Size at maturity, sexual dimorphism, gonad development, and abundance of white-dotted skate, Bathyraja albomaculata, were assessed along its geographic range in the Southwest Atlantic, from Uruguay to Tierra del Fuego (Argentina). In all, 291 specimens from eight research cruises conducted between 1998 and 2003 were examined. Length–mass relationships were sexually dimorphic, adult females being significantly heavier than males of the same length. The largest female observed was 762 mm total length (LT), and 50% maturity (LT50) was attained at 653 mm LT. The largest male was 732 mm LT, and male LT50 was 628 mm LT. Female and male LT50 were not significantly different and were about 86% of observed maximum size, similar to other Bathyraja species. The lack of sexual dimorphism in size at maturity and the slight difference in maximum size between sexes support the hypothesis of relaxation of selection for large female size in oviparous elasmobranchs. Liver size was not sexually dimorphic. Mature female gonads were symmetrical in terms of mass, and similar in ovarian follicle number and size. Females carried egg cases in April, September, and October, consistent with previous observations around the Islas Malvinas and suggesting year-round egg laying. Bathyraja albomaculata was most abundant on the outer continental shelf and slope between 36°30′S and 45°S (northern area). In contrast, it was generally absent between 48°S and 52°S (southern area), a region where it was formerly reported as abundant. These differences are coincident with the development of a fishery targeting skates in the southern area, whereas in the north B. albomaculata taken as bycatch are mostly discarded alive.

ABSTRACT.Competition for food could be a major force driving changes in the community structure of skates (Rajidae) subjected to fishing exploitation. Under this hypothesis, small skates are released from competition with larger skates after fishing has depleted the larger species. Here, we compare the abundance patterns of two sympatric skates with similar niches but different life histories, Bathyraja albomaculata (larger and slow-reproducing) and Bathyraja macloviana (smaller and faster-reproducing), before (1971, 1978) and after (1998)(1999)(2000)(2001)(2002)(2003)(2004) a 108% increase in industrial bottom trawling on the southeastern South American shelf in order to test the prediction that B. macloviana should competitively exclude B. albomaculata after the increase in fishing mortality. In 1971 and 1978, there was no relationship between the abundance of both species, indicating that they coexisted over large scales. In 1998scales. In -2004, the relationship between the abundances of these skates was bell-shaped, indicating that both species increased in abundance at low densities until peaking, after which B. albomaculata decreased when B. macloviana became more abundant, consistent with resource competition. We tested whether food may be a potential limiting resource by comparing the diet of both species. The two species consumed mostly polychaetes, differing only in the consumption of polychaetes from the family Nephthyidae, which was much higher for B. macloviana. Bathyraja macloviana could replace B. albomaculata at high densities when food resources may become scarce. These results support the hypothesis that competition release is an important factor explaining the changes in skate communities in overexploited areas. Keywords: Bathyraja albomaculata, Bathyraja macloviana, Rajidae, diet overlap, resource competition, elasmobranch fisheries, Argentina.De la coexistencia a la exclusión competitiva: ¿Puede la sobrepesca cambiar el resultado de la competencia en rayas (Chondrichthyes, Rajidae)?RESUMEN. La competencia por el alimento podría ser una fuerza importante detrás de los cambios en la estructura de las comunidades de rayas (Rajidae) bajo explotación pesquera. Según esta hipótesis, las rayas pequeñas son liberadas de la competencia por las rayas de mayor tamaño, al disminuir la abundancia de éstas últimas por la pesca. En este trabajo, se comparan los patrones de abundancia de dos rayas simpátricas con nichos similares pero con diferentes historias de vida, Bathyraja albomaculata (mayor y de reproducción lenta) y Bathyraja macloviana (más pequeña y de reproducción más rápida), antes (1971, 1978) y después (1998)(1999)(2000)(2001)(2002)(2003)(2004) de un incremento del 108% en el arrastre de fondo industrial en la plataforma sudeste de América del Sur, para evaluar la predicción que B. macloviana excluiría competitivamente a B. albomaculata después del aumento en la mortalidad por pesca. En 1971 y 1978, no hubo relación entre las abundancias de ambas especies, indicando que, a escalas grandes,...

The present paper analyzes the diet, feeding strategy and niche width of Bathyraja albomaculata over the Patagonian continental shelf and upper slope. The diet comprised a variety of 14 small invertebrates, although the most important prey were polychaete worms. The opheliid polychaete Travisia sp. and the gammarid amphipod Cirolana sp. were the main prey present in stomach contents of B. albomaculata. No differences were found between the diets of males and females, however ontogenetic changes were detected. Skates <40 cm of disk width (DW) preyed on amphipods, and sizes >40 cm DW largely consumed polychaetes. As size increased, polychaetes became more important and the importance of amphipods decreased. B. albomaculata showed a narrow food niche and a marked specialization towards polychaetes.

Many myliobatoid rays are important mesopredators, having significant effects on coastal benthic communities. In tropical and subtropical high-diversity oligotrophic ecosystems, they partition their trophic resources, which results in high ecological singularity. However, it is unknown whether this is true for temperate low-diversity eutrophic ecosystems. In the present study, we tested, for the first time, the hypothesis that myliobatoid mesopredators are ecologically redundant in a temperate low-diversity eutrophic ecosystem. We quantified diet and measured intra-and interspecific trophic overlap in the three species that regularly occur off Uruguay and northern Argentina, namely Myliobatis goodei, Myliobatis ridens and Dasyatis hypostigma. M. ridens had a typical durophagic diet composed of bivalves and gastropods, M. goodei fed primarily on polychaetes and decapods, diverging from the durophagic diet typical of its genus, and D. hypostigma preyed primarily on amphipods and decapods. There were ontogenetic and seasonal dietary differences in all three species. It is concluded that ecological singularity is present in this temperate myliobatoid assemblage, with each species having a different trophic niche. The practice of pooling together myliobatoid mesopredators in trophic models must be abandoned unless there is evidence of ecological redundancy.