Possible biases in experiments evaluating the consumption of discards by seabirds in the North Sea

Discard utilization by seabirds in the North Sea has been quantified by so-called discard experiments on board fishery research vessels. In this paper we evaluate possible biases in this method, although discard practices vary considerably between commercial fishing fleets, seasons and areas. Whereas the total consumption of discards remained relatively unaffected by the different phases of the fish catch procedure, the length of fish discarded, the different methods of compiling the data and the quantities of fish discarded at once all significantly influence the results. The differences were particularly apparent between single- and multi-item trials in rapidly sinking fish. Correction factors for single-item experiments are suggested. Even more obvious were differences between the methods when focusing on the proportions obtained by the different bird species. Correcting the results of discard experiments on research vessels as presented in this paper appears necessary when quantifying the consumption of discards by seabirds in the North Sea. Received: 10 September 1997 / Accepted: 9 March 1998