Figure 5. In these experiments we examined the possibility that Acartia spp. are responding to the presence of chemical exudates (or kairomones) of zooplanktivorous fish (threespine sticklebacks, Gasterosteus aculeatus). Recent studies have shown several species of zooplankton to respond to predator exudates, including cladocerans (Dodson, 1988; Ringelberg, 1991a;1991b; Dawidowicz and Loose, 1992; De Meester, 1993; Loose, 1993; Loose et al., 1993), chaoborid larvae (Dawidowicz et al., 1990; Tjossem, 1990; Dawidowicz, 1993), brine shrimp larvae (Forward and Hettler, 1992; Forward and Rittschof, 1993) and freshwater copepods (Neill, 1990; 1992). Our past work (Bollens and Frost 1989, Bollens et al. 1994, 1995), however, implicated visual or mechanical cues, not chemical exudates, as responsible for triggering changes in vertical distribution and migration in Acartia. In our more recent experiments utilizing this new experimental system (Fig. 5), we found no differences in the vertical distribution of Acartia spp. in the presence or absence of caged fish. These results corroborate our earlier findings on Acartia.