Acoustic assessment and distribution of anchovy and sardine in ICES Subdivision IXa South during the ECOCADIZ 2015‐07 Spanish survey (July‐August 2015) with notes on the distribution of other pelagic species

Abstract

The present working document summarises a part of the main results obtained from the Spanish (pelagic ecosystem‐) acoustic survey conducted by IEO between 28th July and 10th August 2015 in the Portuguese and Spanish shelf waters (20‐200 m isobaths) off the Gulf of Cadiz onboard the R/V Miguel Oliver. The 21 foreseen acoustic transects were sampled. A total of 19 valid fishing hauls were carried out for echo‐trace ground‐truthing purposes. CUFES sampling (117 stations) was carried during the survey in order to describe the extension of the anchovy spawning area. A census of top predator species was also carried out along the sampled acoustic transects. This working document only provides abundance and biomass estimates for anchovy and sardine which are presented without age structure. The distribution of all the mid‐sized and small pelagic fish species susceptible of being acoustically assessed is also shown from the mapping of their back‐scattering energies. Sardine was the most frequent species in the fishing hauls, followed by horse mackerel, chub mackerel, anchovy and mackerel. However, the most abundant species in these hauls was anchovy, followed at quite a distance by blue jack mackerel, sardine, horse mackerel and chub mackerel. As usual, the bulk of the anchovy population was concentrated in the central part of the surveyed area, with the smallest anchovies mainly occurring in the surroundings of the Guadiana and Guadalquivir river mouths and Bay of Cadiz, and larger/older anchovies occurring in the westernmost waters. The total biomass estimated for anchovy, 21.3 kt (2 506 million fish), was slightly below the historical average, but it still in the range of population levels featuring to a recovered population. The comparison of these estimates with their spring counterparts from the PELAGO survey evidences almost identical values for the Portuguese waters, whereas the ECOCADIZ survey estimated in summer at about 1000 million and 11800 t less of anchovy in the Spanish waters. Such differences might be attributable to a possible overestimation of the acoustic energy attributed to anchovy in the Spanish waters of the Gulf by the PELAGO survey because of the difficulties in the discrimination of anchovy echoes in this area from a dense plankton layer where the species was embedded. Sardine was widely distributed all over the surveyed area but in the easternmost waters closer to the Strait of Gibraltar and showed two main nuclei of density: the coastal waters of the central part of the Gulf, and the inner‐mid shelf waters between Cape San Vicente and Cape Santa Maria. Sardine yielded a total of 23.5 kt (883 million fish), population levels which have showed some recovery from the lowest historical values recorded in the two previous years but still below the historical average. In contrast to the abovementioned for anchovy, ECOCADIZ survey estimated in summer 4 fold more sardine in Spanish waters than PELAGO survey in spring, with the juvenile fraction being the dominant in both seasons. The progressive incorporation (recruitment) of juveniles coming from successive spawning events may be the reason for such seasonal differences. INTRODUCTIO