Estimating the mortality rate of humpback whale calves in the central North Pacific Ocean

Sighting histories of individually identified female humpback whales (Megap tera novaeangliae) in their winter and summer ranges were used to investiga te mortality of North Pacific humpback whale calves. We compiled records co llected between 1979 and 1995 by eight independent research groups, which y ielded 29 cases where 25 different mothers sighted in Hawai'i were identifi ed later the same year in Alaska. In 7 of 29 cases, a calf sighted with its mother in Hawai'i was missing from its mother's Alaska sighting(s). After investigating many factors, we determined that the largest potential bias w ould occur in late-autumn observations, when calf absences might indicate w eaning or temporary mother-calf separation rather than calf mortality. Our minimal and most robust estimate excluded all mortalities and survivals bas ed on sightings of the mother after October 31; 3 of 20 cases or 0.150 (95% confidence intervals (CI) = 0.032, 0.378). The maximal calf mortality rate , derived from all the available data, was 7 of 29 cases or 0.241 (95% CI = 0.103, 0.434). An intermediate estimate that excluded all cases based on s ingle Alaska sightings and omitted late-season sightings (2 of 11 cases or 0.182; 95% CI = 0.023, 0.518) is perhaps closest to the actual first-year m ortality rate for humpback whale calves, although it is compromised by its small sample size. Our results demonstrate both the value and the limitatio ns of using longitudinal data to determine the life-history parameters that are essential for documenting the recovery of endangered populations.