Assessment of the fishing impact on the silver eel stock in the Baltic using survival analysis

Abstract

Restoration of the depleted stock of the European eel (Anguilla anguilla (L.)) requires anthropogenic impacts to be quantified, reduced, and controlled. In this article, we assess the impact of the silver eel fishery on the Baltic Coast in Sweden, applying survival analysis to 60 years of mark–recapture experiments, involving 8000 recaptures out of 18 000 releases. The hazard of being recaptured (overall 46%) varies along the coast and declined substantially over the decades. But, most notably, the hazard for the individual diminishes strongly after the first kilometres en route. This individualized hazard disqualifies the more traditional mark–recapture methodology, which assumes random recaptures. We advocate the general use of survival analysis for conventional mark–recapture data. The result of our analysis indicates that the impact of the fishery just prior the 2009 fishing restrictions was in the order of 10%—in itself well within sustainability limits, though only but one of the factors contributing to the mortality in the Baltic Sea.