No abstract

Research on the ecology of sympatric dolphins has increased worldwide in recent decades. However, many dolphin associations such as that between common dolphins (Delphinus delphis) and dusky dolphins (Lagenorhynchus obscurus) are poorly understood. The present study was conducted in the San Matías Gulf (SMG) ecosystem (North Patagonia, Argentina) where a high diet overlap among both species was found. The main objective of the present work was to explore the niche overlap of common and dusky dolphins in the habitat and temporal dimensions. The specific aims were (a) to evaluate the habitat use strategies of both species through a comparison of their group attributes (social composition, size and activity), and (b) to evaluate their habitat preferences and habitat overlap through Environmental Niche modeling considering two oceanographic seasons. To accomplish these aims, we used a historic database of opportunistic and systematic records collected from 1983 to 2011. Common and dusky dolphins exhibited similar patterns of group size (from less than 10 to more than 100 individuals), activity (both species use the area to feed, nurse, and copulate), and composition (adults, juveniles, and mothers with calves were observed for both species). Also, both species were observed travelling and feeding in mixed-species groups. Specific overlap indices were higher for common dolphins than for dusky dolphins, but all indices were low, suggesting that they are mainly segregated in the habitat dimension. In the case of common dolphins, the best habitats were located in the northwest of the gulf far from the coast. In the warm season they prefer areas with temperate sea surface and in the cold season they prefer areas with relatively high variability of sea surface temperature. Meanwhile, dusky dolphins prefer areas with steep slopes close to the coast in the southwestern sector of the gulf in both seasons.

RESUMEN.A nivel internacional, paralelo a la declinación y al colapso de pesquerías de gran escala, surgió un interés creciente por el estudio de las interacciones entre mamíferos marinos y pesquerías. El golfo San Matías (Patagonia, Argentina) es considerado un ecosistema pesquero independiente de las aguas de la Plataforma Continental Argentina, con condiciones oceanográficas y biológicas particulares. La condición de sistema semi-cerrado podría generar escenarios particulares para la interacción entre la flota pesquera de arrastre de fondo y la población de lobos marinos de un pelo Otaria flavescens. En el presente trabajo se caracterizó la dieta del predador y la composición de las capturas pesqueras a fin de evaluar el solapamiento trófico entre ambos componentes. A partir de este análisis y la comparación de las tallas de las presas consumidas, se encontró que la posibilidad de una interacción competitiva entre la flota pesquera y los lobos marinos, a partir de la utilización de recursos similares, sería baja en el ecosistema del golfo San Matías. Palabras clave: Otaria flavescens, lobo marino, golfo San Matías, pesquería de arrastre, solapamiento trófico, Patagonia argentina.Trophic overlap between the South American sea lion Otaria flavescens and the demersal trawl fishery in San Matías Gulf, Patagonia, Argentina ABSTRACT. As world fisheries began to decline and massive collapses were observed, the competition between marine mammals and fisheries became an issue of growing concern. San Matías Gulf (Patagonia, Argentina) is considered to be a fishery ecosystem independent of the Argentine Continental Shelf waters, with particular oceanographic and biological properties. As a semi-enclosed ecosystem, this gulf may generate particular scenarios for interactions between the demersal trawl fishery fleet and the population of South American sea lions Otaria flavescens. In this paper, the diet of the top predator and the composition of fishery catches were characterized in order to assess the trophic overlap between these two components. This analysis and a comparison of the sizes of prey consumed revealed a low probability of competition for similar resources between the fishing fleet and the marine mammals in the San Matías Gulf ecosystem.

An understanding of the underlying processes and comprehensive history of population growth after a harvest-driven depletion is necessary when assessing the long-term effectiveness of management and conservation strategies. The South American sea lion (SASL), Otaria flavescens, is the most conspicuous marine mammal along the South American coasts, where it has been heavily exploited. As a consequence of this exploitation, many of its populations were decimated during the early 20th century but currently show a clear recovery. The aim of this study was to assess SASL population recovery by applying a Bayesian state-space modelling framework. We were particularly interested in understanding how the population responds at low densities, how human-induced mortality interplays with natural mechanisms, and how density-dependence may regulate population growth. The observed population trajectory of SASL shows a non-linear relationship with density, recovering with a maximum increase rate of 0.055. However, 50 years after hunting cessation, the population still represents only 40% of its pre-exploitation abundance. Considering that the SASL population in this region represents approximately 72% of the species abundance within the Atlantic Ocean, the present analysis provides insights into the potential mechanisms regulating the dynamics of SASL populations across the global distributional range of the species.

The performance of management measures adopted to reduce the catch of small-size fish in trawl fisheries requires a proper evaluation taking into account that different factors (e.g., the behavior of fishermen, gear efficiency, market forces, and weak controls) may affect the results. The high bycatch of juvenile Argentine hake Merluccius hubbsi recorded in the trawl fishery of San Matías Gulf, Argentina, could be an indication of a suboptimal harvesting pattern that should be addressed. Regulations to mitigate this problem (i.e., a minimum mesh size and the seasonal closure of a nursery area) were imposed in 1997-1998. The purpose of this study was to assess the efficacy of the size-selectivity measure 10 years after its implementation based on two key indicators: the presence of juvenile Argentine hake in catches and discard rates. Our results show that this measure was ineffective in reducing the catch and discard of juveniles and that both indicator values increased during the period analyzed. This failure could be the result of the limited scientific advice available when the measure was designed or poor implementation, monitoring, and control strategies.

The diet composition and feeding strategy of the Argentine hake Merluccius hubbsi in the San Matías Gulf were analysed in order to use this information for the sustainable management of the fishery. Merluccius hubbsi behaved as an opportunistic predator. Small M. hubbsi consumed planktonic crustaceans, whereas medium and large fish ate numerous prey taxa with low frequency of occurrence and variable specific abundance. Intra- and intercohort cannibalism were detected in all size groups and were particularly significant in large M. hubbsi. Medium-sized M. hubbsi consumed small conspecifics and large-sized M. hubbsi consumed both small and medium M. hubbsi. These results indicate that the removal of large M. hubbsi by fishing may increase the risk of overfishing by two combined effects: a direct effect of recruitment-overfishing and an indirect effect of growth-overfishing through an enhanced cannibalism of medium M. hubbsi on small M. hubbsi. Intra- and intercohort cannibalism and other trophic relationships in the M. hubbsi should therefore be considered explicitly in stock assessment models.

We surveyed the gastrointestinal tracts of 6 bottlenose dolphins Tursiops truncatus from Patagonia to check for helminth parasites and characterize dolphin diet. All dolphins harbored parasites (6477 helminths). We recorded 7 species, including nematodes Anisakis simplex s.l., Pseudoterranova decipiens, acanthocephalans Corynosoma cetaceum, C. australe, and digeneans Braunina cordiformis, Pholeter gastrophilus and Synthesium tursionis. Among the gastric helminths, the most prevalent species were C. cetaceum and A. simplex while C. australe and S. tursionis inhabited the intestine at low prevalence. This is the first report of C. australe and P. decipiens in bottlenose dolphins. Regarding diet, 5 stomachs contained food remains (consisting of 103 prey items). The most important prey species were Geotria australis and Stromateus brasiliensis, but their role in parasite transmission is unclear. At the community level, the gastrointestinal parasite community of T. truncatus was depauperate and strongly overlapped the community described for pelagic dolphins inhabiting Patagonia, suggesting a strong local influence in shaping helminth communities. Nevertheless, these observations are at odds with the notion that oceanic cetaceans have comparatively poorer helminth fauna than neritic species such as bottlenose dolphins, due to the lower likelihood of parasite exchange.

Understanding the recovery of whale populations is critical for developing population-management and conservation strategies. The southern right whale (SRW) Eubalena australis was one of the baleen whale species that has experienced centuries of exploitation. We assess here for the first time the population dynamics of the SRW from the southwestern Atlantic Ocean at the regional level to measure numerically the effect of whaling and estimate the population trend and recovery level after depletion. We reconstructed the catch history of whaling for the period 1670–1973 by an extensive review of different literature sources and developed a Bayesian state-space model to estimate the demographic parameters. The population trajectory indicated that the pre-exploitation abundance was close to 58,000 individuals (median = 58,212; 95% CI = 33,329–100,920). The abundance dropped to its lowest abundance levels in the 1830s when fewer than 2,000 individuals remained. The current median population abundance was estimated at 4,742 whales (95% CI = 3,853–6,013), suggesting that the SRW population remains small relative to its pre-exploitation abundance (median depletion P2021 8.7%). We estimated that close to 36% of the SRW population visits the waters of the Península Valdés, the main breeding ground, every year. Our results provide insights into the severity of the whaling operation in the southwestern Atlantic along with the population´s response at low densities, thus contributing to understand the observed differences in population trends over the distributional range of the species worldwide.