Estimating the trapping area and density of common whelk (Buccinum undatum) using experimental fishing

Abstract

For many species targeted by static gear fisheries, the relationship between catch per unit effort (CPUE) and density on the seabed is poorly understood. One aspect that is crucial in understanding this relationship is the area from which catches are drawn, known as the trapping area, yet this remains largely unquantified. In this paper we investigate the size of the trapping area for the commercially important common whelk (Buccinum undatum) by studying variations in CPUE in relation to the distance between pots. With declining spacing, the sphere of attraction will increasingly overlap, leading to reductions in catches and allowing the density of catchable animals to be estimated. Experimental fishing took place with pot distances ranging from 4 to 50 m, with catches recorded for over 500 pots over a week-long period. In addition to pot spacing, deployment location and haul date were significant factors influencing CPUE. For the study site, and based on environmental conditions at the time, the trapping area was estimated to be 120 m², indicating whelk pots spaced at least 12–13 m apart are likely to fish independently of each other. This resulted in a density estimate of 0.9 individuals m⁻², with spatial variation between 0.4 and 1.2 individuals m⁻². Results suggest pot spacings in the local fishery are sufficient to avoid interactions, and there is potential to predict densities from commercial CPUE, although further work is needed to understand variability in the size of the trapping area in relation to environmental and biotic factors.