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Endangered Species Research

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ESR 13:173-180 (2011)  -  DOI:

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

Holly Fearnbach1,*, John W. Durban2,3, Dave K. Ellifrit2, Ken C. Balcomb III2

1School of Biology, University of Aberdeen, Lighthouse Field Station, Cromarty, Ross-shire IV11 8YJ, UK
2Center for Whale Research, Friday Harbor, Washington 98250, USA
3Protected Resources Division, Southwest Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, 8604 La Jolla Shores Drive, La Jolla, California 92037, USA

ABSTRACT: 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 (±SE) of 6.9 ± 0.2 m, with growth slowing notably after the age of 18 yr; this was significantly larger than the asymptotic size of 6.0 ± 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.

KEY WORDS: Killer whale · Photogrammetry · Size · Growth · Salmon

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Cite this article as: Fearnbach H, Durban JW, Ellifrit DK, Balcomb KC III (2011) Size and long-term growth trends of Endangered fish-eating killer whales. Endang Species Res 13:173-180.

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