Density-dependent changes in the distribution of Southern Right Whales (Eubalaena australis) in the breeding ground Peninsula Valdés

Abstract

BackgroundThe Southern Right Whale (Eubalaena australis) population of the South–western Atlantic Ocean is recovering. In the breeding ground of Peninsula Valdés, as a consequence of the population growth, expansion to new areas by some types of groups and a change in the habitat use patterns at the coastal area were recorded.MethodsWe analysed information gathered from aerial surveys conducted along the coast of Peninsula Valdés in 15 years of effective sampling in a 19-year span. These surveys were divided into four periods (1999–2000; 2004–2007; 2008–2012 and 2013–2016) and estimated the density of whales in a 620 km of coast divided into segments of five km.ResultsThe density of the whales increased to near three whales per km2 (averaged over each period) in the high-density areas. When this mean number was reached, the significant changes in density in the adjacent areas were detected in the following period. These changes were a decrease in density in the high-density areas and an increase of density in the low-density areas.DiscussionWe propose that a threshold in density elicits a response in habitat use, with the Mother-calf pairs remaining in the area, while the other groups are displaced to new areas.

Historical reconstruction of the population dynamics of southern right whales in the southwestern Atlantic Ocean

“…Changes in the catchability in the surveyed area could explain the low maximum rate of increase estimated. As mentioned previously, the observed redistribution of the whales in the Península Valdés area 32 , 39 , 90 suggests that the area is getting close to its carrying capacity in the survey area as optimal habitat while solitary individuals and breeding groups are forced to use deeper waters or to move out of the region, in particular to Golfo San Matías 39 , 90 . Although `Index` scenarios exploring time-varying and/or density-dependent catchability were not able to estimate high rates of increase, models that assume a negative trend in catchability might favour higher values.…”

“…This notion is also supported by evidence of recolonization of previously inhabited migratory destinations 33 , 39 – 41 . Sueyro et al 90 , assessing the change in the distribution of SRWs in the breeding grounds of Península Valdés, proposed that a threshold in the whale’s density within breeding areas triggered a density-dependent response, with the mother-calf pairs remaining in the area and the other groups being displaced to new regions. Currently, the underlying process of the observed recovery of the SRW population from the southwestern Atlantic Ocean appears to involve a slower increase within the traditional feeding ground 32 accompanied by a higher probability of recolonizing ancient habitats 39 , 90 .…”

“…Sueyro et al 90 , assessing the change in the distribution of SRWs in the breeding grounds of Península Valdés, proposed that a threshold in the whale’s density within breeding areas triggered a density-dependent response, with the mother-calf pairs remaining in the area and the other groups being displaced to new regions. Currently, the underlying process of the observed recovery of the SRW population from the southwestern Atlantic Ocean appears to involve a slower increase within the traditional feeding ground 32 accompanied by a higher probability of recolonizing ancient habitats 39 , 90 . These findings, together with model estimates for the entire study area, lead us to propose that the SRW population will continue growing for many decades, though density-dependent effects on population parameters can be expected.…”

“…`Index` scenarios evaluated the potential for time-varying and/or density-dependent catchability. In the breeding ground of Peninsula Valdés, because of the population growth, expansion to less dense regions ( i.e ., suboptimal habitats) by some types of groups was recorded 32 , 39 , 90 . This change in distribution could mean that catchability of animals in the sampling area is varying over time.…”

Wildlife and Marine Mammal Spatial Observatory: Observation and automated detection of Southern Right Whales in multispectral satellite imagery

“…These whales perform yearly migrations from wintering calving sites to summering sub-Antarctic waters where they feed on large aggregations of Antarctic Krill (Euphausia superba) and copepods. The largest winter aggregations occur in Peninsula Valdes (Argentina) [16,17], southern Cape coast (South Africa) [18], Australia particularly between Head of Bight and Fowlers Bay [19], as well as in New Zealand [20,21,22], Brazil [23,24,25] and Uruguay [26,27]. In smaller numbers sightings have been reported of the coasts off Peru and Chile [28,29,30] where photo-identification [31]and UAV-based fieldwork led to the hypothesis of the existence of a nursery ground [32].…”