Abstract
Identifying mortality sources and mitigation solutions is crucial in species management and conservation. In killer whales (Orcinus orca), mortality events may pose a serious concern for the conservation of small discrete populations, especially if they involve entire groups. This study investigated 19 incidents involving 116 killer whales from a minimum of five populations becoming naturally entrapped in inshore areas of the North Pacific (n= 12) and North Atlantic (n= 7) oceans between 1949 and 2019. Here, we aim to provide an assessment of possible causal factors, lethality and human responses to these events. Site characteristics and group size identified three categories of entrapments. InCategory 1, nine cases involved small groups of killer whales (median = 5, range: 1–9) at sites characterized by severe geographic and food constraints. Four cases inCategory 2included larger groups (median= 14, range: 6–19) and entrapment sites with no obvious geographic constraints but at which man-made structures could have acted as deterrents. Five cases assigned toCategory 3involved lone, often young individuals settling in a restricted home range and engaging in interactions with people and boats. Overall, all or some of the killer whales swam out on their own after a mean of 36 d of entrapment (range: 1–172, SD = 51,n= 9 cases), died of nutritional/physiological stress after 58 d (range: 42–90, SD = 21,n= 3 cases) or of injury after ~5 years of daily interactions with boat traffic (n= 1 case). Indication of the killer whales' declining condition or being at risk of injury, and of poor habitat quality, led to the decision to intervene in seven cases where a variety of methods were used to guide or relocate remaining individuals back to open waters after 39 d (SD = 51, range = 8–150). Monitoring protocols, which aided in identifying entrapment situations, and intervention methods which enhanced the health and survival of entrapped killer whales, are discussed.
Highlights
In wildlife management and conservation, identifying causes of mortality is important for status assessment and for implementation of mitigation measures
Reviewing available information for 19 cases of natural entrapment of killer whales recorded for the period 1949–2019, this paper examines the characteristics of, and mortality induced by, these events
We compiled 19 events of natural entrapments involving a total of 116 killer whales from a minimum of five populations for the period 1949–2019: 12 in the eastern North Pacific and seven in the North Atlantic (Tables 1, 2; Supplementary Table 1; Figures 1, 2)
Summary
In wildlife management and conservation, identifying causes of mortality is important for status assessment and for implementation of mitigation measures. Motivated by the desire to enhance animal welfare, and perhaps to conserve species in some cases, responding to cetacean strandings has become common practice worldwide (Moore et al, 2018). Other entrapments involve cetaceans found outside of their typical habitat (e.g., a Bryde’s whale Balaenoptera edeni entrapped for 100 days in a river of New South Wales, Australia, see Priddel and Wheeler, 1997, 1998) or trapped in confined waters due to falling tide and topographical obstacles (e.g., spinner dolphins Stenella longirostris entrapped in small lagoons on Saipan Island upon falling tide, see Trianni and Kessler, 2002). Intervention to assist trapped cetaceans in finding their way to open waters may be required, especially if the species is of high conservation concern
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