Distribution of pantropical spotted dolphins in Pacific coastal waters of Panama

Abstract

Spotted dolphins (Stenella attenuata) have been subjected to large removals as bycatch in the purse-seine tuna fishery. While pelagic stocks are relatively well known, information on coastal populations is scarce. This study attempted to quantify seasonal differences in the distribution of a coastal population in Bahía Honda, on the Pacific coast of Panama (7°50'N, 81°35'W). Field work (117 days) was conducted from January to June 2002. To analyse distribution, the study area was divided into a grid of 4.84km² squares. Survey effort was logged by recording a GPS position every two minutes. Number of sightings in each square was divided by effort and corrected according to the proportion of sea/land. Dolphin presence and group size distribution were analysed using geostatistics and density contours were created through kriging. Only slight differences in distribution were detected between dry (Jan-Mar) and rainy (Apr-Jun) seasons. Dolphins preferred the area around Cativo bay, with a secondary core area next to Canales de Tierra Island. Encounters were rare towards Rosario bay. In general, sightings per unit effort seemed to be higher during the rainy season, although this may be a result of the dolphins getting used to the boat and improved observer experience. Group size was very variable, ranging from solitary animals to around 50 individuals with a median of 12 with no seasonal difference (U = 7142, p = 0.83). Most groups found in Bahía Honda (83.4%) contained fewer than ten individuals. This study, in conjunction with those elsewhere along the coastal eastern Pacific suggests that group size could be related to depth and/or to the presence/absence of the sympatric bottlenose dolphin, Tursiops truncatus. Further studies on biotic and abiotic correlates of dolphin distribution are needed, and information on potential prey, diet and feeding habits would help clarify the interspecific relationship with bottlenose dolphins.