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

Abstract

Right Whale (Eubalaena australis) population of the South–western Atlantic Ocean is recovering. In the breeding ground of Peninsula Valdés, because 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. We 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. Information on the group composition was recorded, including Mother-calf pairs, Solitary Individuals or Breeding groups. For the analysis in this work Solitary Individuals and Breeding groups were pooled in Other groups, as a group category opposite to the Mother-calf pairs. Preliminary examination of data led us to define two high densities zones, one inside Golfo Nuevo and other inside Golfo San José, defining the low-density zones as those outside the high-density zones. The density of the whales increased to near three whales per km2 (averaged over each period) in the high-density areas. When this mean density was reached, 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. We 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.