Abstract
Entanglements affect marine mammal species around the globe, and for some, those impacts are great enough to cause population declines. This study aimed to document rates and causes of entanglement and trends in local haulout abundance for Steller and California sea lions on the north coast of Washington from 2010–2018. We conducted small boat surveys to count sea lions and document entangled individuals. Rates of entanglement and entangling material occurrence were compared with records of stranded individuals on the Washington and Oregon coast and with packing bands recorded during beach debris surveys. The rate of entanglement for California sea lions was 2.13%, almost entirely composed of adult males, with a peak rate during June and July potentially due to some entangled individuals not migrating to their breeding grounds. For Steller sea lions, the rate of entanglement was 0.41%, composed of 77% adults (32.4% male, 63.3% female), 17.1% juveniles, 5.9% unknown age, and no pups. Steller sea lions exhibited a 7.9% ± 3.2 rate of increase in abundance at the study haulouts, which was similar to that seen in California sea lions (7.8% ± 4.2); both increases were greater than the population growth rates observed range-wide despite high rates of entanglement. Most entanglements for both species were classified as packing bands, followed by entanglement scars. Salmon flashers were also prevalent and only occurred from June–September during the local ocean salmon troll fishery. Packing band occurrence in beach debris surveys correlated with packing band entanglements observed on haulouts. However, no packing band entanglements were observed in the stranding record and the rate of stranded animals exhibiting evidence of entanglement was lower than expected, indicating that entanglement survival is higher than previously assumed. Future studies tracking individual entanglement outcomes are needed to develop effective, targeted management strategies.
Highlights
The prevalence of man-made marine debris is of global concern and has been gaining attention from media, researchers, and the public in recent decades as the impacts to marine life become better understood [1,2,3,4,5]
California sea lions exhibited some seasonal variability with a peak in entanglement rates in the summer, coinciding with the lowest months for haulout abundance (Fig 5)
The California sea lions in this study experienced the second highest entanglement rate for any otariid in the published literature and the highest otariid entanglement rate documented in the United States (Table 3)
Summary
The prevalence of man-made marine debris is of global concern and has been gaining attention from media, researchers, and the public in recent decades as the impacts to marine life become better understood [1,2,3,4,5]. Entangling materials can come from terrestrial and marine pollution, and from derelict and active fishing gear. Packing bands and rubber bands are likely encountered passively as debris, while monofilament line, rope, and net fragments can be a sign of either passive encounters with derelict gear or a sign of interaction with an active fishing set. Salmon flashers and other hook and line setups are likely encountered as actively fished gear and are evidence of fishery depredation behaviors, which cause harm both to the entangled animal and to the fisher’s catch [10,11]. The frequency and nature of entangling interactions with marine debris might be governed by ocean currents, upwelling patterns, and marine traffic patterns, while interactions with active or derelict fishing gear are driven by fishing effort, gear types, and prey distribution [9,14,15,16,17]. Each type of entangling material poses different challenges and opportunities for mitigation, so identifying the source of entangling materials is crucial to building effective and targeted prevention plans
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