Wöhler, O.C.; Giussi, A.R.; García de la Rosa, S.B; Sánchez, M.F.; Hansen, J.E.; Cordo, H.D.; Alvarez Colombo, G.L.; Incorvaia, I.S.; Reta, R.; Abachián, V. 1999. Resultados de la campaña de evaluación de peces demersales australes efectuada en el verano de 1997.
INIDEP Informe Técnico, 24: 70 p.
An assessment survey on the austral fish was carried out during the summer 1997 on both the shelf and slope off southern Argentina. The cruise covered 101,508 nm2 between 45° and 54° 30’S and depths between 50 and 400 m. The survey did not include the area controlled by the United Kingdom. Considering the total area as an undivided stratum, a random sampling was used to estimate demersal fish biomass and a delta distribution-based model to evaluate mean densities. Out of the forty five species caught 28 were bony fish and 17 elasmobranch. Four cephalopod mollusc species were also identified. A biomass of 3.2 million tons (CI95%: 1.90 – 6.06 millions) proved that longtail hake was the most abundant resource within the region followed by common hake (590,000 tons), patagonian cod and notothenids (slightly over 200,000 tons each). Pink cuskeel reached almost 190,000 tons. Among other fish species, only Psammobatis spp rays were over 100,000 tons. The main longtail hake concentrations (100 t/nm2) were found between 48° and 52° 30´S at water depths between 50 – 100 m. Size distribution of longtail hake ranged 18 – 110 cm TL with a mode at 54 cm. Fitting of length (cm)- weight (g) relationship parameters resulted in W = 0.0099 * TL 2.7215. Length at 50% first maturity (TL50%) was estimated at 59.4 cm. Distribution patterns, maximum densities, size structures, length – weight relationship and length at 50% maturity related to the other above mentioned species are also reported. From 2,669 otolith readings, length growth parameters corresponding to longtail hake were estimated as follows: L∞= 88.15 cm TL; K = 0.245 year-1 and t0 = -0.425 years (males) and L∞ = 100.92 cm TL; K = 0.190 year-1 and t0 = -0.610 years (females). Significant differences between sexes were observed. Age group 3 (year class 1993) was the most abundant (2,600 millions of individuals). Natural mortality (M) estimates for the species ranged 0.22 – 0.269 whereas total mortality (Z) between ages 4 and 11 was estimated to be 0.335. Studies on fish feeding showed that longtail hake, southern blue whiting and common hake ate mainly zooplankton species such as amphipod Themisto gaudichaudii and euphausiid Euphausia lucens as well as small fish (notothenids, longtail hake juveniles, anchovies, sprats). Other predators, including pink cuskeel, southern hake, patagonian cod, spiny dogfish and Raja flavirostris fed mainly on macrocrustaceans, cephalopods (squid and octopus) and fish (notothenids and longtail hake). Cannibalism, representing 65% of the total longtail hake ingested, would be exerted only on young-of-the-year fish whereas predation would be suffered by 0 to 3 year old fish. Nematod Anisakis sp. was responsible for the high degree of parasitism found in southern blue whiting. Different parasites were identified in gills, mesentery, liver, stomach, pyloric caeca and gut. Sea surface temperature ranged 7.5 – 12.5°C. A thermic front (0.02°C/km) was found between 50° and 51°S. Bottom temperature decreased as waters became deeper. Encounter on the shelf of low salinity coastal waters (33.2 psu) and outer waters from the Malvinas current (33.8 to 34.2 psu) generated saline fronts mostly between 53° and 54°S. A strong thermocline was found between 47° and 51°S. It registered a maximum gradient of 5°C/m at 50 m deep waters in the central shelf region. Haloclines were found south of 51°S at depths between 50-70 m. Sea water density within the northern region was mainly influenced by temperature values whereas salinity was the key factor southwards. Zooplankton densities ranged 18.5 – 2488.8 mg/m3. The largest biomasses were found in coastal waters within the area of Bahía Grande (50°-52°S) where T. gaudichaudii was the most abundant species. Differences between day and night zooplankton distribution produced important echo traces only during dark hours.