Inverse patterns in abundance of Illex argentinus and Loligo gahi in Falkland waters: possible interspecific competition between squid?

Abstract

Fishery statistics for two abundant commercial squid, Illex argentinus (Ommastrephidae) and Loligo gahi (Loliginidae), in Falkland Islands waters (southwest Atlantic) between 1987 and 1999 were analysed. The areas of the highest densities of the two species are usually separated, with I. argentinus most abundant to the north-west of the Islands in February–May and L. gahi to the south-east in February–May and August–October. However, in some years, I. argentinus intrude in great numbers into nursery or feeding areas of L. gahi in April and May. It could, therefore, affect, either directly (via predation) or indirectly (by competition for food), the abundance and recruitment of the second cohort of L. gahi, exploited around the Islands between April and October. Catch per unit effort (CPUE) of I. argentinus in February–March did not correlate with that of L. gahi in February–May. In contrast, the CPUE of I. argentinus in April–May was negatively correlated with that of L. gahi in April–May and August–October of the same year. Possible reasons for these inverse abundance patterns of the two squid species, and their implications for fisheries management are discussed.