Environmental impacts on recruitment migrations of Patagonian longfin squid (Doryteuthis gahi) in the Falkland Islands with reference to stock assessment

Doryteuthis gahi squid stocks in the Falkland Islands are assessed by depletion models which may include in-season recruitment pulses. Occurrences of in-season recruitment pulses are inferred by sharp peaks in CPUE, but this ad hoc approach does not distinguish between true recruitments into the fishery and aggregations of squid already in situ. Data were therefore investigated from 13 fishing seasons in which true recruitments could be identified by changes in D. gahi size distributions, and related to environmental variables of sea wind, sea surface temperature, and geostrophic currents. Changes in size distributions showed 30% of CPUE peaks to be true recruitments. Recruitment peaks followed a significant parabolic relationship between east-west and north-south sea wind vectors. Recruitment peaks occurring under west sea winds were characterized by a positive correlation between west and north wind speeds. Recruitment peaks occurring under east sea winds were characterized by a positive correlation between east and north wind speeds. The wind speed criteria were used to classify CPUE peaks in all D. gahi fishing seasons from 2002 to 2012. Among winter seasons, D. gahi abundance estimates correlated best with CPUE when depletion models included all recruitment peaks. Among summer seasons, which are shorter, abundance estimates correlated best with CPUE when depletion models included only the last overall peak. We hypothesize that identifying recruitment peaks improves the accuracy of depletion models, but time series must be sufficiently long for multi-peak models to fit optimally.