PREDATION RISK FROM PISCIVOROUS FISH INFLUENCING THE DIEL USE OF MACROPHYTES BY PLANKTIVOROUS FISH IN EXPERIMENTAL PONDS

This study investigated interactions between zooplanktivores (roach an d perch) and piscivores (pike and large perch) in experimental ponds ( 16 m(2)) with open water habitat and three densities of natural macrop hytes. Fish habitat selection was determined both day and night and wa s supported by daytime observations to study anti-predator behavioural patterns. Diel migration out from among macrophytes was seen in the a bsence of predators, particularly for roach, which changed from 13% of individuals being in open water during the day to 90% at night. The r isk of predation from piscivores influenced the habitat selection of t he zooplanktivores. Roach seemed to be the most vulnerable to predatio n from pike and selected the open water (90-92%) during daylight hours , but kept a 1-m distance from the macrophytes edge. The presence of p ike thus reduced the use of macrophytes by roach, which in turn may im prove macrophytes and the edge area as a refuge for zooplankton. Pike appeared to have less impact on the gross habitat selection of 0+ perc h, which were associated with the macrophytes (58-89%), though they we re still vulnerable to predation. Adult perch, which was a generally l ess effective predator than pike, showed subdued behavior, concealing themselves in the macrophytes most of the time. No effect of the simul taneous presence of two predator species was found.