The influence of biomanipulations (fish removal) on the structure of lake foodwebs, case studies using the LakeWeb-model

Abstract

This paper presents results on how intensive fishing (fish removal) islikely to influence the structure of lake foodwebs. The work is based on acomprehensive dynamic lake ecosystem model, LakeWeb, which accounts forproduction, biomasses, predation and abiotic/biotic interactions of nine keyfunctional groups of organisms: phytoplankton, bacterioplankton, two types ofzooplankton (herbivorous and predatory), two types of fish (prey and predatory),zoobenthos, macrophytes and benthic algae. The model uses ordinary differentialequations, the ecosystem scale and gives seasonal variations (the calculationtime is 1 week). It is designed to account for all fundamental abiotic/bioticinteractions and feedbacks for lakes in general for the nine target groups. TheLakeWeb-model has been calibrated and critically tested using empirical data andregressions based on data from many lakes. It has been shown that the model canclosely capture typical functional and structural patterns in lakes, whichshould give credibility to the results presented in this work. Obtaining suchresults using traditional methods, i.e., extensive field studies in one or a fewlakes, would be very demanding (in terms of money, persons involved and time).In this paper, results are presented for two lakes, one Swedish and oneBelarussian. The intensive fishing operations carried out in LakeBlacksastjarn, Sweden, to reduce Hg-concentrations in fish did notsucceed. A typical cost of an intensive fishing is about 10,000–30,000 USDper lake of this size (≤ 0.25 km2). The costs toremove fish would be about 40–120 USD per kg ww fish removed! Intensivefishing simulated for Lake Batorino, Belarus, to reduce the fish biomass willlikely increase the prey fish biomass as long as the predation pressure on preyfish is lower than during the prefishing stage. The biomass of predatory fishwill recover only slowly. However, this operation is not likely to succeed inlowering the algal volume in lakes with a high biomass of predatory zooplankton.This is easy to state qualitatively and the LakeWeb-model offers a practicallyuseful tool to quantify such changes and identify lakes where biomanipulationsare likely to fail or succeed.