Trends In Eel Habitat Abundance In The Netherlands During The 20th Century

No abstracts are to be cited without prior reference to the author.

Loss of fresh water habitats is one potential explanation for the decline of the stock of the European eel Anguilla anguilla (L.) throughout the second half of the 20th century. Assessing its role in the decline, as well as developing targets for habitat restoration, requires that the loss of habitat is first quantified. For the Netherlands, information on changes in water surface for the larger water bodies is readily available, but (the change in) the contribution of the smaller waters (canals, ditches etc) is completely unknown. A first quantitative assessment of the decrease in fresh water area throughout the 20th century is compiled for the Netherlands. Both surface area and bank length are investigated. Quantification took place by measuring the length of ditches on historical and modern land maps, in two selected reference areas, selected to be representative for the major part of the country. The cumulative bank length has decreased to about half its former value, which decline was almost exclusively concentrated in the small ditches. The steepest decline occurred in the 1950s and 1960s, in relation to land re-allocation programmes. The water surface, however, greatly increased during the 1930s and 1940s, due to the reclamation of part of the Waddensea (now Lake IJsselmeer), but later on gradually declined, due to the construction of polders and the decrease in ditch abundance. The increase in open water surface and the loss of bank length are both potential causes for the decline of the eel stock. The creation of Lake IJsselmeer resulted in a large fresh water lake close to the sea, acting as a kind of trap for immigrating glass eel; the abundance of the resulting eel stock gave rise to an intense fishery, overexploiting the local stock. The loss of bank length will have resulted in a direct loss of habitat, particularly since female eel tend to concentrate in these areas.