Santiago A. Barbini

Fish abundance and distribution patterns related to environmental factors in a choked temperate coastal lagoon (Argentina)

O objetivo deste estudo foi avaliar a influencia de fatores ambientais sobre a distribuicao e abundância de peixes na lagoa costeira Mar Chiquita. Estacoes sazonais de amostragem de peixes foram distribuidas ao longo da lagoa e agrupadas em tres zonas de uma zona de aguas Mixo-oligohalino com elevado aporte de agua doce para uma zona de aguas Mixo-eurihaline com alta influencia da agua do mar. Uma zona de aguas Mixo-mesohalino caracteriza a parte do meio da lagoa. Os resultados mostraram que a composicao dos peixes foi dominada por especies marinhos estuarinos dependentes (ED) em todas as tres zonas e em todas as estacoes: Odontesthes argentinensis, Brevoortia aurea, Mugil liza e Micropogonias furnieri foram as especies mais representativas. A categoria seguinte mais importante foi peixes de agua doce (FW), com Platanichthys platana e Jenynsia multidentata sendo as especies mais representativas. De acordo com o melhor modelo selecionado pelo ajuste de modelos lineares generalizados, a abundância de ED aumentou de acordo com a temperatura da agua durante o verao e outono, em todas as tres zonas. A maior abundância foi registrada na zona de aguas mixo-eurihalinas. Por outro lado, a abundância de FW aumentou com o aumento da pluviosidade e tambem quando os ventos de direcao leste sopraram. Em conclusao, a utilizacao diferencial do habitat pelas especies que ocorrem na lagoa costeira Mar Chiquita foi observada. Poucas especies ED dominaram a composicao de especies de peixes, enquanto FW fizeram uso da lagoa quando as condicoes de alta pluviosidade foram observados.

Feeding habits of the apron ray, Discopyge tschudii (Elasmobranchii: Narcinidae), from off Uruguay and northern Argentina

The feeding habits of the apron ray, Discopyge tschudii, were investigated, off Uruguay and northern Argentina, and we tested the hypothesis that the diet changes with increasing body size, between sexes and seasons using a multiple-hypothesis modelling approach. Discopyge tschudii preys mainly on polychaetes (88.77% index of relative importance (IRI)) followed by siphons of the clam Amiantis purpurata (8.13% IRI) and amphipods (3.08% IRI). Ontogenetic, sexual and seasonal changes were found. Larger individuals of D. tschudii consumed buried polychaetes more often. The consumption of errant polychaetes was higher in males and in the cold season. Also, amphipods were preyed on more heavily by females and the number of siphons of A. purpurata consumed was higher in the cold season.

Ontogenetic diet shifts and food partitioning between two small sympatric skates (Chondrichthyes, Rajidae) in the Southwestern Atlantic

Knowinghowsympatricspeciespartitiontheirresourcesisveryimportantforunderstandingthemechanisms thatinfluencemarinecommunitystructure.Wetestthehypothesisofresourcepartitioningbetweentwosympatric,closely related, morphologically similar, small skates, Psammobatis bergi and P. extenta. The diets of the two species were different. P. bergi fed predominantly on crabs, amphipods, isopods and polychaetes. P. extenta consumed mainly amphipods and shrimps. Changes in the diet with body size, maturity stage and season varied among species. The consumption of amphipods decreased with increasing size of both species. The consumption of crabs increased with increasingsizeofP.bergi,butdecreasedwithincreasingsizeofP.extenta.Theconsumptionofcumaceansincreasedwith size of P. extenta and decreased with size of P. bergi. Adults preyed on isopods more heavily than juveniles for both species. P. extenta preyed on shrimps more heavily in the warm season and, on the contrary P. bergi preyed on shrimps more often in the cold season. The dietary breadth was higher inP. bergithan inP. extentaand was higher in females than in males. Our results showed that these sympatric and closely-related species have clearly different trophic traits. Additional keywords: dietary breadth, elasmobranch, food partitioning, Psammobatis bergi, Psammobatis extenta.

Geographic distribution of the short-tailed river stingray (Potamotrygon brachyura): assessing habitat loss and fishing as threats to the world’s largest obligate freshwater elasmobranch

Freshwater elasmobranchs are threatened but little is known about them. Potamotrygon brachyura is the world’s largest obligate freshwater elasmobranch. This makes it popular among fishermen and suggests a susceptibility to anthropogenic threats. We collected records of P. brachyura (n=70) from media, fishermen and scientific sampling, to estimate its global geographic distribution (using both generalised additive and MaxLike models). Then, we estimated the species’ exposure to habitat modification and fishing pressure, by applying multinomial ordinal models with threat levels as response and the presence or absence of P. brachyura as an independent variable. Distance to coast, depth, water temperature range, salinity range and mean water temperature were the main determinants of the distribution in the Rio de la Plata. This resulted in a narrow coastal distribution. In the Uruguay and Parana River basins, the probability of occurrence was positively associated with flow accumulation, percentage of open water and submerged vegetation, and lowland ecoregions, and negatively to wetness index and altitude. This limited the distribution to large lowland rivers. Approximately 41% of the range of P. brachyura in the Rio de la Plata was subject to high habitat modification. In the Uruguay and Parana River basins, this percentage reached 7%; however, the overlap with high fishing pressure was 59%. For conservation purposes, P. brachyura may function as an umbrella species.

Effects of intrinsic and extrinsic factors on the diet of Bathyraja macloviana, a benthophagous skate

The effects of intrinsic (sex, maturity stage and body size) and extrinsic (depth and region) factors on the diet of Bathyraja macloviana, in the south-west Atlantic Ocean, were evaluated using a multiple-hypothesis modelling approach. Bathyraja macloviana fed mainly on polychaetes followed by amphipods, isopods and decapods. Effects of intrinsic and extrinsic factors on diet composition of this species were found. The consumption of polychaetes had a humped relationship with total length (L(T), and isopods and decapods increased with increasing L(T). Immature individuals preyed on amphipods more heavily than mature individuals. Furthermore, region and depth had an important effect on the consumption of isopods, decapods and amphipods. Such ontogenetic changes and spatial patterns may provide insights into understanding the regulatory mechanisms of marine communities.

The presence of southern fishes in the Argentinian continental shelf and adjacent areas

A list of Southern Ocean fishes captured in the Argentinian continental shelf and adjacent areas is presented. The list comprises a total of 41 species. They represent 15% of the ichthyofauna registered in the Southern Ocean. The geographic position of the samples was considered in order to show the northward displacement of some Antarctic species toward southwestern Atlantic waters.