Can length structure of English Channel cephalopod landings reveal trends in exploitation?

No abstracts are to be cited without prior reference to the author.

The need of alternative methods for the assessment of fishing pressure is often advocated in Cephalopod stocks in relation with their short life span and presumably variable life traits. English Channel Loliginid squid and cuttlefish are among the first Cephalopod resources in the Northeast Atlantic. English Channel fishery data collection programmes have already enabled to apply slightly modified versions of 'classical single species methods' like monthly cohort analysis. The present study revisits landings length frequency sampling (which is carried out at the Port-en-Bessin fish market and now extends over a number of months) and analyses if length structures can reveal changes in cohort exploitation patterns. Changes in length structure are explored via multivariate correspondence analysis (CA) in which variables (i.e. length-classes) and records (i.e. sampling dates) can be plotted simultaneously. The largest dataset concerns Loligo forbesi with 11 fishing seasons sampled consistently from June-N to march N+1 with length classes ranging from 7 to 50+ cm DML). Factorial analysis splits differences in length structures into a series of independent factors of decreasing importance. Factor 1 describes changes in modal length and underlines that the main source of variability in length data is seasonal modal progression. Factor 2 reveals differences in length variance and projections in the F1-F2 plane show an arch effect (which is even clearer in Loligo vulgaris). Interannual differences between fishing seasons are less clear. Length-structure changes are discussed in the light of recruitment strength and fishing mortality estimates (in previously assessed cohorts). Fishing mortality and recruitment also show month-to-month variations that correspond to specific length structures. However, length-structures do not seem adapted to predict inter-annual changes in the fishery and no significant relationship could be found at the inter-annual scale.