Effects on the food web of an optimised fish passage in the Afsluitdijk
In 1932 the former Zuiderzee was closed by the Afsluitdijk in order to obtain new land byreclamation programmes, and to assure safe conditions in case of big storms or floods. The major changes that caused the construction of the Afsluitdijk were the absence of tides and the decline of salinity. The latter fact provoked the extinction of the mostestuarine resident fish species. The Afsluitdijk forms a physical barrier, which difficultsthe migration of diadromous fish, as well as the displacements of marine fishes, from theWadden Sea to the IJsselmeer. A new fish passage is considered to be built to facilitate the migration, and an Environmental Impact Assessment of the new fish passage is required. This survey is a literature research to know the possible effects on the food web of the IJsselmeer, as a consequence of the construction of a fish passage in the Afsluitdijk.The short period of time available and the urgency of an answer made any field research impossible to conduct.
The species involved in this survey are the group of diadromous species, marine juvenile and marine seasonal which are found in the Netherlands Coast of the Wadden Sea or int he IJsselmeer. Estuarine resident species were extinct as a consequence of the closure of the IJsselmeer, and as a result were not added in this research.
The species which can provoke an effect in the food web of the IJsselmeer were selected by a set of criteria. The opinion of the experts obtained by interviews were a useful additional tool to reach conclusions.The first criterion applied depends on population size and the fulfilled spawning requirements in the IJsselmeer. In this group are the following species: Salmo salar, Alosaalosa, Coregonus lavaretus oxyrhincus, Acipenser sturio and Alosa fallax. The secondcriterion of selection permits to know if the Afsluitdijk forms a huge barrier for the migration. Because, in that case, the construction of a fish passage will lead to aconsiderably increase of fish in the IJsselmeer. Different factors have been selected inorder to obtain that knowledge. The factors are the possibilities of forming a self-sustainable population and the swimming capacity of different species. The latter ischosen to know the present possibilities of different species to cross the Afsluitdijk viasluices and to compare this with the future situation when a fish passage will be built. In this group there is: Salmo trutta, Platichthys flesus, Anguilla anguilla and the marine species, Clupea harengus, Sprattus sprattus, Chelon labrosus and Liza ramada. The species which are expected to increase considerably their number are Osmerus eperlanus and Gasterosteus aculetus. To identify the possible effects on the food web in the IJsselmeer the role in the food web and the use (corridor or feeding habitat) of the IJsselmeer have been taken into account. The reached conclusion is that no effects are expected on the food web in the IJsselmeer.
0 comments:
Post a Comment