Monitoring of Anchovy Schools by a Glider‐Mounted Echo Sounder in the South‐Eastern Bay of Biscay

ABSTRACT Effective monitoring of pelagic fish populations is crucial for sustainable fisheries management. Traditional vessel‐based acoustic surveys often lack the temporal coverage needed to capture dynamic fish distribution and behaviour in response to short‐term environmental drivers, such as gales. This study evaluates the reliability of an active‐acoustic glider for monitoring European anchovy ( Engraulis encrasicolus ) schools in the south‐eastern Bay of Biscay, Spain, aiming to complement existing vessel surveys. During the 2022 JUVENA sampling campaign (September–October 2022), concurrent glider and vessel acoustic data were collected. A novel glider‐adapted protocol for acoustic data processing was developed, revealing high consistency between both methodologies. Glider observations successfully captured anchovy school spatial displacement in response to a gale event. Anchovy exhibited a coastward migration of 4 to 6 km/day, which is two to three times faster than under stable conditions. This coincided with an initial intense episode of deepening (25 to 45 m) of the thermocline, accompanied by a gradual decrease in its depth during the entire campaign. Although the maximum depth of anchovy remained at 30 m, their vertical distribution shifted towards the surface during the gale and progressively deepened again afterwards, remaining above the thermocline. This study showcases a cost‐effective observational strategy for extended, uninterrupted monitoring, demonstrating the high potential of glider‐mounted echo sounders to enhance our understanding of fish distribution and behaviour in challenging conditions. Although further refinement of acoustic processing protocols and coordinated vessel‐glider campaigns are explored in this paper to optimize future missions, the configuration considered constitutes a robust base.