Abstract: This study reports new information on Grampus griseus (G. Cuvier, 1812) distribution with depth and slope, group size, general and interspecific behaviour, and calving intervals for the Azorean archipelago. Observations are in agreement with scarce previous work and most other regions in the North Atlantic and Mediterranean. Data were collected on 107 sightings south of Pico Island that took place between 17 May and 29 August 2003 from opportunistic research platforms. Information on occurrence, provided by an observer with 50 y of land-based cetacean experience, was included for comparison. Risso’s dolphins preferred areas between 497 and 1,233 m depth (modal class 600 to 650 m; N = 69), with slopes between 27 and 35%, although these data require validation. The majority of groups were composed of up to 20 individuals (modal 6 to 10), averaging 12.3 (1 to 55; N = 74). Large socializing aggregations observed during July and August (55 to ~175 individuals) were similar to land-based observations between 1992 and 2005. G. griseus‘s diurnal activities in this study were mostly traveling (77%) and socializing (13%), with feeding (5%) and resting (3.7%) occurring less frequently. The first newborn calves are reported. Two species groups and interactions with six cetacean species are described. Harassment behaviours with Globicephala spp. and Physeter macrocephalus suggest competitive interference. Year-round observations between 1992 and 2005 near Pico Island, together with birth reports and recent data on site fidelity, suggest population residency.

Key Words: Risso’s dolphin, Grampus griseus, ecology, behaviour, aggregations, newborn, mixed-species groups, harassment

Document Type: Research article

DOI: 10.1578/AM.34.4.2008.426

Page Numbers: 426-435