ESR 20:109-119 (2013)  -  DOI: https://doi.org/10.3354/esr00490

ABSTRACT: After a serious decline in the past, some twaite shad Alosa fallax populations have significantly recovered during the last decade. Although increasing numbers of the species occur in areas of the southern North Sea, detailed quantitative analyses have not been published on spawning stocks of specific North Sea estuaries. In this study, the development of the twaite shad spawning stock of the Elbe estuary, its spawning period and spawning area were determined using abundance data, gonad maturity information and the gonadosomatic index. Abundances were compared between the periods 1992-1993 and 2009-2010, and environmental parameters were included in the analysis. Abundances have increased significantly in recent years, which could be a result of more favourable climatic conditions. The spawning period, from at least early May until mid-June, has not changed since the early 1960s; however, an abundance increase in April suggested a compact immigration of spawners. The spawning area was located in the middle and upper estuary, but has been extending downstream since the 1990s and is now found in freshwater as well as in areas of low salinity (0.5 ? 0.1). This might be caused by significantly lower salinities in the middle and upper estuary in comparison with the conditions in the 1990s. The current vulnerability status of twaite shad in the German North Sea region should be re-evaluated; however, stocks should be monitored continuously to ensure maintenance of the population.

KEY WORDS: Twaite shad · Anadromous fish · Population recovery · Elbe estuary · Spawning period · Spawning area