The following M.Sc. thesis was recently accepted by Hawai‘i Pacific University. Aschettino, J.M. 2010. Population size and structure of melon-headed whales (Peponocephala electra) around the Main Hawaiian Islands: evidence of multiple populations based on photographic data. M.Sc. Thesis, Hawai‘i Pacific University. 117 pp. A pdf copy available at http://www.cascadiaresearch.org/hawaii/melonheadedwhale.htm ABSTRACT: Despite the presence of melon-headed whales in tropical and sub-tropical waters world-wide, little is known about this species. Melon-headed whales frequent offshore waters surrounding the Main Hawaiian Islands where aerial surveys by Mobley and colleagues suggest a relatively small population (154 individuals (CV=0.88)). A 2004 near mass-stranding in Hanalei Bay, Kaua‘i occurred when an embayment of 150 – 200 animals coincided with naval mid-frequency sonar use. To assess population size and movements in Hawai‘i, more than 40,000 photos were collected from 47 encounters around the Main Hawaiian Islands between 2002 and 2009, and additional photos were obtained from collaborators from 1986 – 2001. Using only good quality photographs, there were 1,433 unique individuals in the photo-identification catalog, of which 1,046 were distinctive. Of these, 330 (31.5%) were seen on more than one occasion. Re-sighting data combined with social network analyses showed evidence of two distinct populations – a smaller, resident population, seen exclusively off the northwest region of the island of Hawai‘i, and a much larger Main Hawaiian Islands population, seen throughout the entire range of the Main Hawaiian Islands. Depth of encounters with the resident population were significantly shallower (median = 381 m) than those with the Main Hawaiian Island population (median = 1,844 m). Re-sightings of individuals have occurred up to 22 years apart for the Hawai‘i resident population and up to 13 years apart for the Main Hawaiian Islands population, suggesting long-term residency for both populations. Dorsal fin disfigurements suggest that fisheries interactions occur with members of both populations. Abundance estimated through mark-recapture analyses (corrected for the non-distinctive individuals), was 447 (CV = 0.12) for the Hawai‘i residents and 5,794 (CV = 0.20) for the Main Hawaiian Islands population. These estimates provide more accurate and precise population estimates of melon-headed whales in Hawai‘i. Although the total population size of melon-headed whales in Hawai‘i is higher than once believed, the restricted range of the small resident population may pose additional management implications for these individuals. _____________________ Jessica Aschettino, M.Sc. jma803@yahoo.com Hawai‘i Pacific University Kaneohe, HI 96744 and Research Associate Cascadia Research Collective