María Alejandra Romero

Scientific trawl surveys are conducted worldwide for fisheries management and ecosystem assessments. However, maintaining constant sampling protocols is rarely achievable due to logistical and financial limitations. Here, we used a data set from the San Matías Gulf (Patagonia, Argentina) to evaluate how variation in sampling effort can bias estimates of species richness and occupancy. We also explored analytical approaches that improve the reliability of biodiversity indicators when standardized data sets are not available. Species incidence data from 7 surveys (2006-2024) were analysed using generalized additive models (GAMs) and species-accumulation and rarefaction/extrapolation curves to assess the effects that varying sampling effort had on species richness at local (haul) and regional (entire Gulf) scales. We also evaluated whether rare species were disproportionally affected by sampling effort reduction and whether there have been significant changes in species occupancy within the gulf. Our results reveal that local species richness estimates are largely explained by depth, longitude and sampling effort. At the regional scale, all surveys showed similar richness values at comparable effort. These findings suggest that the lower richness observed in the 2 most recent surveys is likely an artifact of reduced sampling effort, leading to the under-detection of rare species. However, asymptotic richness shows a constant decrease from 2018 onwards, which indicates that loss of biodiversity cannot be ruled out. Additionally, we detected changes in the occupancy of some species. Our results highlight the importance of explicitly modelling effort-related variables and provide a methodological framework applicable to biodiversity surveys facing sampling effort constraints.

Understanding the population dynamics of marine mammals is essential for assessing their resilience to historical harvesting pressures and current anthropogenic threats. The South American sea lion (SASL, Otaria byronia ), widely distributed along the South American coasts, represents the largest biomass of northern and central Patagonia. This species faces threats such as incidental fishing captures, aquaculture, and tourism, with well‐documented impacts on its population. However, the effects of Highly Pathogenic Avian Influenza (HPAI) H5N1 on its population dynamics are poorly understood. This study provides new insights into the impact of the 2023 avian influenza outbreak on the SASL in northern Patagonia. Population trends and age‐sex class composition of colonies were updated, incorporating the HPAI outbreak in the Río Negro rookeries to compare trends before and after the epizootic event. A positive trend in SASL abundance was observed from 2007 to 2022 across most age and sex classes. However, in the 2024 census, 7175 ± 145 individuals were counted across three colonies, with the largest, Punta Bermeja, recording a 27.6% decline. During the 2023 outbreak, 1476 deaths were reported, affecting all age and sex classes. These findings suggest the HPAI H5N1 outbreak significantly impacted the previously positive population trend.

Crustacean fisheries have increased worldwide, leading to changes in fishing practices. In San Matias Gulf, Argentina, the Patagonian red shrimp replaced Argentine hake as the main target since 2012. We analyzed this shift from 2004 to 2020 using vessel monitoring systems and landings. Fishing positions were identified using the speed rule. Gravitational centers of fishing intensity were used to assess seasonal spatial changes of the fleet, and bottom‐trawling footprint was used to quantify the area of the seabed affected. Landings were pooled monthly to assess multivariate changes in landing composition. Previously concentrated around a frontal system, the fleet moved northward between 2014 and 2020, while reducing hake landings but increasing shrimp landings. The total area trawled did not change significantly, but the footprint nearly doubled. Our results can be used for fishery management by increasing understanding of spatial and temporal changes in fishing strategies for policymakers.

Bycatch is recognized as the primary threat to the viability of marine mammal populations. Given that bycatch levels display strong variations in space and time, knowledge of individual fishery‐marine mammal interaction will likely achieve working solutions. Here, we quantify marine mammal bycatch in the trawl fishery of the San Matías Gulf, Argentina, for the first time using data from onboard observers and fishing effort records. We infer expected annual mortality, evaluate the probability of exceeding conservation thresholds, and explore the importance of relevant covariates to account for broadscale differences in bycatch rates. The South American sea lion (Otaria byronia) was the most incidentally caught species (46 individuals), while only one common dolphin (Delphinus delphis) was taken. Bycatch of sea lions was primarily associated with the catch of the target species, proximity to colonies, and the identity of the target species. The maximum estimated bycatch rate was 0.12 animals/haul and 0.08 animals/tonne of target species landed, with significantly higher bycatch rates observed in the hake fleet. For the whole period, the annual bycatch mortality were below the reference points. Our findings highlight potential hotspots of marine mammal bycatch and provide valuable insights for refining future mitigation measures and enhancing conservation efforts.

Understanding the recovery of whale populations is critical for developing population-management and conservation strategies. The southern right whale (SRW)Eubalena australiswas 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 depletionP₂₀₂₁8.7%). 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.

Allometric power equations are generated using otolith length and width in order to estimate the total length of five demersal species inhabiting the San Matias Gulf, the second largest gulf in Argentina. The selected fish species are economically important resources in Argentina and have been identified as prey for the system’s main predators. A total of 2795 specimens were sampled, and data were fitted using the nonlinear least squares analysis. It is shown that the length and width of the otolith are good predictors of the total fish length. A slightly better regression fit is observed when otolith length was considered as a predictor. Statistically significant morphometric differences in regression models were observed between sexes, except for Acanthistius patachonicus. Results are discussed and compared with those reported by other authors.

Marine mammals and humans are apex predators and both may compete for fish in ecosystems under continuous fishing pressure. We assessed the degree of trophic overlap between prey species found in the diet of 5 marine mammals (39 specimens of sea lion Otaria flavescens, 61 fur seals Arctocephalus australis, 76 franciscana dolphins Pontoporia blainvillei, 25 bottlenose dolphins Tursiops truncatus and 28 Lahille’s bottlenose dolphins T. gephyreus) and the catches of the 6 main commercial fishing gears used in southern Brazil (coastal gillnets, oceanic gillnets, purse seine, demersal pair trawling, bottom [single] trawl and double-rig trawling) between 1993 and 2016. An adjusted general overlap index indicated an overall moderate to high overlap. Specific overlap analysis showed that O. flavescens and T. truncatus presented high trophic relationships with fisheries, followed by T. gephyreus. Smaller interactions were observed for A. australis and P. blainvillei, even though they also exploit commercial fishing resources. Coastal gillnet and pair bottom trawling are the fisheries that most target the fish species favoured by O. flavescens, T. gephyreus and T. truncatus. The information presented in this study on trophic interactions may assist decision making for both fishery management and conservation measures for these apex predators. Commercial fishing activities are a major threat to marine mammals both regionally and globally. Current levels of fishing or its intensification may lead to dramatic changes in the coastal marine food web, including additional threats to coastal marine mammal populations in southern Brazil.

With the unprecedented rate of biodiversity change in the world today, understanding how diversity gradients are maintained at mesoscales is a key challenge. Drawing on information provided by 3 comprehensive fishery surveys (conducted in different years but in the same season and with the same sampling design), we used boosted regression tree (BRT) models in order to relate spatial patterns of α-diversity in a demersal fish assemblage to environmental variables in the San Matias Gulf (Patagonia, Argentina). We found that, over a 4 yr period, persistent diversity gradients of species richness and probability of an interspecific encounter (PIE) were shaped by 3 main environmental gradients: bottom depth, connectivity with the open ocean, and proximity to a thermal front. The 2 main patterns we observed were: a monotonic increase in PIE with proximity to fronts, which had a stronger effect at greater depths; and an increase in PIE when closer to the open ocean (a ‘bay effect’ pattern). The originality of this work resides on the identification of high-resolution gradients in local, demersal assemblages driven by static and dynamic environmental gradients in a mesoscale seascape. The maintenance of environmental gradients, specifically those associated with shared resources and connectivity with an open system, may be key to understanding community stability.