Bass distribution has expanded north in the past 25 years in response to a series of warmer summers, mild winters and against a general background of rising water temperatures. This also indicates the establishment of new local spawning grounds in the southern North Sea.
Due to the northward expansion of bass populations, seasonal migration occurs less frequently in recent years. Research has revealed that more and more adult bass seem to hibernate and presumably also spawn in the southern North Sea. Indicators for this assumption are the discovery of early stage bass eggs and bass larvae around the Dogger Bank and west of the Voordelta (the coastal area located to the west of the islands of the Dutch provinces of South Holland and Zeeland, around the deltas of Haringvliet, Grevelingen and Oosterschelde) during IMARES surveys. The idea that bass spawn in the Dutch and Belgian part of the North Sea is confirmed by market sampling research of landed bass in the Netherlands (Bierman et al. 2010).
If bass spawning in these areas is an annual recurring phenomenon, is not clear. The new, more northern spawning activity is likely to be much more dynamic in terms of timing and location, reflecting changing conditions in the southern North Sea. Moreover the presence of bass larvae in the southern North Sea is difficult to interpret. Either they originate from the English Channel or from closer to the shore. It is difficult to establish what’s what as the standard larvae and egg surveys have two big deficiencies. Firstly they are not conducted during spring (April and May), which is bass spawning season in the southern North Sea. Secondly they don’t cover enough possible spawning grounds. The area around the Dogger Bank is only partially surveyed and the shallower coastal zones are a blind spot as no surveys have been carried out there.
