Risso's dolphins of La Herradura, Coquombo, Chile

Abstract

The Risso’s dolphin (Grampus griseus G. Cuvier, 1812), one of the largest in the family Delphinidae, with groups ranging from a couple to over 80 individuals, can be found in temperate and tropical regions worldwide. They prefer waters 400 - 1000 meters deep and proximity of continental slopes. Their social structure was suggested to be based on age and sex, with strong bonds between adult males and between adult females. In Chile, prior to my research, there is only one publication on Risso’s dolphins; according to this report, they occur continuously along the Chilean coast.

La Herradura, a small and shallow bay, is located on the central-north coast of Chile, facing the Pacific Ocean. During a one year of study (April 2011 to April 2012), daily scans of the bay were conducted, covering the interior of La Herradura, and photo-identification boat surveys were undertaken within and outside the bay. In the course of this 1-year study, a total of 13 encounters were recorded outside the bay and 16 encounters inside the bay. The occurrence of Risso’s dolphin outside the bay did not show any clear pattern across the year. Dolphin groups encountered outside had a mean group size of 20 individuals (SD=19.5) and a median of 16. Of these groups, 38.5% had calves and the main behavior showed by the groups was travelling and foraging. Inside La Herradura bay, the groups of dolphins were seen primarily during the morning (8:00-12:00); this pattern was consistent across the study period. The mean group size was 63 individuals (SD=45.3) and a median of 46 individuals. The 87.5% of these groups had calves among them and the main behavior observed was milling.

A total of 747 individuals were photographically identified across 44 encounters and were used for mark-recapture analyses. Supplementary data, collected opportunistically between 2007 and 2010, provided by Mr. Oliver Yates, were included to increase the robustness of the analyses. Most of the individuals were sighted just once or twice during the 5-year period (2007-2012). The estimated size of the population generated by POPAN parametrization in program MARK was 3468 individuals (SE=188.5; 95% CI 3144 to 3889). Overall, Risso’s dolphin in the La Herradura area had very low site fidelity and the associations among individuals were generally weak (mean HWI=0.17 ±±SD=0.06). However, HWI picked at 0.2-0.3 and the results of principal coordinate analyses (5 clusters, modularity= 0.24) seem to indicate a grouping pattern that, although not rigorously tested due to the limitation of the sample size, might be indicative of preferential cluster-type associations within an overall fluid society.

Overall, in Coquimbo, Risso’s dolphin groups with calves seemed to use the La Herradura Bay as a resting site during the morning, probably after having spent the night foraging. The animals display typical fission-fusion dynamics. However, it is possible that the dolphins form a substantially more structured social grouping, which could only be tested with a longer study period and larger sample size.