Sardine (Sardina pilchardus Walbaum) characterisation off the Spanish Atlantic coast

In 1983 both Spain and Portugal begun to conduct acoustic surveys in the Atlantic waters of the Iberian Peninsula. The main goal for these surveys was the assessment of the main pelagic fish species, but focussed on sardine. Some years the surveyed area was extended as far as the distribution of blue whiting, but in general covering only the continental shelf. Since 1988 the Spanish surveys are undertook in spring, during the spawning period of this fish species. As most of the pelagic fish species sardine occurs in schools. From the school data base gathered during the acoustic surveys, the echo-traces were allocated into fish species following a criteria. This scrutiny is based on the fish proportion found at the fishing station, but a learning process was also used which consisted in relating school characteristics (shape and energy) and its location (geographical location, distance to the coast, sea bottom typology among others) with fish species. This paper describes the main characteristics of the sardine schools, which have been extracted manually from paper echograms from 1992 to 1997 surveys (except 1994). A series of variables for each school were obtained: position (latitude, longitude, time, distant to the next school, distant to coast, minimum school depth and total water column depth), morphologic (height, length, area and perimeter), energetic (school energy -SA value- and density -energy/ area-) and environmental parameters (temperature and salinity). These were described by box plot, scatter plot and other basic exploratory methods. From this analysis, differences among years and geographic areas have been found in sardine schools. The relation of such changes with the total sardine abundance estimates as well as the implication in the survey design were also discussed