Three-dimensional structure and avoidance behaviour of anchovy and common sardine schools in central southern Chile

We studied the avoidance behaviour and three-dimensional (3-D) structure of anchovy (Engraulis ringens) and common sardine (Strangomera bentincki) schools mixed in high concentrations in a coastal area of central southern Chile. Observations were carried out during an acoustic survey in January 2002 by means of a vertical echosounder and a multi-beam sonar. The sonar harvested around 900 series of 3-D school images, and 3000 2-D school images were collected with the echosounder. The results showed that all fish aggregations presented the same internal structure, but different global morphologies, from single small schools (with length three times the height) on the edges of the distribution to large dense layers (length more than seven times the height) in its centre. Observation of avoidance in the vertical and horizontal planes indicated that limited vertical diving occurred close to the ship (fish dive from the surface to the 5-10-m depth layer below the vessel), while no horizontal avoidance was observed.