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

Abstract

Recovering small, endangered populations is challenging, especially if the drivers of declines are not well understood. While infrequent births and deaths may be important to the outlook of endangered populations, small sample sizes confound studies seeking the mechanisms underlying demographic fluctuations. Individual metrics of health, such as nutritive condition, can provide a rich data source on population status and may translate into population trends. We examined interannual changes in body condition metrics of endangered Southern Resident killer whales (SRKW) collected using helicopters and remotely operated drones. We imaged and measured the condition of the majority of all three social pods (J, K, and L) in each of seven years between 2008 and 2019. We used Bayesian multi‐state transition models to identify relationships between body condition changes and both tributary‐specific and area‐based indices of Chinook salmon abundance, and K‐fold cross‐validation to compare the predictive power of candidate salmon covariates. We found that Fraser River (tributary‐specific) and Salish Sea (area‐based) Chinook salmon abundances had the greatest predictive power for J Pod body condition changes, as well as the strongest relationships between any salmon covariates and SRKW condition across pods. Puget Sound (tributary‐specific) Chinook salmon abundance had the greatest predictive power for L Pod body condition changes, but a weaker relationship than Fraser River or Salish Sea abundance had with J Pod body condition. The best‐fit model for K Pod included no Chinook covariates. In addition, we found elevated mortality probabilities in SRKW exhibiting poor body condition (reflecting depleted fat reserves), 2–3 times higher than whales in more robust condition. Collectively, these findings demonstrate that (1) fluctuations in SRKW body condition can in some cases be linked to Chinook salmon abundance; (2) the three SRKW pods appear to have distinct patterns of body condition fluctuations, suggesting different foraging patterns; and (3) aerial photogrammetry is a useful early‐warning system that can identify SRKW at higher risk of mortality in the near future.

Key questions in marine mammal bioenergetics

“…Body condition, defined as the amount of energy reserves relative to structural size, is a physiological unit of considerable interest in conservation-focused bioenergetic studies. As the physical manifestation of energy balance, body condition provides essential information about the health of individuals and populations ( Brock et al, 2013 ; Williams et al, 2013 ; Christiansen et al , 2020 ; Raverty et al , 2020 ; Stewart et al , 2021a ). A variety of metrics have been developed to estimate body condition of free-ranging marine mammals, such as those derived from blubber measures, morphometrics, biochemical or chemical markers, body composition and omics (see reviews in Bowen and Northridge, 2010 ; Castrillon and Bengtson Nash, 2020 ).…”

“…While the focus here is on bioenergetic pathways, relationships between body condition and vital rates may thus incorporate the effects of non-bioenergetic pathways as well. In marine mammals, body size and condition metrics are positively related to foetal growth ( Christiansen et al, 2014b ), pregnancy rates ( Williams et al, 2013 ; Smout et al, 2020 ), offspring growth ( Christiansen et al, 2018 ) and survival probability ( Hall et al, 2001 ; Beauplet et al, 2005 ; Harding et al, 2005 ; McMahon and Burton, 2005 ; Bowen et al, 2015 ; Cheney et al, 2018 ; Oosthuizen et al, 2018 ; Stewart et al, 2021a ). For most marine mammals, these relationships and thresholds of body condition that equate to failed reproduction or imminent mortality remain largely unknown.…”

“…It can also affect energy expenditure through changes in habitat availability ( Pagano and Williams, 2021 ) or thermal landscapes ( Cunningham et al, 2021 ). Evidence is mounting that the inability to obtain sufficient energy is affecting individual growth, reproduction, and survival of marine mammals ( Stirling and Derocher, 2012 ; Ferguson et al, 2017 ; Christiansen et al, 2020 , 2021 ; Stewart et al, 2021a ). If a sufficiently large number of individuals are affected, this can ultimately affect population growth rates ( New et al, 2014 ; Baylis et al, 2015 ), as illustrated by the Population Consequences of Disturbance (PCoD) framework ( Pirotta et al, 2018a ).…”

Requirements and availability of prey for northeastern pacific southern resident killer whales

“…Our model predicted that the SRKW population has been in energetic deficit for six of the last 40 years, notably from 2018 to 2020. Several previous studies have highlighted positive correlations between Chinook salmon abundance and SRKW body condition [ 73 ], survival rate [ 39 ], and fertility [ 40 ]. Although our analysis did not permit to support those findings with certainty, our model suggests that SRKW could have a higher birth rate and net population and a lower mortality rate in years during which their DPER was met.…”

Foreword to the Special Issue on ‘The rapidly expanding role of drones as a tool for wildlife research’

“…nutritional status) of a range of species, particularly in marine environments (e.g. Pirotta et al 2017;Allan et al 2019;Stewart et al 2021a). Recent advances have shown that the precision of photogrammetric methods can be dramatically improved using deep learning, reducing the amount of time spent inputting information manually, which will facilitate expansion and development of photogrammetric methods to automatically measure individual animals with the greatest precision (Gray et al 2019).…”