Predation of whiting and haddock on sandeel:: aggregative response, competition and diel periodicity

The predation of haddock Melanogrammus aeglefinus and whiting Merlangius merlangus on sandeel Ammodytes marinus was investigated based on intensive sampling [performed with a bottom trawl (GOV) equipped with a small meshed codend cover; sediment samples were taken with a van Veen grab] in a restricted area of' c. 15 x 20 nautical miles in the northern North Sea during a 5 day period in July 1996. The analysis of the spatial distribution of predators revealed a pronounced aggregation of whiting in the south-west part of the area, where sandeel catches were also highest. This pattern was thought to be the result of all aggregative behaviour of whiting. The sandeel concentration most likely reflected a restricted patch of coarse sediment of the preferred grain size for sandeels to bury in. In haddock the aggregation was less obvious. Both predators fed almost exclusively on sandeel in the southwest part of the area with haddock stomachs containing more sandeel than whiting stomachs. The stomach contents in both predators increased rapidly during the night, indicating that the predators were targeting burying sandeels. This would explain the competitive advantage of the benthivorous haddock. (C) 2004 The Fishenes Society of the British Isles.