Spatial and temporal variation in age and growth in juvenile Loligo forbesi and relationships with recruitment in the English Channel and Scottish waters

In many cephalopod stocks, resource abundance and fishing yields depend on the recruitment of the annual cohort. Loliginid squids such as Loligo forbesi are present in the English Channel and in Scottish waters. Variation in age and growth in juvenile Loligo forbesi of these two areas was investigated at the spatial and temporal scale, and inter-annual differences in growth were related to recruitment variation to test density dependence in squid growth in both populations. Spatial and temporal differences in growth were analysed in five cohorts that showed marked differences in recruitment abundance. Biological sampling showed that recruitment occurred in late autumn and spring in Scottish waters and in summer in the English Channel. In both fishing seasons, monthly samples collected at fish markets were analysed and ages of juveniles (mantle length < 200 mm) were determined using daily statolith growth increments. Age determination indicated that recruits were older than previously thought (about 8 to 11 months). Back-calculated hatching dates were used to estimate growth variation during the pre-recruitment stage. Exponential growth models adequately described size-at-age data. Linear modelling demonstrated inter-annual and spatial significant differences in growth rates. Influence of the hatching month (within or between cohorts) on growth was detected. To improve our understanding of recruitment variability, this study addresses the question: Does early growth vary in relation with recruitment? Available recruitment estimates appeared to be related to annual growth rates; density dependence in squid growth is suggested for the English Channel population.