Environmental drivers of spawning and recruitment of anchovy in the Bay of Biscay

To prevent future collapse events as observed in the mid-2000s, environmental and biological drivers of anchovy (Engraulis encrasicolus) spawning habitat variability in the Bay of Biscay need to be better understood and monitored. To this end, this study uses an extended time series of egg density observations, new high-resolution Essential Ocean Variables (EOVs), and complementary statistical and mechanistic modelling approaches. The relationship between the spawning habitat and the adult spawning stock biomass (SSB) is also analysed. A delta generalized additive model (delta-GAM) and a mechanistic SEAPODYM-Hs model were applied to evaluate the relative roles of temperature, prey availability, predation, spawning stock biomass (SSB), as well as species biology (seasonal reproduction) and behaviour (coastal attraction). Bottom temperature emerged as a more relevant indicator of spawning habitat suitability than sea surface temperature, possibly reflecting conditions during spawning or egg development. Zooplankton availability and more particularly micronekton predation, essentially by migrant mesopelagic species, were identified as key mechanisms shaping egg mortality and the spatial distribution of successful spawning between coastal and slope areas. However, the seasonality of the reproduction remained essential to account for the monthly spatial distribution, whereas SSB was the primary predictor of recruitment interannual variability. Low SSB combined with persistently unfavourable environmental conditions over the species’ lifespan (~3 years) increased the risk of population collapse as observed in the mid-2000s. The mechanistic model, despite its parsimonious parameterization, showed strong predictive skills comparable to that of the delta-GAM. The resulting index (Hs) provides a robust indicator of the suitability of environmental conditions for anchovy spawning habitat and can be predicted in real-time using the Copernicus marine service variables.