Threat-sensitive predator avoidance by pike larvae

The threat-sensitive predator avoidance (TSPA) hypothesis was tested on hatchery-reared pike Esox lucius larvae. Larval swimming activity, food attacks upon zooplankton, escapes, immobility behaviour, vigilance, as well as time in vegetation under different predation risk were measured. Single larvae were video-filmed in aquaria allowing them to have visual contact with a small or a large perch Perca fluviatilis as predator. The odds of fleeing increased significantly if the larvae were exposed to a large predator instead of a small one. Swimming activity and foraging decreased significantly in the presence of a predator compared to the control. On the other hand, no significant effect was detected in time spent in the vegetation, and in vigilance of larvae, measured as time intervals between food attacks. The findings suggest that pike larvae primarily flee in the presence of a large predator, whereas they remain immobile, and forage, when possible, in the presence of a small predator. Sometimes a lack of response between the large and the small predator treatments was detected, which suggests that the antipredator response has a threshold, and when increasing the threat, the prey response does not increase. In the present paper it is demonstrated that hatchery-reared pike larvae respond to the level of threat, and thereby seem to be risk adjusting as predicted. (C) 2004 The Fisheries Society of the British Isles.