Seasonal variation in the consumption of food by fish in the North Sea and implications for food web dynamics

Abstract

Previous North Sea food web studies are reviewed. These studies used estimates of primary production and fish energy requirements, then manipulated the energy flow pathway between the two to balance the budget. The resulting pathways largely ignored actual fish diets and consumption rates. In the present paper, North Sea biomass flow food webs are constructed for each quarter of the year from published estimates of fish abundance, diet and daily food consumption, and using long-term Continuous Plankton Recorder and North Sea benthos survey data. Estimates of daily production of each component of the web are derived from specific daily growth and gross growth efficiency measurements. The flow of biomass from secondary production to fish is sufficient to supply the food requirements of planktivorous pelagic fish and benthivorous demersal fish. Piscivorous pelagic fish obtain much of their food requirements from outside the North Sea. The food requirements of piscivorous demersal fish also do not appear to be adequately supplied from within the North Sea, but, in this case, immigration is not thought to provide the shortfall. The high predation pressure on demersal piscivores may explain why this group appears to be especially vulnerable to fishing. The supply of biomass from primary production to secondary producers appears to be sufficient without the need to postulate import into the North Sea. Indeed, the supply to the benthos is such that a detritivore trophic level can be inserted between organic settlement and production of macrobenthos.