Chaoborus behavioural responses to changes in fish density

Abstract

The response of Chaoborus to biomanipulation-induced changes in fish community structure was studied in two small south-central Ontario lakes which had similar physical and chemical characteristics. The biomanipulation involved: (a) three years (1991-1993) of pre-manipulation characterization of the food webs in two lakes with very different food web structures (Ranger Lake was dominated by piscivores and Mouse Lake had no piscivores), (b) removal of approximately 90 % of the piscivores from Ranger Lake and their addition to Mouse Lake, and (c) three years (1994-1996) of post-manipulation characterization of the resulting changes in food web structure. The analysis presented here focuses on behavioural changes in Chaoborus, with specific reference to changes in Chaoborus density and migration patterns. At Ranger Lake, during both the pre- and post-manipulation phases of the experiment, the absolute density of Chaoborus (C. albatus, C. punctipennis, C. trivittatus 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 inter-annual variability was also much higher, there were no significant pre- to post-manipulation changes in density, suggesting that changes in fish community structure had little direct influence on Chaoborus standing stocks. At Ranger Lake, diel migration patterns remained constant through the entire experimental period (1991-1996), but at Mouse Lake, there were clear changes in migratory behaviour. At Mouse Lake, during the pre-manipulation years (1991-1993), all four Chaoborus species migrated to the sediments during the day and ascended to the epilimnion at night. Post-manipulation, all four species remained in the water column.