Beschreibung: 1. As a preliminary step towards a fishery management strategy for the squid Loligo forbesi, indices of reproduction, egg production and recruitment were derived for this species in Irish (1991-93) and Scottish (1990-95) waters to give a quantitative estimate of peaks in spawning and fishery recruitment. 2. The indices were derived by combining landings or catch per unit effort (CPUE) data with the estimated proportions of mature squid (reproducers) and immature squid (recruits) to give an index of the number of squid spawning in and recruiting to the fished population each month. 3. The reproductive index peaked between November and December in Irish waters and between December and January in Scottish waters, earlier than predicted by simply examining peaks in the proportion of mature squid. 4. The index of recruitment showed peaks in the autumn in both Irish and Scottish waters, but there was also an additional April peak in Scottish waters. The two periods of recruitment in Scottish waters, together with a decrease in the size of mature females during the spawning season, are not compatible with a single breeding season and possible explanations for this are discussed. 5. By helping to define the timing of the fishing season (to avoid overfishing) and by providing estimates of natural mortality (for assessment of population size), the indices provide important input for population models for fisheries management of squid and short-lived fish species. 6. A possible model for a directed fishery for L. forbesi is proposed. In the model, the start of the fishing season is delayed to prevent growth overfishing and the season curtailed during spawning to prevent recruitment overfishing. However, on the basis of the indices developed here, this leaves only a narrow time window in which such a fishery could operate.

Beschreibung: Microsatellite DNA markers were applied for the first time in a population genetic study of a cephalopod and compared with previous estimates of genetic differentiation obtained using allozyme and mitochondrial DNA (mtDNA) markers. Levels of genetic variation detected with microsatellites were much higher than found with previous markers (mean number of alleles per locus=10.6, mean expected heterozygosity (HE)=0.79; allozyme HE=0.08; mtDNA restriction fragment length polymorphism (RFLP) HE=0.16). In agreement with previous studies, microsatellites demonstrated genetic uniformity across the population occupying the European shelf seas of the North East Atlantic, and extreme genetic differentiation of the Azores population (RST/FST=0.252/0.245; allozyme FST=0.536; mtDNA FST=0.789). In contrast to other markers, microsatellites detected more subtle, and significant, levels of differentiation between the populations of the North East Atlantic offshore banks (Rockall and Faroes) and the shelf population (RST=0.048 and 0.057). Breakdown of extensive gene flow among these populations is indicated, with hydrographic (water depth) and hydrodynamic (isolating current regimes) factors suggested as possible barriers to migration. The demonstration of genetic subdivision in an abundant, highly mobile marine invertebrate has implications for the interpretation of dispersal and population dynamics, and consequent management, of such a commercially exploited species. Relative levels of differentiation indicated by the three different marker systems, and the use of measures of differentiation (assuming different mutation models), are discussed.