Large-scale experiment demonstrates potential benefits of restocking glass eels in coastal waters of the southern Baltic Sea

Coastal waters are important habitats for the endangered European eel. In a large-scale experiment marked glass eels were released in two geographically distinct coastal areas in the southern Baltic Sea. Generalized linear mixed models were used to test for effects of restocking on yellow eel density as well as eel condition and growth following a before-after-control-impact analysis design. Alongside of increased natural immigration, restocking distinctly increased the yellow eel density by around 1.5–3 times in both restocked areas compared with densities recorded before the release. Increased densities by restocking did not generally lead to slower growth, altered body condition, or a change in the sex ratio of naturally immigrated eels. Restocked eels had comparable condition and grew somewhat faster than naturally immigrated eels in both restocked areas within the first three years after restocking. These results suggest, that the habitat capacity for eels in the restocked areas was not reached by the current natural settlement in combination with the released eels. Therefore, eel restocking in coastal waters seems to be a useful management option to locally increase the escapement rate of mature silver eels.