Changes to lake ecosystem structure resulting from fish cage farm emissions: Fid-Bau Portal

Changes to lake ecosystem structure resulting from fish cage farm emissions

Håkanson, Lars

This study demonstrates how a given lake (Lake S. Bullaren, Sweden) would respond to emissions from a fish cage farm. The main questions to be answered are: How would emissions of feed spill and faeces from a fish farm influence the production and biomass of key functional organisms and how long would such changes remain if the fish farm is closed down? The work is based on a comprehensive lake ecosystem model, LakeWeb, which accounts for production, biomass, predation, abiotic/biotic interactions of nine key functional groups of organisms, phytoplankton, bacterioplankton, two types of zooplankton (herbivorous and predatory), two types of fish (prey and predatory), as well as zoobenthos, macrophytes and benthic algae. The LakeWeb model gives seasonal variations (the calculation time is one week). It has been calibrated and critically tested using empirical data and regressions based on data from many lakes. These tests have demonstrated that the model can capture typical functional and structural patterns in lakes very well, lending credibility to the results presented in this study. To the best of the author's knowledge, this is the first time that effects from fish cage farm emissions on lake ecosystem structure have been quantified at the ecosystem scale. To carry out studies such as these in the traditional manner, by extensive field work in one or a few lakes, would be very demanding, in terms of costs, personnel and time. This work has demonstrated that fish farm emissions would cause significant increases in the biomass of wild fish, without corresponding increases in algal volume. Thus, it is concluded that the fish farm emissions influence the lake's secondary production more markedly than primary production. Although this finding might seem to be a paradox, it is related to the fact that wild fish directly consume food spill and faeces from the fish farm, thereby creating a specific foodweb pathway, as described in this study.