Jager, Z.; Kranenbarg, J.; Vethaak, D. (2004). Vissen tussen zoet en zout. Levende Nat. 105(5): 204-208
In: De Levende Natuur: tijdschrift voor Natuurbehoud en Natuurbeheer. Stichting De Levende Natuur: Oosterbeek; Amsterdam; 's-Graveland. ISSN 0024-1520, meer
The Dutch estuarine fish fauna comprises about 100 species, which can be grouped according to so called ecological functional guilds that make different use of the estuary. One of these guilds is the group of diadromous species, migratory fish which spend part of their live in fresh water and part in marine waters. Another relevant guild of estuarine fish are the resident species that can spend their whole life in brackish water and are tolerant to the dynamic and harsh estuarine environment.The fishes that inhabit our (former) estuaries have suffered by the changes that occurred in the transition between fresh and marine water. These were brought along by a number of developments. 'dry feet', 'safety' and 'human impact'. The Netherlands are a low-Iying country in the delta of the rivers Rhine, Meuse, Scheldt and Ems. Starting in the Roman era the sea level rise forced the inhabitants to raise artificial mounts and to construct dikes. The water that could flow to sea unhindered before now was discharged through sluices or by pumps. As a consequence the out flowing water reached high current velocities which surpassed the swimming capacity of fish, and the diadromous fish coming from the sea could no longer enter the inland fresh water bodies. Their life cycle was disrupted. On the other side, fresh water fish were flushed out to sea and were not able to return. Flounders developed ulcers in the vicinity of sluices.During the 2Oth century safety became a big issue, and the coastal defence was enforced. The impact of these changes are explained by the developments that occurred in the populations of two species, the anadromous Smelt (Osmerus eperlanus) (which spends most of his life in estuarine waters) and the Twaite shad (Alosa fallax).A box illustrates the trends in the incidence of ulcers in Flounder (P/atichthys flesus).
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