Abstract
This study investigates survival and abundance of killer whales (Orcinus orca) in Norway in 1988–2019 using capture–recapture models of photo‐identification data. We merged two datasets collected in a restricted fjord system in 1988–2008 (Period 1) with a third, collected after their preferred herring prey shifted its wintering grounds to more exposed coastal waters in 2012–2019 (Period 2), and investigated any differences between these two periods. The resulting dataset, spanning 32 years, comprised 3284 captures of 1236 whales, including 148 individuals seen in both periods. The best‐supported models of survival included the effects of sex and time period, and the presence of transients (whales seen only once). Period 2 had a much larger percentage of transients compared to Period 1 (mean = 30% vs. 5%) and the identification of two groups of whales with different residency patterns revealed heterogeneity in recapture probabilities. This caused estimates of survival rates to be biased downward (females: 0.955 ± 0.027 SE, males: 0.864 ± 0.038 SE) compared to Period 1 (females: 0.998 ± 0.002 SE, males: 0.985 ± 0.009 SE). Accounting for this heterogeneity resulted in estimates of apparent survival close to unity for regularly seen whales in Period 2. A robust design model for Period 2 further supported random temporary emigration at an estimated annual probability of 0.148 (± 0.095 SE). This same model estimated a peak in annual abundance in 2015 at 1061 individuals (95% CI 999–1127), compared to a maximum of 731 (95% CI 505–1059) previously estimated in Period 1, and dropped to 513 (95% CI 488–540) in 2018. Our results indicate variations in the proportion of killer whales present of an undefined population (or populations) in a larger geographical region. Killer whales have adjusted their distribution to shifts in key prey resources, indicating potential to adapt to rapidly changing marine ecosystems.
Highlights
Life history and other population parameters are key elements in status assessments of animal populations
This study investigates survival and abundance of killer whales (Orcinus orca) in Norway in 1988–2019 using capture–recapture models of photo-identification data
Our results show that the NSS herring remains a major ecological driver of killer whale dynamics in this region
Summary
Life history and other population parameters are key elements in status assessments of animal populations. Recent studies in seasons and locations not previously investigated have documented new prey types, that is, Atlantic salmon (Salmo salar; Vester & Hammerschmidt, 2013), Atlantic mackerel (Nøttestad, Sivle, Krafft, Langard, et al, 2014), harbor porpoise (Phocoena phocoena; Cosentino, 2015), lumpfish (Cyclopterus lumpus; Jourdain et al, 2019), and pinnipeds (Jourdain et al, 2017; Vongraven & Bisther, 2014) for killer whales in Norway, including for individuals known as herring-eaters (see Jourdain et al, 2019, 2020) These new observations could be the result of enhanced research effort but could reflect behavioral responses to a changing marine ecosystem. Supplementary photographs, with reliable information on date and time, collected from wildlife photographers and members of the general public within the study area were used for identification purposes in 2012–2019 (Table 1; Figure 1)
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