CAN WE DETECT THE EFFECTS OF ENVIRONMENTAL VARIATIONS ON FISH POPULATIONS THROUGH VPA OUTPUTS? THE NORTH ATLANTIC ALBACORE CASE

SUMMARY Past studies have indicated that numbers-at-age 1 of albacore may be correlated with the winter NAO index in the previous year. Two main hypotheses have been stressed to explain such observation, i.e. an impact of the NAO on the recruitment or on migration patterns and consequently in the availability and catchability of specific age classes to the surface fishery. The objective of the present paper is to test whether such processes can be distinguished using VPA outputs. We first computed simple cross-correlations between the NAO at various lags, catches-at-age, Numbers-atage and F-at-age estimated in the last assessment. Since results were inconclusive, simulations were undertaken. The results showed that the probability of detecting spurious correlations between a given environmental variable, such as the NAO, and VPA outputs appears to be strongly related to the level of errors due to the observation process and modelling assumptions made within the assessment procedure. We concluded that correlations based solely on VPA outputs should be interpreted with great care.