Changes in the fish communities of the Kawartha Lakes in response to anthropogenic stressors

Recently, a great deal of concern has been expressed by fishery managers and user groups on the perceived decline in fishing yields in some of the Kawartha Lakes (central Ontario), but it is not known whether such declines are system-wide, and whether or not they are symptomatic of historical changes in the fish assemblages in these lakes. The objectives of this research were to characterize the patterns in the fish community structure in these lakes, document the changes in the fish assemblages throughout the past 25 years, and to assess whether such changes are related to changes in nutrient concentrations and/or the productivity of lower trophic levels. Extensive data on the historical fish communities were collected using standardized trapnetting surveys; gillnetting and funnel trapping were used to assess the current fish communities. Total phosphorus, chlorophyll a and zooplankton were collected to identify productivity differences between the lakes. Secchi depth was collected as measure of water clarity. There was no significant change in the overall abundance of piscivores and no consistent changes in the species composition of the piscivore guild based on CUE over the period from 1980 to 2003. There has been a regional scale decline in the relative abundance of walleye and an increase in the relative abundance of Micropterus species. An increase in water clarity due to reduced nutrient loading and the introduction of zebra mussels and warmer summer temperatures were factors that corresponded with a decline in walleye relative abundance and an increase in the relative abundance of Micropterus species in these lakes.