Sex differences in residency patterns of Risso's dolphins (Grampus griseus) in the Azores: Causes and management implications

Abstract

Knowledge of the residency patterns of marine mammals is an important element for management and conservation strategies. Here we investigate a population of Grampus griseus off Pico Island, Azores. Our data set covers the period 2004–2007, based on at‐sea observations of 1,250 individually identified animals, 303 of known or assumed sex. Using photo identification and GPS locations we calculated mean monthly sighting rates and lagged identification rates to analyze temporal patterns, and estimated kernel density to study the home range. Our results show site fidelity and relatively restricted home ranges, which corroborate the existence of a resident population on the study site. We further document sex differences, including a higher number of males present in the area at any given time but females staying for longer consecutive periods, and male home ranges with significantly less overlap than those of females. These observations are consistent with a mating system based on multimale pods defending areas where females periodically return. We hypothesize that squid distribution is a major factor in structuring these patterns. These findings reinforce the need for a precautionary management approach that would include limiting pressure from commercial activities.

Annual variability in dentin δ15N and δ13C reveal sex differences in weaning age and feeding habits in Risso's dolphins (Grampus griseus)

“…Further, females exhibited larger networks of associations with other females, whereas males preferred to associate with particular males (Lin 2003). Similar home range preferences for 760 groups of females and for males were also observed in Risso's dolphins in the Azores, Portugal (Hartman et al 2015). Genetic analyses on skin samples of Risso's dolphins in the Ligurian Sea found kin-based affiliations within groups of females but not within groups of males (Gaspari 2004).…”

“…Similar home range preferences for groups of females and for males were also observed in Risso's dolphins in the Azores, Portugal (Hartman et al . ). Genetic analyses on skin samples of Risso's dolphins in the Ligurian Sea found kin‐based affiliations within groups of females but not within groups of males (Gaspari ).…”

“…) and exhibit sex related differences in residency patterns (Hartman et al . ) and home range width (Lin ).…”

“…Risso's dolphins tend to remain with their natal group after weaning and subsequently leave to form groups of juvenile animals at approximately 5 yr of age (Amano and Miyazaki 2004). Individuals form long-term bonds of~2-12 animals of the same sex and similar age (Hartman et al 2008) and exhibit sex related differences in residency patterns (Hartman et al 2015) and home range width (Lin 2003).…”

Modeling the Potential Distribution and Richness of Cetaceans in the Azores from Fisheries Observer Program Data

“…The Risso's dolphin model indicates an expansion of the potential distribution up to June and then a contraction after that month. Coastal habitats, however, seem to be important during most of the period, which may be related to the presence of resident groups using these areas as foraging, calving and nursing habitats (Hartman et al, 2014(Hartman et al, , 2015. In contrast, the short-finned pilot whale model indicates a potential distribution that is spatially and temporally homogeneous.…”