Fish catch and water quality in small lakes

We examined the relationship between water quality and fish catch in 80 small Finnish lakes. Generally, fishing effort and gear may vary considerably among lakes; therefore, the detection of any relationship between water quality and fish catch may be hampered. To avoid this bias, fish in the present study were trapped with a standardised gill net (catch refers to the pooled biomass of trapped fish, catch per unit effort (CPUE). Water quality was assessed by measuring lake-specific values of pH, calcium, dissolved and total aluminium, conductivity, alkalinity and water colour. From these data, we extracted two principal components, together they retained 78% of the variation in the original data. Despite our efforts to minimise noise in the catch data, we could not find any predictive relationship between water quality and total biomass of fish. We conclude that in Finnish lakes it is difficult, if not impossible, to predict the lake-specific fish catch based on water quality. However, even with the present data there were differences in the total catch of fish originating from lakes with diverse values along the first principal component (a gradient composed of pH, calcium, conductivity and alkalinity). Lakes in the low end averaged a catch of 6.4 kg, while those at the high end averaged 9.2 kg. This was due to roach having increasing catches along the gradual transition from acid to neutral lakes.