Decadal changes in adult size of salmon-eating killer whales in the eastern North Pacific

Abstract

Two populations of killer whales aggregate around Vancouver Island to feed primarily on Chinook salmon. Aerial photogrammetry of endangered southern residents has documented some adults growing to smaller lengths in recent decades, suggesting that early growth may have been constrained by low Chinook availability in the 1990s. We investigated whether growth and adult length were also constrained in the more abundant northern residents. Photographs were collected from an unmanned hexacopter at altitudes of 30 to 37 m over 4 yr, 2014 to 2017. Images were linked to 78 individuals of known age and sex based on distinctive saddle patch pigmentation. The length of each whale was estimated by measuring pixel dimensions between both the snout and dorsal fin and the dorsal fin and fluke; these were scaled to real size using camera lens focal length and altitude, determined by a laser or pressure altimeter. Total length, derived by summing the longest (flattest) of each measure, ranged from 2.42 m for a first-year calf to 7.45 m for the largest adult male. A Bayesian change point analysis revealed that adult whales <40 yr old were on average shorter by 0.44 m than older adults, which grew to typical lengths of 6.28 and 7.14 m for females and males, respectively. This mirrors the growth trends reported for southern residents, supporting demographic evidence of correlated prey limitation in both populations. The growth data suggest that the effects of nutritional stress are not only acutely lethal but also have long-term consequences for the condition of whales in both populations.