Vertical distribution and morphology of small pelagic fish schools in the Gulf of California

In the Gulf of California, small pelagic fish like sardines, anchovies and mackerels are of ecological and economic importance. A particular feature of these species is their tendency to form schools, which can be described by their size, acoustic density, position and location in the water column. The aim of this work was to analyse the characteristics of the Gulf of California small pelagic schools and their dynamics in the water column in two different seasons during the same year. The information from two acoustic surveys was obtained with a scientific echosounder SIMRAD EK60 (split beam, 38 kHz) during the winter (February) and spring (May and June) 2014. To analyse the degree of grouping in the schools, echo-integration by shoal was done using Movies+ post-processing software, obtaining variables corresponding to morphological, energetic and bathymetric aspects of the detected schools. The analysis allowed the recognition of 301 acoustic schools, the majority of which (81 %) were observed during winter, while 67 % were detected at night, and 23 % during twilight periods. Spring schools showed higher mean volume backscattering strength (S_v, dB re 1 m^-1) and a more compact morphology (lower length, height and area) compared to winter detections; only during night-time similar conditions of acoustic energy and morphology were observed in both seasons. The school vertical distribution in winter was similar to that commonly reported for small pelagic fish for other regions, where a great number of schools is observed at depth during the day. Winter detections were also observed at greater depths. On the other hand, during the spring, the diurnal schools showed higher values of S_v, nevertheless, in this season there seems to be no day and night differences between vertical distribution of the schools.