Fish movements and schooling behavior across the tidal channel in a Mediterranean coastal lagoon: An automated approach using acoustic imaging

•We proposed a method for fish monitoring in transitional waters based on an acoustic camera system coupled to an automated image processing.•Fish movements across the tidal channel in a Mediterranean lagoon were described, quantifying fish numbers and schooling behavior.•Large and small fish schools occur overnight following different hourly and tidal patterns.•Acoustic sonar operating underwater at high frequency is a reliable tool for fish long-term monitoring in lagoons. A method for fish monitoring by high-frequency acoustic camera, coupled with an automated method for counting fish, was implemented that allowed to analyze very large amounts of data with reduced costs. The acoustic camera was mounted in the tidal channel of the Caprolace lagoon (central Italy) and programmed for 12 h. continuous recording at night time, between October 2016 and February 2017, for a total of 413 h, that were automatically processed by a specific software routine. A total of 266,717 fishes passed across the acoustic cone of the camera, and two typologies of schooling events were documented, based on fish numbers making up the groups. Large (n° ≥ 10 fish) school events (n = 884) maximum density occurred in early morning hours, in coincidence with low tide; on the other hand, small (n° ≥ 5 and < 10 fish) schools were more numerous (n = 5,394), and did not show a clear distribution pattern related to night hour and tidal phase. The individual acoustic area was different in the two schooling typologies, average single fish size being significantly higher in large schools. Acoustic images recording effectiveness was tested by evaluating fish movements dynamics between a Mediterranean coastal lagoon and the adjacent sea. The method proved to be an efficient, versatile and portable system for fish monitoring in coastal lagoons and in general in transitional environments, and it allowed to describe schooling behavior and abundance of marine migratory fish during the winter migration season.