THE EFFECT OF BROWN TROUT (SALMO-TRUTTA L) ON STREAM INVERTEBRATE DRIFT, WITH SPECIAL REFERENCE TO GAMMARUS-PULEX L

The effect of brown trout (Salmo trutta L.) on invertebrate drift density was examined in three previously fish-free springbrooks. Drift density was studied during the day following the sequential introduction of: 1) empty cages, 2) cages containing trout, and 3) empty cages. Each period lasted three whole days, and drift density was measured at daytime and night-time every day. Control drift density was determined using empty cages in two similar springbrooks. Night-time drift density of large Gammarus pulex L. (> 3.9 mg DW ind-1) decreased compared to that of smaller size-groups following trout introduction, and was significantly lower (p<0.05) than the pre-introductory level after the trout had been removed again. The daytime drift density of large G. pulex remained unchanged, as did both daytime and night-time drift density of smaller size-groups of G. pulex, and the other taxa investigated (Leuctra hippopus (Kempny), Leuctra nigra (Olivier), Amphimura sp., Nemoura flexuosa Aubert, and Baetis rhodani (Pict.)). No significant changes in drift density were observed in two control springbrooks.