Acoustic assessment of the austral sardine, Sprattus fuegensis, in the inner sea of the Chilean Patagonia Luis A. Cubillos 1,4 , Milton Pedraza 2 , Mariano Gutierrez 3 , Hern´ an Rebolledo 2 , Cristian Parra 4 Sandra Curin-Osorio 1,4 , Ma. Cristina P´ erez-Cuesta 1,4 , Claudio Carrasco 1,4 and Carolina Alarc´ on 4 1 Programa COPAS Sur-Austral, Universidad de Concepci´ on Casilla 160-C, PC: 4070043, Concepci´ on, Chile (Email: lucubillos@udec.cl) 2 Subsecretar´ ıa de Pesca y Acuicultura Bellavista 168, Piso 16, Valpara´ ıso (Email: mpedraza@subpesca.cl; hrebolledo@subpesca.cl) 3 Universidad Nacional Federico Villarreal, Facultad de Oceanograf´ ıa, Pesquer´ ıas y Ciencias Alimentarias, Calle Francia 726, Miraflores, Lima, Per´ u (Email : marianog@unops.org) 4 EPOMAR, Universidad de Concepci´ on, V´ ıctor Lamas 1290, Casilla 160-C PC: 4070043, Concepci´ on, Chile (Emails: sacurin@udec.cl, mperezc@udec.cl ccarrasco@udec.cl, calarcon@udec.cl) Abstract—The austral sardine (Sprattus fuegensis) is the target fish of a small-scale fishery operating in the Patagonian waters off Chile (41 ◦ 30’S-46 ◦ 00’S). The annual catches have fluctuated from 14,344 to 48,589 tons between 2006 and 2014. The stock assessment of the austral sardine utilizes a Statistical Catch-at- Length model which uses independent estimates of abundance. In autumn 2013, an acoustic assessment was carried out to evaluate the abundance of the austral sardine in the inner sea of the Chilean Patagonia. The study area covered the coastal zone of two administrative regions of Chile, the Los Lagos region in May 2013 and the Ays´ en region in June 2013. Acoustic data was obtained by using an EK-60 echo sounder with 38 and 120 kHz transducers. The echograms were examined with Echoview to obtain NASC data. A total of 25 fishing haul allowed the estimation of species composition and length structure of the pelagic fish. In addition, austral sardine specimens confined in a cage allowed the estimation of target strength (TS). The fishing data and the NASC permitted evaluating the acoustic biomass of austral sardine, as well as the biomass of anchovy (Engraulis ringens) and common sardine (Strangomera bentincki). Biomass was estimated according to a stratified sampling design, the Gaussian Likelihood Geostatistics technique, and then fitted to probability density models. 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 Ays´ en region. Estimates of biomass based on Gaussian geostatistics were 122,469 ton (CV=7.6%) in the Los Lagos region and 92,890 (7%) ton in the Ays´ en region. Probability density models, describing the acoustic density data, estimated 100,145 ton in the Los Lagos region and 81,965 tons in the Ays´ en region. The biomass of anchovy was 50,079 tons in the Los Lagos region and 18,668 tons in the Ays´ en 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 obtained here must be regarded as relative abundance. This study method will be of importance for annual monitoring of abundance changes in the sardine austral. I. I NTRODUCTION The austral sardine (Sprattus fuegensis), is the main fish exploited by a small-scale fishery operating in the inner sea of the Chilean Patagonia (41 ◦ 30’S-46 ◦ 00’S), with Calbuco (41 ◦ 73’S,73 ◦ 12’S) and Puerto Montt (41 ◦ 28’S,72 ◦ 56’S) as the main ports for landings in the Los Lagos administrative region of Chile. In addition, the common sardine (Strangomera bentincki) and anchovy (Engraulis ringens) small pelagic fish, which are caught as secondary species in the fishery. The austral sardine fishery started in 2006 when the species was correctly identified in the catches [1]. Subsequently, fishermen from the Ays´ en administrative region reported catches of the austral sardine, but catches have been low and sporadic. Between 27 and 39 boats per year operate in the Los Lagos region [2]. The total catch biomass has fluctuated between 14,344 and 48,589 tons from 2006 and 2014, majority of which is the contribution is due to the austral sardine, followed by anchovy and common sardine [2]. The fishery is managed by setting an annual quota on the ba- sis of a Statistical Catch-At-Length (SCAL) stock assessment model [2] [3]. Besides fishery-dependent information such as total catch, length composition of fish in the catch, and catch per unit effort, the model also requires fishery-independent data. According to the management cycle, the quota for the following year is decided in November of a given year. The quota is set based on the average recruitment and target fishing mortality estimated from the fishery-dependent and independent data. Once the quota is established, any further review requires new recruitment information. Therefore, it is important to develop a quick and independent assessment of abundance. One of the methods used to estimate austral sardine abundance is acoustic measurements of biomass [4] [5] [6] [7]. Thus, Niklitschek et al. [8] used this method to estimate the