Oceanographic conditions off coastal South America in relation to the distribution of Burmeister's porpoise, <i>Phocoena spinipinnis</i>

Abstract

Historical data (1965-2000) of temperature, salinity, and dissolved oxygen (0 and 50m) were analyzed to understand the seasonal variability of the oceanographic conditions associated to the distribution of P. spinipinnis from Paita, Peru (05°01'S, 81°W), in the Pacific Ocean, to Santa Catarina, Brazil (28°48'S, 49°12'W), in the Atlantic. The variability of historical average data was associated with different processes as a function of the geographical position and seasonality. These variations have different expressions along each coast as a function of the circulation system in each ocean. The northern boundary of the distribution of P. spinipinnis for the Pacific coast at Paita (5°S) is coincident with the westward turn of the Humboldt Current, as it is incorporated into the South Equatorial Current. In the Atlantic, the northern boundary for the species seems to be associated with the Atlantic Subtropical Convergence (30-40°S). The high temperature (<24°C) and salinities (<36psu) registered at the surface and at 50m between the coast and 20nm were coincident with the known northern limit of the distribution of this species on both coasts of South America. We propose the existence of three oceanographic areas within the distributional range of P. spinipinnis: (1) from Paita, Peru to south of Arauco Gulf, Chile, that has the influence of the Humboldt Current, and a developed oxygen minimum zone (OMZ); (2) from south of Arauco Gulf to south of La Plata River, Argentina, which shows the influence of the Cape Horn and Malvinas Currents, respectively, as well as downwelling processes, freshwater contributions from fjords, glaciers and rivers; and (3) from La Plata River to Santa Catarina, Brazil, which is characterized by the influence of the Brazil Current, and the freshwater contributions of the basin of La Plata River and the estuarine system of Patos Lagoon, south Brazil. The presence of the OMZ is possibly a factor in the separation of groups (1) and (2) along in the Chilean coast. In addition, we propose that Burmeister's porpoise presents a continuous distribution throughout this range from Paita, Peru to La Plata River basin, Argentina, being able to reach Uruguayan and Brazilian waters under certain oceanographic conditions (intrusion of colder and less saline waters toward the north associated with the Subtropical Convergence).