Roach habitat shifts and foraging modified by alarm substance - 2. Reasonsfor different responses of fish in field and laboratory studies

The Schreckstoff-induced fright reaction (shift in fish depth distribution) was studied in roach from a European lowland lake in two twin vertical tub es of 0.85 m diameter and 11.5 m depth (Plon Plankton Towers) in an attempt to find possible reasons for the weakness of roach responses to the alarm substance in the field, as compared to those in laboratory conditions. Roac h (fork length 15.4 +/- 1.6 cm), fed Daphnia and chironomids under diel 9/1 5 h light/dark periodicity, displayed a transitory but firm response to the alarm-substance treatment in one of the towers, while no change was seen i n the reference one. The mean depth of fish within I h of dusk following Da phnia supply to the towers' epilimnia (0-2.5 m depth), as well as 3 h later in the dark, was significantly greater in the treatment than in the refere nce tower. The response was more pronounced in prey (Daphnia) density than in the depth distribution of roach. The indirect effects on Daphnia were ex tremely strong (Daphnia many-fold more abundant in the tower with alarm sub stance than in the reference one), but shortlasting. Frightened fish were m ore reluctant to feed than the reference fish in the daylight, but the init ial evening difference in Daphnia abundance vanished overnight due to feedi ng in the dark.