Spatio‐temporal distribution of early life stages of the European anchovy Engraulis encrasicolus L. within a European temperate estuary with regulated freshwater inflow: effects of environmental variables

Abstract

The spatio‐temporal distribution of the anchovy Engraulis encrasicolus in its early life stages was related to environmental conditions within the Guadalquivir River estuary (south‐west, Spain) by monthly field surveys during nine periods of recruitment (May 1997 to December 2005). Anchovy post‐larvae showed a consistent seasonal pattern of abundance throughout the study period: each year, a high anchovy recruitment period (HARP) occurred from late May to November, with a main peak in July, coinciding with the period when environmental conditions were more stable. Results of stepwise multiple regression analysis indicates that density of its main prey, Mesopodopsis slabberi, explained 45% of total anchovy density variance. Furthermore, after removing the effect of this variable, interannual differences in the anchovy estuarine recruitment were not significant. Clear spatial and tidal trends were also observed for both prey and predator: at each sampling date, density decreased from the outer to inner estuary; at each sampling site, density was higher during ebbs than during floods. Field distributions related to environmental gradients during HARP indicated that the populations of E. encrasicolus and M. slabberi seemed to hold a steady position at a point on the salinity gradient by longitudinal displacement within the estuary. Since freshwater management actions affect the longitudinal position of the salinity gradient as well as the input of nutrients to the estuary and adjacent coastal areas, it is hypothesized that this factor may also be relevant to anchovy recruitment.