Schooling in pelagic fish: An approach in three dimensions using vertical scanning multibeam sonar

In fish resource assessment, it is very important to know about the behavior and the form in that schools are occupying the space. The present paper describes the morphology and internal structure in pelagic schools using vertical scanning multibeam sonar. This sonar provides with data making it possible to reconstruct the school in three-dimensions. Acoustic surveys were carried out in Venezuela, Senegal, and Mexico. Geometrical (position of the school), morphological, density and internal structure parameters were extracted from a total of 668 schools: 257 from Mexico, 343 from Venezuela, and 68 from Senegal. The analysis of fish schools avoidance effect caused by vessels showed two modes in school frequency distribution in the case of Venezuela and Senegal schools and three modes for Mexico schools. This frequency distribution was decreasing as a vessel distance increase. In general, the schools show an amoeboid shape, which cannot be simply described with some geometric volume (e.g., spheres or ellipsoids). Also, it shows a contagious internal structure, i.e., fishes are not positioned at random inside the school. Another interesting characteristic is the presence of empty spaces inside the school that are called vacuoles.