Population indices of reproduction and recruitment in Loligo forbesi (Cephalopoda: Loliginidae) in Scottish and Irish waters

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.