Size and long-term growth trends of Endangered fish-eating killer whales

The Endangered southern resident population of killer whales Orcinus orca has been shown to be food-limited, and the availability of their primary prey, Chinook salmon Oncorhynchus tshawytscha, has been identified as a key covariate for the whales' individual survival and reproduction. We collected aerial photogrammetry data on individual whale size, which will help to better inform energetic calculations of food requirements, and we compared size-at-age data to make inferences about long-term growth trends. A helicopter was used to conduct 10 flights in September 2008, resulting in 2803 images from which useable measurements were possible for 66 individually identifiable whales, representing more than three-quarters of the population. Estimated whale lengths ranged from 2.7 m for a neonate whale in its first year of life, to a maximum of 7.2 m for a 31 yr old adult male. Adult males reached an average (asymptotic) size estimate ( plus or minus SE) of 6.9 plus or minus 0.2 m, with growth slowing notably after the age of 18 yr; this was significantly larger than the asymptotic size of 6.0 plus or minus 0.1 m for females, which was reached after the earlier age of 15 yr. Notably, there was no overlap between the ranges of estimated sizes of adult males (6.5 to 7.2 m) and females (5.5 to 6.4 m). On average, older adults (>30 yr) were 0.3 m (n = 14, p = 0.03) and 0.3 m (n = 5, p = 0.23) longer than the younger whales of adult age, for females and males, respectively; we hypothesize that a long-term reduction in food availability may have reduced early growth rates and subsequent adult size in recent decades.