Long-term strategies for studying rare species: results and lessons from a multi-species study of odontocetes around the main Hawaiian Islands

Context. Funding agencies are often unlikely to fund research on rarely-encountered species and limited time is usually spent with such species when they are not the focus of research. Thus, knowledge of these species often lags behind their encounter rates. Aims. To gain information on rarely-encountered odontocetes in Hawai'i while simultaneously studying common ones. Methods. During a long-term small-boat based study, we prioritised time spent with rarely-encountered species, collecting photos and biopsy samples, and satellite tagging. Sample sizes were augmented with photo contributions from members of the public and other researchers, and genetic samples from stranded animals and other researchers. Results from genetic and tag data analyses were interpreted in the context of social network placement and re-sighting histories. Key results. Pygmy killer whales (Feresa attenuata) represented <2% of odontocete sightings, and sighting rates varied by depth and among islands. Photo-identification shows that 318 of 443 identified individuals are linked by association in the main component of the social network. Movements among islands were limited, with individuals off O'ahu and Hawai'i exhibiting high site fidelity, although resident groups from each island share a common mitochondrial haplotype. Three groups involved in mass strandings in two different years were not linked to the main component of the social network, and did not share mitochondrial haplotypes with known resident groups. Conclusions The approach of prioritising rarely-encountered species for additional sampling is an effective way of learning more about poorly-known species. Implications. Such an approach may be critical for filling data gaps for populations potentially at risk from human activities.