Commensalism Between Guiana Dolphins Sotalia guianensis and Sea Birds in the North Bay of Santa Catarina, Southern Brazil

Commensal associations between Guiana dolphins Sotalia guianensis and sea birds, Larus dominicanus, Sula leucogaster, Sterna sp. and Fregata magnificens have been frequently observed during the feeding behavior of the dolphins in the North Bay of Santa Catarina, (26°00’-28°25’S, 48°30’-48°49’W), Southern Brazil. Boat and land surveys were performed, using focal group sampling method, in blocks of five minutes. In 81 surveys, between 1993 to 1997, associations with birds were observed in 67% of the days (n= 54 days). The most representative years were 1993 (63.16%) and 1994 (73.68%), with a total frequency of 62.72%. In 1995, less associations were registered, only 8.5% of the total. In the following years associations were registered more frequently (16.13% in 1996 and 12.64% in 1997), but with lower intensity and numbers than the early years. The frequency of occurrence of the bird species also varied each year. Sterna sp. showed the highest time of occurrence in observed association (1587 minutes, 33.32% of the total time observed) and the highest frequency days in associations (n = 46, 85.18% of the total (= 54)). The second most representative bird species was F. magnificens with 1218 minutes of associations, almost 26% of the total, and observed in 40 days (74.07%of the total). S. leucogaster appeared with 951 minutes in association (19.97%) and it was observed in 29 (53.70%) of the 54 total days of associations registered. L. dominicanus showed the smaller time of occurrence, 925 minutes (19.42%) and it was registered in 29 days (53.7%) of associations, mainly in the austral winter. These associations occurs principally when the dolphins are executing coordinated feeding strategies, railing the fishes in big groups, making easy the obtainment of food to the birds, that is characterize this relation like commensalism.