Population structure of the Argentine shortfin squid Illex argentinus (Castellanos 1960, Family Ommastrephidae)

Cephalopod fisheries have increased in importance over the last decade, partly due to reduced yields of finfish fisheries. The Argentine shortfin squid Illex argentinus is the target of one of the largest fisheries in the Southwest Atlantic. This species is characterised by large annual population fluctuations and a complex population structure that consists of up to four seasonal spawning groups. Illex argentinus has a wide geographic distribution range (22˚S to 55˚S) and individuals undertake long-range ontogenetic migrations. This species is exploited by numerous coastal states and on the high seas where harvesting is unregulated, making it a “straddling stock”. All these factors and characteristics combined make I. argentinus particularly vulnerable to exploitation. Therefore, the overall goal of this thesis is to disentangle how environmental factors affect key population characteristics and investigate the population structure of I. argentinus to inform future management of the fishery. The first chapter of this dissertation investigated the response of I. argentinus population- and individual-level characteristics to changing environmental conditions, namely temperature. The second chapter investigated the connectivity between the seasonal spawning groups of I. argentinus, particularly whether seasonal spawning groups identified through statolith ageing and microstructure represented a single interbreeding genetic metapopulation. The third and final chapter of this dissertation aimed to reveal differences in the early ontogeny of I. argentinus spawning groups. The findings of this dissertation confirmed the high level of spatial and temporal population connectivity, and plasticity of I. argentinus in the Southwest Atlantic