MEPS 573:229-236 (2017)  -  DOI: https://doi.org/10.3354/meps12156

ABSTRACT: Over the last decade, North Sea stocks of lesser sandeel Ammodytes marinus have undergone a dramatic decline caused by intensive local fishing pressure. Lesser sandeel is a key species in the North Sea, supporting the largest fishery in this ecoregion. This species has developed the particular ability to burrow in sandy seabeds, while forming large pelagic schools to feed on zooplankton. In this study, we analyzed acoustic data from traditional and multibeam echosounders collected during several annual April-May acoustic surveys in sandeel grounds in the northeastern North Sea. Our results showed that schools in the upper pelagic zone maintain contact with the seabed refuge by forming persistent bridge-like collective structures consisting of several million individuals. Moreover, our simulation approach revealed that all large lesser sandeel schools likely have a seabed connection, and comparisons between conventional and multibeam echosounders showed that conventional echosounders markedly underestimate the occurrence of pelagic schools with a seabed connection. This unique adaptation of forming bridges, which act as natural anchors, likely plays an important role in lesser sandeel ecology, preventing post-settled individuals from being detached from suitable bottom substrate. The resulting limited horizontal migration also increases the vulnerability of lesser sandeel to fishery and other anthropogenic impacts.

KEY WORDS: Lesser sandeel · Ammodytes marinus · Collective behavior · North Sea · Acoustics· Anti-predator strategies