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In fish resource assessment, it is very important to know about the behaviour and form of fish schools. This paper describes the three-dimensional (3D) morphology and internal structure of pelagic schools observed using vertical-scanning multibeam sonar. The acoustic data were collected in waters off Venezuela, Senegal, and Mexico. The data were used to derive metrics of school location, density, shape and internal structure from a total of 668 schools: 257 from Mexico, 343 from Venezuela and 68 from Senegal. In general, school shapes were amoeboid-like (e.g. spheres or ellipsoids) and not simply geometric. Also, the fish were patchily-distributed, forming both nuclei (groups) and vacuoles (empty spaces) within the schools. The results support the hypothesis of 'auto-organization of the fish inside schools', meaning that the distribution of fish within a school arises from the elementary reaction of the individuals. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

The Multi-Beam Echo Sounder (MBES) is a tool that allows getting information about bathymetric, morphological and compositional characteristics of the seabed surface. Although this is still one of the most important features in hydrography, MBES can now also discriminate the acoustic imaging of the water mass by recording sampled reflectivity measurements along each beam. In this paper, two MBES, namely the Kongsberg Simrad EM3002D (300 kHz) and the Reson Seabed8125-H (455 kHz), which are generally employed to study seafloor characteristics, have been used for the identification of fish schools in the water column at very high resolution. In particular, a methodology to identify and analyse fish schools through the Water Column Backscatter (WCB) is presented. This methodology made it possible to obtain a wide variety of fish schools shape in 3-D (in our case, anchovies and sardines), which allowed a first study on the ecological behaviour of the schools according to their shape. All data were collected along with Kongsberg Simrad EK60 scientific echo sounder, allowing also a comparison of different approaches to fish schools identification. The data presented in this paper were collected during two oceanographic surveys within the project MEDIAS (MEDiteranean International Acoustic Survey), Ancheva 2010 and Ancheva 2011. Beyond the mere visualization of school shapes, we also attempted to gain numerical values of the school volumes and to realize a more detailed study of anchovies and sardines school shapes. In line with the current literature, this study showed that there are three main factors influencing the shape of a school, namely the depth of the bottom, the time of the day when data are acquired and the density of individuals in the investigated area. [ABSTRACT FROM AUTHOR]