Key environmental factors influencing the early life history of clupeoid fishes, adding nutrients and heat

Abstract

The present PhD thesis focuses on the potential impacts of human-induced environmental changes on the early life history of clupeoid species in the North Sea and Baltic Sea. In addition to the more direct effects of fisheries, other anthropogenic impacts like eutrophication, habitat degradation or global warming directly and indirectly influence all six respectively four clupeoid species which inhabit the temperate waters of northern Europe. Allis shad (Alosa alosa), Twait shad (Alosa fallax), sprat (Sprattus sprattus), herring (Clupea harengus) are found in the North and Baltic Sea while European sardine (Sardina pilchardus) and European anchovy (Engraulis encrasicolus) only occur in the North Sea and to bsome extent in the western Baltic Sea. It is projected bthat some of the anthropogenic impacts will increase in the future (e.g. warming) while others will remain at high level (e.g. habitat degradation, eutrophication) for a longer period of time. How these factors actually lead to stock changes, for example in abundance and distribution, is often not resolved in sufficient detail. The present dissertation aims to investigate the impacts of spawning habitat degradation in one of the key spawning areas of Atlantic herring in the Baltic Sea and to analyze the influence of warmer water temperatures on early life stages of this species. Additionally, the situation in the southern North Sea is investigated, where nowadays larvae of two “new” clupeoid species (anchovy and sardine) can be found within catches of sprat and herring.