Acoustic assessment of the austral sardine, Sprattus fuegensis, in the inner sea of the Chilean Patagonia

The austral sardine (Sprattus fuegensis) is the target fish resource of a small-scale fishery operating in the Patagonian waters of Chile (41°30'S-46°00'S). Purse seine boats composed the fleet and landings have fluctuated between 14,344 and 48,589 tons during the period between 2006 and 2014. The stock assessment of the austral sardine utilizes a Statistical Catch-at-Length model and this method requires independent estimates of abundance. In order to evaluate the abundance of the austral sardine in the inner sea of the Chilean Patagonia, an acoustic assessment was carried out in Autumn 2013. The study area covered the coastal zone of two administrative regions of Chile, the Los Lagos region in May 2013 and the Aysen region in June 2013. An EK-60 echo sounder permitted to obtain the acoustic data with transducer of 38 and 120 kHz from which echograms were examined to obtain NASC data. A total of 25 fishing set allowed to estimating the species composition and the length structure of the pelagic fish. In addition, fish confined in a jail allowed to estimating the target strength (TS) of the austral sardine. The fishing data and the NASC permitted to evaluate the acoustic biomass of the austral sardine, as well as the biomass of the anchovy (Engraulis ringens) and the common sardine (Strangomera bentincki). The biomass was estimated according to a stratified sampling design, the Gaussian Likelihood Geostatistics technique, and the fit to probability density models. The stratified design included six strata in the Los Lagos region and five strata in the Aysen region, as well as four depth strata (0-50, 50-100, 100-150 and 150-200 m). The stratified estimates of the austral sardine biomass were 118,173 tons (CV=11.6%) in the Los Lagos region and 101,630 tons (CV=9%) in the Aysen region. The estimates of biomass based on Gaussian geostatistics were 122,469 tons (CV=7.6%) in the Los Lagos region and 92,890 (70%) tons in the Aysen region. The best fitting of probability density functions to the acoustic density was log-normal, and allowed to estimate 100,145 tons of austral sardine in the Los Lagos region and 81,965 tons in the Aysen region. The biomass of anchovy was 50,079 tons in the Los Lagos region and 18,668 tons in the Aysen region. The biomass of common sardine was minimal with 5,327 tons in the Los Lagos region only. The estimate of the TS for the austral sardine was -67.59 dB and represented fish between 9 and 17 cm. The study area is spatially heterogeneous, and hydrographical features could determine behaviors of the austral sardine that have not yet described. Therefore, the estimates of biomass here obtained must be regarded as a relative abundance. The basis here obtained will be of importance for monitoring yearly changes in the abundance of the sardine austral.