Seasonal distribution of cephalopod species living in the central and southern North Sea

During the International Bottom Trawl Surveys and International Beam Trawl Surveys from 1996 to 2003, ten cephalopod species were encountered in the central and southern North Sea in six families: Loliginidae (Alloteuthis subulata, Loligo forbesi, Loligo vulgaris), Sepiolidae ( Sepiola atlantica, Rossia macrosoma, Sepietta oweniana), Sepiidae (Sepia officinalis), Ommastrephidae (Todaropsis eblanae), Onychoteuthidae (Onychoteuthis banksii), and Octopodidae ( Eledone cirrhosa). Apart from A. subulata, none of the species lived in the investigated area in large numbers. In general, the central and southern North Sea is not a favourable habitat for cephalopods due to the shallowness of the water. The occurrence of individual species is further restricted by water temperature or salinity requirements, as shown by their seasonal migration patterns. These parameters depend on sea depth, time of the year, and influx of water from outside. Inside the central and southern North Sea, deep waters are relatively cool in summer but relatively warm in winter, while the shallow coastal waters of Belgium and The Netherlands are warm in summer and cold in winter. This explains the seasonal migration of Alloteuthis subulata, a species indigenous to the North Sea that prefers relatively warm waters. It migrates northwestwards before winter and southeastwards in late spring. A similar migration pattern exists for the two other species in the Loliginidae, Loligo forbesi and L. vulgaris. Temperature requirements of Eledone cirrhosa differ, and so does its migration pattern. For other cephalopods, water salinity rather than temperature is critical. Rossia macrosoma is restricted to the northern range of the central North Sea because it requires high salinities. Moderate salinities are endured by Todaropsis eblanae, a species that occurs in moderate numbers in the western part of the central North Sea. Low salinities are endured by Sepia officinalis and Sepiola atlantica, which spend part of their life in the estuaries and may even survive in water with a salinity of only 27‰. While Sepiola atlantica lives in the entire central and southern North Sea, Sepia officinalis is restricted to the coastal waters and estuaries. Numerous other cephalopod species occur in the deeper waters of the Atlantic and the Channel. Incidental findings of unusual cephalopod species in the central and southern North Sea, such as those of Onychoteuthis banksii in the present study, are likely due to animals being taken along with the invading currents of the Gulf Stream.