Chaoborus behavioural responses to changes in fish density

The response of Chaoborus to biomanipulation-induced changes in fish commun ity structure was studied in two small south-central Ontario lakes which ha d similar physical and chemical characteristics. The biomanipulation involv ed: (a) three years (1991-1993) of pre-manipulation characterization of the food webs in two lakes with very different food web structures (Ranger Lak e was dominated by piscivores and Mouse Lake had no piscivores), (b) remova l of approximately 90 % of the piscivores from Ranger Lake and their additi on to Mouse Lake, and (c) three years (1994-1996) of post-manipulation char acterization of the resulting changes in food web structure. The analysis p resented here focuses on behavioural changes in Chaoborus, with specific re ference to changes in Chaoborus density and migration patterns. At Ranger L ake, during both the pre- and post-manipulation phases of the experiment, t he absolute density of Chaoborus (C. albatus, C, punctipennis, C. trivittat us and C, flavicans) varied little, remaining at or near 150 individuals m( -3). At Mouse Lake, densities were more than twice as high, and although in ter-annual variability was also much higher, there were no significant pre- to post-manipulation changes in density, suggesting that changes in fish c ommunity structure had little direct influence on Chaoborus standing stocks . At Ranger Lake, diel migration patterns remained constant through the ent ire experimental period (1991-1996), but at Mouse Lake, there were clear ch anges in migratory behaviour. At Mouse Lake, during the pre-manipulation ye ars (1991-1993), all four Chaoborus species migrated to the sediments durin g the day and ascended to the epilimnion at night. Post-manipulation, all f our species remained in the water column during both the night and the day. Given that direct effects of fish on Chaoborus densities were not obvious, we suggest that changes in behaviour may have been mediated by the indirec t effects of kairomones exuded by fish. Our data suggested that changes in Chaoborus migratory behaviour were non-linear and that the boundary in fish densities that elicited full diel migrations, lay between 700 and 2,400 pl onktivores/ha.