Abundancia relativa y estacionalidad de dos poblaciones de toninas (Tursiops truncatus) en la región de Guaymas, Sonora, Golfo de California

Abstract

Introduction: The study of the ecology of the bottlenose dolphins ( Tursiops truncatus ) in the Gulf of California has been based on the distribution and abundance of the coastal ecotype; therefore there are no comparative studies of the ecology and seasonality of coastal and oceanic ecotypes in the central coast of Sonora. The aim of this study is to determine if seasonal changes have an effect on the relative abundance of the number of herds and the number of individuals of the two ecotypes (coastal and ocean) that inhabit the Guaymas Basin in the Gulf of California. Methods: 13 trips to the coast and 16 to the ocean area where the number of herds and individuals were registered during one year to determine their seasonality. Seasonal relative abundance was obtained by the number of dolphins/hour, and the number of herds/hour. To determine the seasonality of dolphins, first we obtained the seasonal abundance by dividing the number of herds and the number of individuals, and second we made a hypothesis tests, with the number of herds and number of individuals and obtained a likelihood ratio for each ecotype. Results: The greater abundance of coastal dolphins was during spring (18.52 dolphins/hour) and for the oceanic ones it was in winter (51.43 dolphins/hour; Tables 1 and 3). The seasonal analysis determined that the expected herd counts could be similar while the individuals count will be different for each season (Coastal: herds D = 4.92 ( P = 0.1781); individuals D = 61.50 ( P < 0.001); and for Oceanic: herds D = 7.78 ( P = 0.0501); individuals D = 547.42 ( P < 0.001; Tables 2 and 4). Seasonally coastal dolphins present a greater number of herds and fewer individuals than the oceanic dolphins with the exception of spring season in both cases (Tables 5 and 6). Discussion and conclusions: Areas of high primary productivity are of great importance to marine mammals and due to that, their distribution and overall relative abundance obtained for coastal dolphins/hour is one of the highest that we have found reported for other coastal regions (Baumgartner et al. 2001; Bazúa and Delgado 2014). These results agree with the characteristics of both ecotypes of dolphins, the oceanic dolphins presenting larger groups; and the coastal dolphins having a larger number of herds, except in spring, this is probably due to the movement of the oceanic dolphin’s prey towards the continental shelf. Key words: Bottlenose dolphins; ecotypes; relative abundance; seasonality.