Yellow eel (Anguilla anguilla) density trends along the German part of the southern Baltic Sea between 2009 and 2020

Abstract

According to the recruitment indices provided by ICES for the years 2011–2021, the natural recruitment of the European eel remains at low level. However, fishery-dependent data series indicate an increasing trend in the southern Baltic Sea within recent years. In the light of the pan-European conservation efforts, the question is whether fishery-independent monitoring approaches can verify this positive trend. To monitor yellow eel density, a 1 ha enclosure system for non-tidal coastal waters was used in eight reference areas along the German part of the southern Baltic Sea (Federal State Mecklenburg-Western Pomerania) from 2009 to 2020. Changes in presence rate, mean yellow eel density and total length distribution were investigated. Additionally, the total stock size of yellow eels and the annual settlement rate of juvenile eels were estimated. Overall, the enclosure data indicate a positive trend in yellow eel stock size in recent years. An increasing mean yellow eel density was observed in five out of eight reference areas sampled. The density change was linked to an increased number of juvenile eels as evidenced by a significant change in the total length distribution. The mean annual settlement rate of juvenile eels was estimated as 13.2 eels of age class 0 + ha−1 year−1 in 2012–2017. Besides the increased settlement rate, other mechanisms of population dynamics or conservation measures might have contributed the observed positive trend in yellow eel densities.