Wind forcing of crab megalopae recruitment to an estuary (Ria de Aveiro) in the northern Portuguese upwelling system

Abstract

Summary Megalopae of the common shore crab, Carcinus maenas (L.), were sampled in the Ria de Aveiro, during spring and summer of 1991. At weekly intervals, plankton samples were collected below the surface, at mid-water and above the bottom during night and day flood tides, with the use of plankton nets. In order to evaluate the relationship between wind speed and direction and input of megalopae to the estuary, a complex correlation coefficient between megalopae concentration and average daily wind stress was calculated. Maximum values of the correlation coefficient were obtained when the concentration of megalopae was lagged after wind stress by 3 d. The direction of the correlation was 2°, meaning that input of megalopae to the estuary increased as southward wind stress decreased. High densities of megalopae were not associated with strong southward wind stress. Correlation between input of megalopae to the Ria de Aveiro and wind stress improved when a cumulative wind stress vector was used, which was constructed as the sum of the wind stresses for time lags of 2, 3 and 4 days before the input of megalopae. The results are consistent with onshore transport of megalopae over the shelf following relaxation of southward, upwelling favourable, wind events.