A review of the past and present status of anadromous fish species in the Netherlands: is restocking the Rhine feasible?

The paper reviews the past, present and future of eight anadromous fish species inhabiting the Lower Rhine (The Netherlands), viz. -sturgeon (Acipenser sturio), whitefish and houting (Coregonus lavaretus, C. oxyrinchus), smelt (Osmerus eperlanus), allis and twaite shad (Alosa alosa, A. fallax), sea trout (Salmo trutta) and salmon (Salmo salar). All species are under threat or became extinct (e.g. sturgeon, allis shad). It is not possible to single out a specific factor for the decline or disappearance. A combination of factors is responsible, as the degradation of the spawning and nursery areas, river correction for shipping, building of sluices and hydropower dams, extraction of sand and gravel and river pollution. The likelihood that a species will return via natural recovery, or restocking is assessed. The return of the sturgeon is unlikely. Present observations of sturgeon can be attributed to releases of unwanted sturgeon hybrids. A natural stock of coregonids in Dutch waters seems not feasible any more due to irreversible habitat degradation. Present day catches originate from German releases. The anadromous smelt, heavily reduced in numbers, still inhabits some of our waters, the non-migratory smelt is still very common. The allis shad is extinct and unlikely to recover. The species never spawned in the Dutch part of the Rhine. Twaite shad, declining in numbers, are still observed in the lower reaches of Rhine and Meuse. Sea trout is presumably still present in the same varying numbers as before. Spawning in our waters has not been documented. The salmon, once fished in large numbers, is now the subject of restocking programmes in Germany. Observations of individuals can partly be attributed to these programmes but also to straying salmon. Restocking programmes should be considerably improved before noticeable success is to be met.