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 firstyear 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.

Survival of the fattest: linking body condition to prey availability and survivorship of killer whales

“…2011, Groskreutz et al. 2019) in the month of September in each of seven years. Images were collected from a manned helicopter in 2008 and 2013 (Fearnbach et al.…”

“…2011, Groskreutz et al. 2019). However, the range of both SRKWs and their salmon prey is enormous, encompassing over 3000 km of coastline, and identifying the prey populations that are most important for SRKWs is challenging.…”

Sexually dimorphic measurements from stranded and bycaught specimens contribute to the characterization of group composition in free‐ranging common dolphins (Delphinus spp.) from aerial images

“…Additionally, drones now offer more opportunities for photographically sampling cetaceans from the air (Durban et al, 2015), with the benefit of minimizing disturbance (Christiansen, Rojano‐Doñate, et al, 2016). Drones are increasingly being used to obtain precise morphometric measurements of cetaceans (Christiansen et al, 2020; Christiansen, Dujon, et al, 2016; Dawson et al, 2017; Durban et al, 2016; Fearnbach et al, 2020; Groskreutz et al, 2019), providing an opportunity to characterize demographic school composition of large dolphin schools. Here we present an example of how data from stranded and fisheries bycaught common dolphins can be used to develop sexually diagnostic measurements of Delphinus spp.…”

Bayesian approach for predicting photogrammetric uncertainty in morphometric measurements derived from drones

“…We then compared measurement uncertainty (as 95% HPD interval widths) and maturity classification between Model 1 and 2, as well as how uncertainty changes with altitude. 1993, Fearnbach et al 2018, Groskreutz et al 2019. We used measurements from up to 4 images per individual, for a total of 40 images.…”

“…This further highlights the importance of using a laser altimeter when estimating morphological relationships from UAS-based imagery. Morphological relationships pertaining to the skull are commonly used for distinguishing species (Leslie et al 2020), tracking ontological growth (Christiansen et al 2016), and estimating TL when direct measurements of TL are not obtainable (Ratnaswamy & Winn 1993, Fearnbach et al 2018, Groskreutz et al 2019. The Bayesian framework we present here demonstrates how morphological relationships are simultaneously estimated with photographically derived measurements, rather than in 2 stages, in which physical measurements are first extracted from photographs and then used to estimate morphological relationship.…”