What caused the decline of the Lake IJsselmeer eel stock after 1960?

• Published in: ICES Journal of Marine Science
• Main Author: Dekker, W.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2004
• Subjects: anguilla-anguilla; european eel; netherlands; Anguilla anguilla
• Online Access: https://research.wur.nl/en/publications/what-caused-the-decline-of-the-lake-ijsselmeer-eel-stock-after-19; https://doi.org/10.1016/j.icesjms.2004.01.003

The European eel (Anguilla anguilla L.) is in severe decline: landings from and recruitment to the stock have fallen off since the mid-1960s and the early 1980s, respectively. Several hypotheses on the causes of the decline in recruitment have been advanced, some predicting an earlier decline of the adult stock. In order to narrow the range of potential hypotheses, this paper contrasts current ones with trends in abundance and length distribution of the local stock in Lake IJsselmeer (the Netherlands) over the period of the decline. The data set consists of research surveys, market sampling, gear development experiments, etc., since 1904, and is uninterrupted since 1950. A statistical analysis is designed in which sampling characteristics (length selectivity of gears and of mesh sizes, and sample selection procedures) are separated out of trends in the local stock over the years (abundance, length composition). The decline of the Lake IJsselmeer stock started in 1960, affected exploited and undersized eels, and was steeper for larger eels. The abundance of the smallest size class in the lake matches the independently recorded recruitment strength, which did not decline before 1980. Excessive exploitation, habitat loss, barriers to migration, introduced parasites, and changes in ocean climate cannot explain the observed trends when taken individually. Therefore, the cause of the decline of the local stock in Lake IJsselmeer is still a mystery and, because the historical information is limited and cannot be added to, is likely to remain so.