Stock assessments of English Channel loliginid squids: updated depletion methods and new analytical methods

The English Channel squid resource (ICES divisions VIId and VIIe) consists of two loliginids, Loligo forbesi and L. vulgaris , which are not distinguished by fishers. Catches are reported almost exclusively by French and British multispecies bottom trawlers. Monthly sampling in the Port-en-Bessin fish market provided biological data about exploited stages that allowed evaluation of catch per species, estimation of catch in numbers and analysis of length frequencies. New species-specific assessments of the 1993–1996 cohorts are presented. Two methods are used with different underlying assumptions. Depletion estimates of recruitment are based on analysis of an abundance index (cpue) in relation to cumulative catch. An analytical approach is also developed using polymodal decomposition of length structures in the context of a given growth curve to obtain catch-at-age data. The estimates derived from both methods show that recruitment can vary by a factor of five. Estimates of fishing mortality from cohort analysis indicate a relatively stable exploitation pattern between years for the dominant species. These parameters are integrated in a Thomson and Bell model to simulate production and biomass of the exploited populations and to generate a diagnostic of the impact of the fishery. For both species, observed exploitation levels were above optimum, but a reduction in fishing mortality would not have resulted in significantly higher yields.