Zooplankton community responses to fish stock management of Lake Vesijaervi, southern Finland: changes in cladoceran body size in 1999-2003

摘要

Planktivorous fish feed selectively on large and conspicuous zooplankton; therefore, small-bodied zoo-plankton prevails in the presence of intense predation by planktivorous fish (Brooks & Dodson 1965). Body size of cladocerans has been shown to be inversely related to fish planktivory, indicating changes in trophic interactions better than biomass (Soranno et al. 1993). The capacity of cladocerans to excrete nutrients and graze algae is strongly size-dependent (Knoechel & Holtby 1986). Communities dominated by large zooplankton tend to have higher grazing rates than those dominated by small zooplankton (Cyr & Pace 1992). Cladoceran body length has been shown as an important predictor of water clarity (Stemberger & Miller 2003). Thus, one goal in lake management is to increase the abundance and body size of filter-feeding cladocerans by controlling planktivorous fish stocks. In the Enonselkae Basin of Lake Vesijaervi in southern Finland (61°02′N, 25°39′E), fish stock has been managed yearly since effective fishing was stopped in 1993 (Kairesalo et al. 1999). Catches declined toward the end of the 1990s and have been intensified since 2000 (Ruuhijaervi et al. this volume) due to water quality deterioration (Keto et al. this vol.). Hy-polimnetic anoxia has further supported the fish management in 2002-03 by leading to a collapse of smelt, Osmerus eperlanus (L.), an important planktivorous fish species in the Enonselkae Basin (Ruuhijaervi et al. this vol.). This study aimed to examine changes in body size of pelagial cladocerans in 1999-2003, and how those changes are related to alterations in planktivorous fish stock and water quality.