The role of prey composition for the condition of grey gurnard (Eutrigla gurnardus)

Abstract

During a fisheries research cruise on the Dogger Bank in the central North Sea, we studied the role of diet for the condition of grey gurnard (Eutrigla gurnardus). Seventeen stations across the Dogger Bank were sampled with the GOV otter trawl in April/May 2006. Each fish sampled was categorised according to three feeding types: a predator specialised on fish, on invertebrates or having a mixed diet, depending on the percentage of fish and/or invertebrates in the stomach. These predator-prey categories were compared by means of three condition indices: Fulton’s condition factor, a hepatosomatic index (HSI) and a gonadosomatic index (GSI). Fulton’s condition factor provided evidence that a specialisation on fish prey increases the condition of grey gurnards in terms of the length-specific individual weight. In contrast, the HSI scored highest for predators feeding on a mixed diet. The GSI showed a tendency to be highest with a diet dominated by invertebrate prey, but differences between the three predator-prey categories were not significant. Based on the topography of the Dogger Bank, stations were allocated in shallow and deep regions. At shallow stations, the brown shrimp Crangon allmani and sandeels, combining basically the taxa Ammodytidae, Ammodytes juv. and Ammodytes marinus, were typically observed in stomach contents of gurnards whereas the amphipod Hyperia galba was predominantly found in gurnards sampled in deep regions.