The southern Brazilian coast hosts various pinniped species, primarily South American fur seals (*Arctocephalus australis*) and South American sea lions (*Otaria flavescens*). While breeding colonies are absent in Brazil, two haulout sites exist within marine protected areas (MPAs): the Wildlife Refuge of Molhe Leste (WRML) and the Wildlife Refuge of Ilha dos Lobos (WRIL). WRIL, a coastal island in Rio Grande do Sul state, is considered the northernmost haulout site for these species on the South Atlantic coast. South American pinnipeds likely originate from Uruguayan and Argentinian breeding sites, arriving during austral autumn and spring. Their presence in WRIL falls within their dispersal period after the breeding season, a life-cycle stage that remains poorly understood. The IUCN lists South American sea lions and fur seals as Least Concern globally, but population trends vary across different regions. Some populations, such as those in Uruguay and the Falklands, are declining, while others, like in Argentina, are increasing. Threats include habitat degradation and interactions with fisheries. This study aims to provide more information on the habitat use and distribution of these species in WRIL during their post-breeding dispersal period by investigating pinniped abundance and spatial distribution using aerial photography, a method superior to land-based counts due to its reduced error and disturbance.

Previous research on pinniped abundance in WRIL primarily used terrestrial and marine census methods from fixed points and vessels. These methods, however, can be susceptible to counting errors due to weather conditions, visibility issues, or the potential for observer bias. A study conducted between 1994 and 1995 used these methods, while further counts were performed from 1993 to 2002. However, none of the previous studies employed aerial photography, a more precise technique offering a broader view and reducing potential biases. Studies show that South American pinnipeds, particularly the sea lions, use several different haulout sites along the Brazilian coast, with movement often associated with the Malvinas Current. The post-breeding dispersal period represents a significant gap in our understanding of their life cycle, hence the importance of this research.

This study utilized aerial photographic censuses conducted between July 2010 and November 2018, encompassing 20 flights over WRIL. Helicopters and a UAV (DJI Mavic Pro I) were used to capture images at altitudes between 100-250 meters. A Canon EOS 7D camera was employed, and the software 'Otariidae' 1.0 aided in counting pinnipeds. Individuals were classified by species, age, and sex based on morphological characteristics. A Generalized Linear Mixed Model (GLMM) with a negative binomial error distribution analyzed monthly differences in pinniped numbers. Year was included as a random effect to account for temporal autocorrelation. A likelihood ratio test compared the GLMM with an intercept-only model to assess the significance of the month variable. Kernel density estimation (KDE) in ArcGIS 10.6.1 analyzed spatial distribution. The island was divided into three sectors (North, Center, South) for analysis, and individual pinniped positions were georeferenced on a 5x5 m grid. KDE produced continuous density surfaces to evaluate spatial preferences and territory occupancy. Ethical approval was obtained under SISBIO licenses 69627 and 35606-1 and PPECEUA05.2019 from UNISINOS's institutional ethics committee.

The highest monthly mean abundance of all pinniped species in WRIL was observed in September (mean: 143.67, SD: ±34.16), followed by July and November. September 2018 exhibited the highest overall abundance (304 individuals), including South American sea lions, fur seals, and a single southern elephant seal. Statistical analysis revealed no significant monthly differences in abundance for South American sea lions and fur seals. South American sea lions were the most abundant species, with subadult males (88.24%) dominating. Adult males comprised 5.98%, potential females 2.17%, juveniles 2.76%, and yearlings 0.85%. South American fur seals were less abundant overall, with yearlings (81.62%) forming the largest age class. Spatial analysis demonstrated a strong preference for the northern portion of WRIL. 95.94% of all pinniped records were concentrated in the north zone, with only 4.06% in the central zone and none in the south. This pattern held true for each species individually. KDE analysis confirmed the clustering of pinnipeds in the northern region, especially during peak months. The higher densities were consistently seen in spring months compared to winter, although statistically not significant.

The study did not support the hypothesis of monthly differences in pinniped abundance in WRIL, contradicting some previous findings. However, it validated the hypotheses of male predominance and spatial preference for the island's northern area. The predominance of subadult male sea lions likely reflects their less intensive parental care responsibilities, allowing for wider dispersal after the breeding season. The high proportion of yearling fur seals points to the importance of WRIL as a resting and foraging habitat during their post-weaning period. The spatial preference for the northern region is likely attributed to its higher elevation, dryness, and protection from waves. The unusually high abundance of fur seals in September and November 2018 requires further investigation, potentially linked to population growth or fluctuations in births and survival rates. The contrast in abundance and mortality patterns between the two species may reflect differences in their ecological niches, feeding habits, and vulnerability to environmental factors and human-induced threats. The relatively low mortality of sea lions compared to fur seals in the region may contribute to their more stable abundance at WRIL.

This study provides the first comprehensive analysis of the spatial and temporal distribution of pinnipeds in WRIL. The findings highlight the importance of this site as a key haul-out area and emphasize the need for careful management of human activities to minimize disturbances to pinnipeds. Future research should focus on elucidating the reasons for the apparent inter-annual variations in fur seal abundance. A more detailed study of the feeding ecology of fur seals along the southern Brazilian coast is also warranted, to better understand their mortality patterns and their use of this northern haul-out site. Monitoring the population dynamics of both sea lions and fur seals in this crucial non-breeding area is crucial for their long-term conservation.

The study's temporal scope was limited to the period between July and November, potentially missing pinniped presence in other months. The classification of potential adult females in South American sea lions based on morphological characteristics from aerial photographs may have limitations regarding accuracy. Finally, the study used aerial imagery only, precluding detailed information on pinniped behavior and individual identification.