.7 m) lake, Little Mere (north-western England). Three densities of Nuphar lutea leaves were manipulated in enclosures to 10%, 50% and 90% coverage of the water surface. Perch (total length ca. 15 cm) was stocked in half of the enclosures in a density 3 fish m-2. The vertical distribution of Chironomus f.l. plumosus larvae (classified in the size classes: 0-5, 5-10, 10-15, 15-20 and >20 mm) in the bottom sediment showed a clear pattern, with bigger larvae burrowing to the greatest depth. Densities of all larval size classes except 0-5 mm were negatively affected both by plant density and by perch in three sediment layers considered (0-2, 2-5 and 5-10 cm). The larvae formerly at the surface did not move deeper in the sediments to avoid fish predation. The size distribution of the larvae, with the prevalence of the 15-20 and 10-15 mm larvae, found in the guts of perch was the most similar to that in the upper sediments at low and medium plant densities, and to that in the intermediate, 2-5 cm, sediment layer at high plant density. The effect of the interaction between plant density and fish was significant for all the larvae except those of 10-15 and 15-20 mm. Although the results of the experiment showed size-selective feeding by perch, the direct overall effect of the fish on the size-structure of the larvae was limited by vertical distribution of the prey in the bottom sediment. In addition, it was modified by nymphaeids, which, at high density, altered food conditions and acted as a refuge for Chironomus larvae both directly (providing shelter) and indirectly (providing fish with the additional, epiphytic prey).