Fishery biology of the squid Loligo forbesi Steenstrup (Cephalopoda: Loliginidae) in Scottish waters
A total of 6512 specimens of Loligo forbesi Steenstrup (Cephalopoda : Loliginidae), obtained from Scottish fishing ports and research vessels during 1984/86, was used to investigate aspects of fishery biology of the species in Scottish waters. L. forbesi exhibits sexual dimorphism in size, males had a larger overall mean size (DML, weight) than females, and for each sex the mean size was larger in those from the north-eastern Atlantic than in those from the North Sea. In length-weight relationship, females had a higher regression slope than males and this relationship for squids from Rockall was significantly different to those from the North Sea. Multivariate analyses on measurements of body parts showed that Rockall squid, also, exhibited less variation in body form but distinct differences from those of the west coast and North Sea. Recruitment of L. forbesi occurred during April/May and November/December, the monthly growth rate after recruitment varied between 12.5 to 28.6mm DML/month for males and 12.5 to 17.5mm DML/month for females; for each sex the November/December cohort had lower growth rate. The logistic growth model best fitted the progression of modes of monthly length frequencies. Age determinations estimated from statolith markings varied from under 3 months for 30mm DML individual to 28 months for 571mm DML male individual. Sex ratio in L. forbesi varied significantly from 1:1 throughout the year with males exceeding females except during winter when the number of females may be equal to/or more than males. Mean size at maturity was 205mm DML for males and 192mm DML for females. Spawning occurred during December to March and the peak spawning period occurred in February throughout the study area. In this species, maturity coefficient and gonad-somatic index were the best indices for identification of spawning season. Fecundity is related to size and the average number of eggs per female was 6500.High proportions of specimens with 'empty' stomach were observed during the winter. The prey of L. forbesi was fish, crustaceans, molluscs and polychaetes in decreasing order of incidence. Identified fish prey were Argentina sphryaena, Trisopterus esmakii, Merlangius merlangus and Ammodytes sp.