Hydroacoustics estimates of sardine (Sardinella brasiliensis) in the Brazilian coast, sampling strategies and the effects on fish distribution and biomass

Abstract

Biomass assessment of small pelagic fish stocks with acoustics depends on the use of scientific echo sounders that transmit/receive acoustical energy, quantify the echoes and generate the parameter Nautical Area Scattering Coefficient (NASC), proportional to the density of the live targets detected in the water column and within the acoustic beam. In order to convert the acoustic density parameter NASC into biological density, it is necessary to identify the organisms detected acoustically, at the level of species, as well as the proportions, in number, between species. NASC is then decomposed into biological density as a function of the species dependent Target Strength (TS) parameter and the proportion among species, within body size classes, that generated the backscattered energy. Therefore sampling along ship track is intrinsic to the method and the mid water trawl is considered the equipment that best meets the needs of biological sampling during acoustics biomass cruises. Between January 2008 and March 2010, four surveys were conducted to estimate the stock biomass of the Brazilian sardine (Sardinella brasiliensis) aboard Oc. V. Atlantico Sul. S. brasiliensis is a small pelagic fish with fusiform body, great swimming ability and consistent avoidance reactions. In all cruises we operated a calibrated SIMRAD EK 500 scientific echo sounder. In cruise I (2008) sampling strategy was based on a mid water trawl hauled from the research vessel. A total of 22 fishing hauls were performed and in only five, sardines were caught, representing just 2.25%, in number, of the total catches. Data processing generated an estimated biomass of 358 t of sardines, incompatible with the 74,630 t caught by the fishing fleet in 2008. Observations during the hauls indicated the escapement of sardine, swimming in front of, or avoiding the net to upper or deeper depths, and not being caught. Such observations demonstrated that the used mid water trawl was not efficient for sardine in that area with that vessel, which defined the need of a more efficient sampler. In Cruise II and III tests were performed with a purse seiner equipped with multi beam sonar. This boat sampled fish schools reported from the research vessel detections with the scientific echo sounder. Purse seiner participation was restricted and only 2 and 3 samples were taken in Cruise II and III respectively, in 2009. Sardine biomass estimates were 12,709 t and 27,928 t respectively, still below the reported year catch of 83,286 t. In 2010 we finally had a purse seiner working full time in parallel with the research vessel, along cruise IV. With this additional sampling system biomass estimation were of 62,569 t, a value very close to the 62,134 t landings of the sardine fleet in the same year. Mid water trawl consistently caught small sardine while the purse seine caught more and larger individuals due to the ability of the larger individuals to avoid the trawling devices and the selectivity of the purse seine. Sub sampling affected the results in two ways, sub estimating area distribution and biomass.