Estimation of roach ( Rutilus rutilus (L.)) and smelt ( Osmerus eperlanus (L.)) stocks with virtual population analysis, hydroacoustics and gillnet CPUE

Abstract

Dynamics of roach ( Rutilus rutilus (L.)) and smelt ( Osmerus eperlanus (L.)) stocks were analysed with virtual population analysis (VPA), hydroacoustics and by analysing gillnet catches per unit effort (CPUE). The research area was the Enonselkä basin of Lake Vesijärvi in southern Finland. In 1989 intensive trawl fishing on previously unexploited roach and smelt stocks was started. Heavy exploitation continued until 1993. Drastic reductions in fish stock densities were observed. The VPA indicated a decline in the roach stock from c. 3900 ind. ha −1 in 1989 to c. 520 ind. ha −1 in 1993. In the smelt stock the VPA indicated a respective decline from c. 18000 to c. 1900 ind. ha −1. In 1994–1996 fishing was sustained on a lower level. Anyhow, hardly any increase in stock density of roach and smelt was observed in 1994–1996. The estimates with hydroacoustics were in concordance with the VPA estimates in both the roach and the smelt stock. The gillnet CPUE-estimates were in agreement with hydroacoustics in roach stock assessment. However, gillnet CPUE was not suitable for analysing the dynamics of the smelt stock.