Unraveling the environmental drivers of blue whiting recruitment: 20 years of observations

Recruitment in marine fish is a complex process influenced by multiple ecological factors that often interact in unpredictable ways, making reliable forecasting challenging. Environmental variability further amplifies this uncertainty. This study analyzed the abundance of 0-year class blue whiting ( Micromesistius poutassou ) from the Spanish bottom trawl survey on the Porcupine Bank (Irish shelf) between 2001 and 2020, focusing on the exceptional recruitment event in 2020. We examined the effects of wind, chlorophyll- a concentration, salinity, temperature, and ocean currents during the spawning season, along with the spawning stock biomass (SSB). Results indicate that recruitment was primarily influenced by the wind-mixing index, chlorophyll- a concentration, and retention index, with no significant correlation to SSB. Although interannual variability in both environmental conditions and recruitment was high, the relationship between environmental factors and recruitment was not always predictable. For instance, while warm and saline years are generally associated with higher recruitment, the period 2002-2012 (characterized by warm and saline waters) only showed strong recruitment in 2003, 2004, 2007, 2009, and 2010. Conversely, although cold and low-salinity conditions are typically linked to lower recruitment, 2020 saw very strong recruitment despite the below-average SSB. Our findings suggest that the exceptional recruitment in 2020 resulted from a unique combination of favorable conditions. Unusually low wind conditions triggered the formation of a stable Taylor column circulation over the Porcupine Bank, promoting phytoplankton accumulation, as evidenced by the elevated chlorophyll- a concentrations. This likely increased food availability for larvae, while the Taylor column also acted as a retention mechanism for larvae and prey. Lagrangian simulations supported this retention hypothesis. Additionally, temperature and salinity conditions during the 2020 spawning season likely optimized the ascent of early life stages from spawning depths to the food-rich surface waters, improving larval feeding success and contributing to the historical recruitment event.