Vertical migration of Chaoborus larvae is induced by the presence of fish

Die1 changes in the depth distribution of ChaoborwfIavicans larvae in thermally stratified aquaria with a distinct light gradient revealed that the larvae responded behaviorally to the presence of fish. Both the midday and midnight mean depths of the population were greater in the presence of fish. Most fish-treated larvae found a daily refuge in bottom sediments. It appears that the stimulus for vertical migration is chemical, not visual or mechanical. The fish effect persisted for more than 15 d but was reversible. Those individuals that previously had been exposed to fish factor were considerably more sensitive to light than untreated larvae and showed a panic response when suddenly illuminated. There is increasing evidence that vertical migration in planktonic animals is an adaptive response to avoid predation by visual predators (Zaret and Suffern 1976; Stich and Lampert 198 1; Gliwicz 1986). It appears that at least in some species the range of die1 vertical migration is correlated with predation intensity (Gliwicz 1986). Luecke (1986) reported that the appearance of migratory behavior in previously nonmigrating larvae of Chaoborusflavicans coincided with cutthroat trout introduction to Lake Lenore, Washington. Two mechanisms could be involved in this behavioral shift. First, in a behaviorally polymorphic population, selective predation may account for mortality of nonmigratory individuals. SecAcknowledgments Our thanks to Maciej Gliwicz for his comments on the first draft of the manuscript, but most of all for his immense contribution to our understanding of the phenomenon of vertical migration. We also thank Chris Luecke and an anonymous reviewer for their criticisms. We profited from discussions with Mike Swift, who also helped improve our English. The drawings were prepared by Danka Kozera. ond, the presence of a predator may induce behavioral change through visual or chemical cues (Dodson 1988) that are likely similar to those inducing morphological changes (see Have1 1987). The aim of our study was to find out which of these two mechanisms could be responsible for migratory or nonmigratory behavior of Chaoborus larvae. We tested in the laboratory whether the animals could respond behaviorally to the presence of fish by changing their distribution pattern and sensitivity to light. In our experiments we used 4th instar larvae of C. flavicans, which are migratory in most lakes of the temperate zone. The larvae were collected from mesoeutrophic Lake RoS (northeastern Poland), where they were exposed to fish. Before we started the experiments, larvae were kept for 2 weeks in lake water at 6"C, in complete darkness. We used three-spined stickleback (Gasterosteus aculeatus L.) from a small stream as our predatory fish. The experiment was performed in thermally stratified aquaria with light gradients to simulate the conditions experienced by migratory animals in a stratified lake. Before fish were introduced the initial vertical distribution of Chaoborus was observed for 24 h. The fish were then introduced and subsequently removed after 4 d. The distribution of larvae was followed for 12 d more to determine whether the fish effect was reversible and how long it lasted. To determine whether the stimulus for migration was chemical, we monitored the distribution and sensitivity to light of larvae exposed to fish-treated water after fish removal.