Long-term trends in the glasseels immigrating at Den Oever, The Netherlands

Immigrating glasseels (Anguilla anguilla L.) have been sampled in Den Oever, the Netherlands, for numbers per dipnet haul (since 1938) and for length distribution (since 1960). The data from 1960 through 1996 were analysed to detect trends over the years. Special attention is paid to the analysis of potential artefacts caused by the sampling strategy. Mean length and numbers were positively correlated, while the timing showed independent, short-term fluctuations. From 1987 onwards, numbers of glasseels were well below the overall average, and they were significantly smaller. Since the minimum in 1991, numbers and mean lengths are both increasing, although they are still below average. It is tentatively concluded that these long-term changes are related to oceanic conditions, which have caused the prolonged and ocean-wide recruitment failure in eels, with the exclusion of suggested continental causes. Further clarification of the recruitment problem in eel is only to be expected when the reproduction problem is properly addressed, at the international level.