Inferring fish abundance in lakes from subfossil midge (Diptera:Chironomidae) remains

Abstract

This study assessed the possibility of using subfossil remains of chironomid head capsules to reconstruct past fish abundances in lakes. Using data from 20 Swedish lakes, a weighted average (WA) model was created to predict the abundance of benthivorous fish (cyprinids and perch). Chironomid head capsules, identified to genus, served as the predictors to model already recorded (catch per unit effort) fish data. The relationship of chironomid species composition to lake morphometry, water chemistry and fish abundance were examined using ordination and variation partitioning methods. Moreover, the relationship between fish abundance and two chironomid species traits: body size and activity mode were examined to obtain information on the influential mechanisms behind these predator-prey interactions. Based on earlier studies, I hypothesized, first, that chironomid body size correlates negatively with fish abundance. Second, I expected that active and mobile species to dominate in the lakes without fish or with low fish abundance and inactive species to dominate in lakes with high fish abundance. The Chironomidae WA models explained 65 % and 60 % of the variation in fish density and biomass, respectively. When Chironomidae models were combined with previously published Chaoborus-based prediction models, both density and biomass models improved slightly. Spearman correlations between the body size and activity modes with fish abundance were significant only for smaller-sized chironomids. The smallest clingers showed a positive correlation with fish density, as did small crawlers with fish biomass. This may be due to the relief of predatory pressure from large-sized invertebrate predators, which are preyed on by fish. The burrowing taxa appeared to be more protected against fish predation, as large-sized burrowers correlated positively with fish biomass, and crawlers of the same size negatively correlated with fish biomass. Redundancy and variation partitioning analyses indicated that fish have a clear and independent effect on the chironomid assemblages.