Publication Abstract

Spatial variation in catch rate and abundance of scallops in the Western English Channel

M.C. Bell and D.W. Palmer

In formulating stock assessment models for stocks of sedentary species, it is important to have a good understanding of stock spatial structure and distribution. Scallops are an example of a species that is patchily distributed over its grounds, with implications for interpretation of fishery statistics. We describe the results of experimental fishing for scallops in the western English Channel, aimed at identifying the spatial scales at which catch rates and abundance vary. An hierarchical survey design was used to examine catch rate variation at scales from 101 to 104 m. Catch rates at 54 locations grouped into six areas within two ICES rectangles varied from 0 to 148 scallops per dredge ha. Approximately equal amounts of variation were attributable to each level of the spatial hierarchy. Catch rates were strongly correlated with ground type, being almost 20 times higher on hard than soft ground. Subsequent depletion fishing showed that around half the variation in catch rates was due to differences in abundance between areas. The remaining variation was due to differences in gear performance. Gear efficiency varied between 8% on soft ground to 51% on hard ground. Future analysis of log-book data from scallop vessels will allow us to describe how targeting of fishing effort relates to these spatial patterns, and thus enable us to identify the appropriate spatial scale at which to consider fishery sampling data for stock assessment.

Reference:

M.C. Bell and D.W. Palmer, 2001. Spatial variation in catch rate and abundance of scallops in the Western English Channel. ICES 2001/J:03: 17pp