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In order to understand the distribution and abundance of a given species, we need to know its evolutionary history, the resources it requires, its demographic vital rates, its interactions with conspecifics and other species, and the effects of environmental conditions (Begon, Townsend, & Harper, 2006). At the edges of the present distribution of a particular species, organisms are exposed to the extreme of variability in the environmental variables that define their natural habitat. In those environments, a small change of any of the variables can strongly affect the survivorship of a given species. The South American fur seal (SAFS), Arctocephalus australis, is found along the Atlantic and the Pacific coasts of South America (Cárdenas-Alayza, Oliveira, & Crespo, 2016). As with several pinniped species, SAFS were exploited in the southwestern South Atlantic intensively during the 18th and 19th centuries, when most of the species of Arctocephalus along the sub-Antarctic seas were driven to the edge of extinction (Bonner, 1982; Gerber & Hilborn, 2001; Ponce de León, 2000). After the cessation of commercial sealing in the early 20th century, the overall population started to recover (Bastida, Rodríguez, Secchi, & Silva, 2007; Crespo et al., 2015). The process of recolonization includes changes in abundance, trend, and distribution of colonies through time (Roux, 1987). Moreover changes in social structure of colonies (i.e., development of haul-out sites, and their transition into rookeries) have been shown to be a characteristic of the "recolonization" phase of several fur seal and sea lion populations (e.g., Bradshaw,

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The free-living marine nematodes of San Julián Bay dataset is based on sediment samples collected in January 2009 during the project PICT AGENCIA-FONCYT 2/33345-2005. A total of 36 samples have been taken at three locations in the San Julián Bay, Santa Cruz Province, Argentina on the coastal littoral at three tidal levels. This presents a unique and important collection for the nematode benthic biodiversity assessment as this area remains one of the least known regions in Patagonia. In total 10,030 specimens of free-living marine nematodes belonging to 2 classes, 9 orders, 35 families, 78 genera and 125 species were collected. The San Julián city site presented a very high species richness.

The dataset of free-living marine nematodes of San Antonio Bay is based on sediment samples collected in February 2009 during doctoral theses funded by CONICET grants. A total of 36 samples has been taken at three locations in the San Antonio Bay, Santa Cruz Province, Argentina on the coastal littoral at three tidal levels. This presents a unique and important collection for benthic biodiversity assessment of Patagonian nematodes as this area remains one of the least known regions. In total 7,743 specimens of free-living marine nematodes belonging to two classes, eight orders, 37 families, 94 genera and 104 species were collected.