Fine scale habitat selection of spinner dolphins in the remote atolls of far-western Hawai‘i: How picky can you be when there is little to choose from?

Abstract

Spinner dolphins frequently use inshore island and atoll habitat for daytime rest and social interactions, and forage over deep waters at night. In far-western Hawaii, groups of spinner dolphins associated with Kure and Midway Atolls have been a subject of long-term research since 1998. In the current study, we applied a local nearest-neighbor convex hull construction (LoCoH) approach to examine area/habitat use pattern in both atolls, equivalent to bounded home ranges based on a non-parametric kernel method. The nearest neighbor value was chosen based on the k-area chart which allows for selecting suitable k values based on the total area of local hulls and shift within them when nearest neighbor increases. In both atolls, dolphin spatial distribution is significantly more constrained than random movement paths (p>99.8004, with n=500 random movement paths), with the core areas (50% isopleth) restricted to small areas over an ‘unconsolidated’ and reflective surface habitat with low levels of ambient noise, in generally shallow water, and in areas sheltered by either neighboring land structure or local bathymetry. Depth contours are used as vectors of movement within the atoll’s lagoon, while areas of extreme local depth and shallows are generally avoided. Several important behaviors, such as resting and socializing, occur over spatially-restricted areas, contrary to behaviors such as travel that might occur over a considerably larger portion of the lagoon. The animals are highly selective in their choice of resting habitat and, with little shelter from low-laying islands, the micro-bathymetry of the atolls determines much of their daytime activities. The availability, accessibility, and size of patches suitable to function as resting sites seem to be an important, perhaps the primary limiting factor, likely determining the atoll’s carrying capacity for a spinner dolphin population, hence affecting local population size and much of their socio-behavioral dynamics.