Crayfish species assemblages in softwater lakes in seven tertiary watersheds in south-central Ontario

In 1989 a study was initiated to document the geographic distribution and relative abundance of crayfish in south-central Ontario lakes and to establish whether or not crayfish can be used to evaluate the health of inland lakes. Several crayfish species that are found in these lakes are known to be intolerant of poor water quality. This report summarizes the results of a survey of 100 lakes of varying size and water quality. This information will be used to estimate the normal range of crayfish abundance in relatively unimpacted lakes in south-central Ontario. Subsequently, crayfish catches from presumably impacted lakes will be compared to this normal range in order to assess the biological significance of human activities.

From 1989 through 1994, crayfish relative abundances were estimated using baited minnow traps. Generally 54 traps were set for a single night in a given lake. Several lakes were re-sampled to examine the repeatability of the resultant abundance estimates. In total, 7 crayfish species were captured. Up to 4 crayfish species were caught in any lake, although no crayfish were captured in 29% of the lakes. By contrast, one species was caught in 23 lakes, two in 33 lakes, 3 in 12 lakes, and 4 species in 3 lakes.

We sampled representative lakes in 7 tertiary watersheds, although the majority of the lakes were from 4 watersheds. Three crayfish species were found in all four of these watersheds. This additional information will provide a correction factor to adjust the normal range in crayfish abundance for situations when a new crayfish species invades a lake. In those lakes where several crayfish species co-occurred, no single species was always the most abundant. As a result, any crayfish species, including an introduced species, may be the most abundant species in a lake. This variation complicated our efforts to predict crayfish abundance.

One of the goals of this study was to characterize the normal range in crayfish abundances with respect to relatively unimpacted lakes in south-central Ontario. Unfortunately, this study failed to reveal any simple patterns. Different combinations of crayfish species were found in the different watersheds, including two introduced species. Moreover, an unusually large number of lakes in several watersheds did not support any crayfish species were correlated with different environmental variables. If crayfish are to be used as biomonitors of the health of small inland lakes in south-central Ontario, it is clear that more information and further data analysis are required before we will fully understand the complex patterns observed in this study.