Temperature history determines the adult size and maturation rates in seasonal cohorts of Loligo forbesii squid

We hypothesised that the large variation observed in squid life-cycle traits, such as growth rate and size and age at maturity, is a consequence of extended breeding throughout the year. Using data from Scotland and Ireland, we tested whether hatching date, specifically exposure to a cooling (October to April) or warming (May to September) temperature period could be associated with these traits in Loligo forbesii, a commercially fished squid species. In agreement with predictions, squid hatched in the warming temperature period matured at smaller size and age and grew more slowly than those from the cooling temperature period, in both sexes. Squid hatched in the cooling temperature period were further sub-divided into those which hatched in October – December and matured at large size/age, and those which hatched in January - April and were mature over a broad range of sizes and ages, although both groups were significantly larger/older than the warming temperature group. All three hatching groups also varied in their timing in the fisheries catches: small/young mature squid were caught from January until May, large/older mature animals were caught in late October to December and the mature animals with variable sizes/ages were captured over a longer period, from October to the end of April. This agreed with a Length-at-Maturity (LM50) time series from Scotland, which showed highest LM50 in squid caught in November, and lowest values in squid caught in February/March. Hatching during warming temperatures (May - September) was mostly absent in Ireland.