Sandeel recruitment in the North Sea: demographic, climatic and trophic effects

The lesser sandeel Ammodytes marinus, which is one of the most abundant fish in the North Sea, has highly variable recruitment dynamics. We evaluate potential associations between their recruitment and some key biological and physical factors. Population data were analysed at the North Sea scale (years 1983 to 1999) and at the regional scale (southwestern section and northeastern section; years 1984 to 1996/7). Relationships between recruitment and spawning stock biomass were weak, but Age-1 sandeels (non-spawners) were related negatively with recruitment, implying that demographic effects are important. A negative relationship was detected between recruitment and the winter index of the North Atlantic Oscillation, which affects sea temperatures during the egg and larval period. Warmer sea temperatures correlated with poorer than average recruitment. This effect was most pronounced in the southwestern part of the North Sea, which is near the southern limit of the species' distribution. There was a positive association between recruitment and Calanus copepod (Stages V and VI) abundance around the time of larval hatching, suggesting that the availability of this prey species is important for young sandeel survival. Our findings suggest that climate change may impact upon sandeel populations in the North Sea.