Risso’s dolphins in Hawai‘i

We’ve seen Risso’s dolphins on only sixteen occasions between 2000 and Fall 2021, with all sightings off the island of Hawai‘i. They are probably seen so infrequently because groups are usually far from shore (ranging from 3 to 61 kilometers; average = 37 km) in deep water (range 1,500 m to 4,700 m; average = 3,482 m). Analyses of sightings by depth taking into account effort show that Risso’s dolphins primarily use very deep water (>3,500 m) in Hawai‘i. Risso’s dolphins in Hawai‘i don’t flee from boats like striped dolphins, but they are very difficult to get close to. Group sizes are typically small, with a median group size of 4 and a range from 1 to 24 individuals.

We have a small photo-identification catalog for Risso’s dolphins with 142 unique individuals, but no individuals have been seen more than once, suggesting that this is a large population with low site fidelity.

In April 2015 we deployed a LIMPET satellite tag on an adult Risso’s dolphin, and we observed that over the next 14 days the animal exclusively used deep water offshore of the islands (med/max depth = 4,217m/4,816m, med/max distance from shore 57.6km/113.9km).

The depth of water where we’ve seen Risso’s dolphins over the past 20 years, combined with photo-identification and LIMPET satellite tag data, suggests that there is no resident insular population of Risso’s dolphins, and that these animals are part of a pelagic population. This is similar to a few other species we’ve encountered in Hawai‘i, including striped dolphins, Fraser’s dolphins, and Killer Whales.

Our sightings have spanned the period from April through November, and we have little research effort in the deep offshore waters during the period from December through March, so Risso’s dolphins may be present throughout the year.