Migratory fish: twaite shad in the Westerschelde
Twaite shad is a migratory fish that went extinct in the Netherlands in the early 20th century. But thanks to an improvement in the water quality, twaite shad are once again swimming in the Westerschelde.
Trends until 1930
The twaite shad is a herring-like fish that spends most of its life in the open sea but swims up estuaries and rivers to spawn. It breeds in the main channel of the river, at the fresh/saline interface. In bygone days the fish used to breed on the sand flats at the mouth of the Schelde's secondary channels (the Rupel and Dender). In the 16th and 17th centuries the twaite shad was intensively fished, not only in the Westerschelde but also in other Dutch rivers.
The twaite shad population has fallen sharply since the beginning of the 20th century because of worsening water quality and the disappearance of the large sand flats on which the fish spawned. It is thought that the twaite shad disappeared from the Schelde in the 1930s.
Trends since 1990
Twaite shad has been recorded again in the Schelde since the 1990s, as can be seen from the data on catches made with a hoop net in the Westerschelde near Bath and in the cooling water from the Doel nuclear power station (see graph). The probable reason for the return of the fish is improved water quality. Numbers of the twaite shad continued to increase until 1999. Thereafter there was a downturn, but in 2002 the catch was large.
The exact cause of the sharp fall in 2000 and 2001 is not known, but it is known that numbers of juvenile twaite shad do fluctuate greatly in nature. It has been established that in the years with small catches the migration peak in May was shorter but stronger than in previous years. When the migration peak is like this, the netting method used is not adequate to obtain proper samples, so the numbers in such years are probably underestimates. Even though the twaite shad is increasing, it is unlikely to breed in the Schelde because of the absence of suitable spawning grounds.
The twaite shad is on the Red List of freshwater fishes.
