Resolution of multiple cohorts in the Loligo forbesi population from the west of Scotland

Length-frequency analysis was employed to resolve multiple cohorts in the L oligo forbesi population from coastal waters to the west. of Scotland. In b oth male and female squid two principal cohorts were identified, recruiting to the fished population in April and November. Both cohorts spawned durin g the winter months (November to April), producing a single extended spawni ng season, with the April recruits of larger size at spawning than the Nove mber recruits. In the males the April recruits appeared to separate into tw o cohorts of different growth rates, thus producing three rather than two s izes at maturity. The abundance of L. forbesi was low during the summer mon ths, indicating that the squid had moved outside the range of the fishery, perhaps into deeper water. Abundance declined during the spawning season, w hich is interpreted as post-spawning mortality. Abundance declined earlier in the males than females, producing a female biased sex ratio during most of the spawning season. The two periods of recruitment are not compatible w ith a 1 year life cycle and single extended spawning season and explanation s of this are discussed. (C) 1999 International Council for the Exploration of the Sea.