During the research programme conducted on the OV Puerto Deseado in the summers of 2011 and 2013, 36 stations were sampled using a demersal net at depths between 53-590 m in the Antarctic Peninsula and South Shetland Islands. A total 3378 fish specimens belonging to 36 species were recorded. Notothenidae was the best-represented family in species number, with Lepidonotothen nudifrons, L. larseni and Trematomus scotti being the most numerous species. Of the fish assemblages, 20% of the species were considered as dominant, 10% as common, 13% as occasional and 57% as rare. Six groups (and two sub-groups) were obtained by the ordination diagram based on geographical location: group 1 = Gerlache Strait, group 2 = Deception Islands, group 3 = Biscoe Island, group 4 = between Elephant and King George islands, group 5 = northern Antarctic Peninsula, and group 6 = South Shetland Islands, with sub-groups 6a shallower South Shetland Islands and 6b deeper South Shetland Islands. Sampling depth and water temperature significantly explained the spatial pattern. A latitudinal pattern of decreasing abundance from north-east to south-west was found in L. larseni and the opposite in T. scotti. The predictability of fish composition in the assemblages' areas could be a useful tool for ecosystem-based management.
Rough scad Trachurus lathami is a key pelagic fish in the Argentinean continental shelf (ACS, south‐west Atlantic Ocean), with recent increases in abundance. It is a main prey of fishes and marine mammals, and shares the environment with commercially relevant pelagic species (Engraulis anchoita and Scomber colias), playing an important role linking lower and upper trophic levels in the ecosystem. This study aims to determine the ontogenetic changes in the diet composition, feeding strategy, trophic niche breadth and trophic level of T. lathami in the North Patagonian Shelf (43°–45°30′S). The stomach contents of adult fish (n = 238) were analysed. The results suggest a clear ontogenetic shift in the diet at a size of ~190 mm. Smaller individuals (160–190 mm) were specialized on misidaceans, and showed the highest trophic level, while larger T. lathami (221–230 mm) consumed decapods (Peisos petrunkevitchi) and teleosts (eggs and larvae). Trophic niche breadth was higher at the medium‐sized class (191–220 mm), which mainly preyed on copepods (Calanoides carinatus) and chaetognaths (Sagitta spp.), evidencing a more diverse diet and a rather generalist strategy. Updated information on the trophic ecology of T. lathami evidences its extremely plastic feeding behaviour, being able to adapt its trophic niche to the most readily available food items from the mesopelagic community.
Summary Geometric‐morphometric and spinulation pattern approaches were employed to assess the discrimination of two species of Sympterygia skates in the Southwest Atlantic: Sympterygia acuta Garman 1877 and Sympterygia bonapartii Müller & Henle, 1841. Two types of variables were employed: linear morphometrics measurements (LMMs), and interlandmark distances (IlDs). The Principal Component Analysis (PCA) and the discriminant analysis (DA) do not overlap between the species. Morphometrically, S. bonapartii is characterized by a wider disc, larger pelvic fins, larger eyes, a greater distance between nostrils and between the first pair of gill slits, a greater interorbital distance between spiracles, and a wider mouth. On the other hand, S. acuta has a larger disc due to the snout size. Regarding the allometric coefficient analysis performed on the IlDs, S. bonapartii showed a positive allometry for the variables that comprise the Box‐Truss 3 (Ventral protocol), which can be related to the mouth width and the first gill slits. This growth type is consistent with those observed in the Box‐Truss 3 (Outline protocol), which is related to the disc width. It has been interpreted that both structures could develop together. In opposition, S. acuta showed isometric growth for the above‐mentioned variables; on the other hand, S. acuta showed positive allometry for all variables that defined the snout in the three protocols, indicating that the snout has a distinctive growth relative to the size through ontogeny. S. bonapartii, however, showed an isometric snout growth. The remaining variables that defined the morphogeometrical protocols displayed the same type of growth. Regarding spinulation, the thorns and dermal denticles proved to be useful to discriminate both species. Thorns of the caudal region were large and oval with smooth edges in S. bonapartii, and oval with lobed edges in S. acuta. Dermal denticles in S. acuta also presented two elongated ridges with a third small ridge between them, whereas those of S. bonapartii presented three elongated ridges of the same size.
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