The function of the Wadden Sea in the life cycle of small pelagic fish

Most fish species require different habitats to complete their life cycle, with coastal areas often playing a crucial role as nursery areas for juveniles. The Wadden Sea is an important nursery area for juvenile fish in the North Sea ecoregion. Despite extensive research on the nursery function of the Wadden Sea for demersal species, its role in the life cycle of small pelagic fish (SPF) remains largely unknown. This limits our understanding of the Wadden Sea ecosystem, as SPF are the dominant component of the overall fish biomass and serve as important food source for higher trophic levels. We studied the role of the Wadden Sea in the life cycle of Atlantic herring ( Clupea harengus ), European sprat ( Sprattus sprattus ), European smelt ( Osmerus eperlanus ) and European anchovy ( Engraulis encrasicolus ) through monthly stow net and seasonal trawl surveys. Our study showed that SPF use the Dutch Wadden Sea primarily as juvenile habitat, with herring being the dominant marine juvenile representative. Length frequency distributions and genetic analysis revealed that the juvenile herring originated predominantly from southwestern waters, such as the English Channel. Additionally, the Wadden Sea still provides spawning grounds for herring and anchovy, with no substantial spawning observed for sprat. While smelt can complete nearly its entire life cycle in the Wadden Sea, it depends on connectivity to nearby freshwater for spawning. In summary, the Wadden Sea functions as a nursery for juveniles, and to a lesser extent as spawning grounds for adults. A thorough understanding of these functions is crucial for identifying bottlenecks and implementing effective conservation and management strategies. • First extensive study on the use of the Dutch Wadden Sea by small pelagic fish (SPF). • SPF, dominated by herring, use the Wadden Sea mainly as juvenile habitat. • Herring originates mainly from southwestern waters, such as the English Channel. • The Wadden Sea still provides spawning grounds for herring and anchovy. • First step in assessing importance of Wadden Sea as nursery area for SPF.