Publication Abstract

The effects of spatial targeting of fishing effort on the distribution of the Norway lobster, Nephrops norvegicus, on the Farn Deeps grounds, northeast England

Spatial targeting of fishing effort - is this a problem for assessing stocks of Nephrops norvegicus?

M.C. Bell, J.M. Elson and J.T. Addison

Fishers exploiting a sedentary stock are likely to visit the most profitable (highest catch rate) areas first. Simulation models demonstrated that this type of spatially non-random ‘foraging’ behaviour can cause the relationship between commercial catch per unit effort and overall population abundance to be highly non-linear. There is thus a risk that stock assessments based on commercial catch data may either exaggerate or be slow to detect important stock changes. Modelling predicted that a sedentary stock should show two measurable indications of effective targeting of fishing effort: (i) decreased spatial variance in density after fishing; and (ii) higher depletion rates in the areas of highest density. To find out whether either of these signs of targeting is apparent in an exploited stock of Nephrops norvegicus on the Farn Deeps Ground, NE England, we used underwater television surveys of burrow systems to describe the stock distribution before and after a winter fishing season. The spatial variance of burrow density was lower at the end of the fishing season, but not significantly so. However, there was a strong positive relationship between initial density and overwinter depletion of density. Above a threshold density of about 0.1 burrows .m^-2, overwinter depletion increased with density, consistent with fishing effort being targeted at the highest densities. Below this threshold, the constant level of overwinter depletion is assumed to represent natural mortality, yielding an estimate of M very close to the value currently assumed in stock assessments. We conclude that Nephrops fishers are able to find and exploit the highest densities of their target species. Our findings suggest that catch per unit effort statistics for this stock should be viewed at a finer spatial scale than is currently possible, and that stock trends inferred from commercial catch data preferably should be corroborated by fishery-independent surveys.

Reference:

M.C. Bell, J.M. Elson and J.T. Addison (2004) Spatial targeting of fishing effort - is this a problem for assessing stocks of Nephrops norvegicus?

M.C. Bell*, J.M. Elson* and J.T. Addison*

New Zealand Journal of Marine and Freshwater Research 39, 1023–1037