Little is known about harbor porpoises at the individual level or local group structure. Group characteristics, site fidelity, and photo‐identification of harbor porpoises were investigated off Fidalgo Island, Washington State. Harbor porpoise presence was affected by season and rip tide strength (Wald χ2 P < 0.04); calf presence was influenced by season and tide (Wald χ2 P < 0.0075). Average group size (2.32 ± 1.38, n = 266) was influenced by season, behavior, and calf presence (F7 = 9.71, P < 0.0001, R2 = 0.294). Fifty‐three individuals were identified using a matrix of primary, secondary, and confirmation markings that were stable over months/years. Over 35% were resighted in more than 1 mo (range 1–7, truexfalse¯ = 1.83); 15.1% were seen in more than 1 yr, suggesting some level of residency. Despite having higher effort, presence and group size were significantly lower in Summer. Variations in the significance of rip tide strength and tides relate to calf presence and support other findings that harbor porpoise population structure is complex and varies at small spatiotemporal scales and may also vary between populations and habitats. This study identifies variables affecting group characteristics and emphasizes the importance of research on local populations of harbor porpoises.
Harbour seals (Phoca vitulina) commonly form larger congregations at haul out locations during times of rest and pupping season, but are generally thought to be solitary at sea. Occasionally larger clusters of individuals may be observed swimming near haul out sites, forced bottlenecking channels or mouths of rivers with concentrated prey and restricted space. Recently, isolated occurrences of mass gatherings of harbour seals have been observed in the Salish Sea that were distanced from haul out sites (over 1 km away) or forced bottlenecking regions. In April-June (but primarily May) 2019–2021 juvenile and adult harbour seals in Burrows Pass (Anacortes, WA, USA) were observed in large groups () ranging in size from 6–50 individuals () within 1-2 body lengths of each other and periodically diving down seemingly hunting and chasing prey. These groupings primarily occurred during flood and slack high tides. Based on the surface level activity observed, habitat type, the frequency of individuals using the area for foraging year round and the tidal preferences during the occurrences, it is likely these are foraging events. Similar large groups have been documented () in the South Puget Sound and Central Puget Sound, first observed in 2016 and officially documented in February of 2017. These groupings (from 20–30 to 150+) occurred year round and at varied tidal states. While some sightings were obviously foraging behaviour, others appeared to be resting, traveling or socializing. Open water behaviour of harbour seals is not well documented, and a literature review found no other published accounts of large in-water groupings. Investigation of ecological relationships (like prey spawning, prey abundance, or other environmental correlates) and observation of underwater harbour seal behaviour will aid in determining the reason for this seemingly novel behaviour.
Stealing of food items from another animal, or kleptoparasitism, has been well studied in bird species. Bald eagles are known kleptoparasites of other birds and occasionally other species, however kleptoparasitic interactions with mammals are relatively uncommon. We describe instances of bald eagles taking, or attempting to take, fish and mammal prey from three species of cetaceans (bottlenose dolphins (Tursiops truncatus), harbour porpoises (Phocoena phocoena vomerina) and killer whales (Orcinus orca)) and one species of pinniped (harbour seal (Phoca vitulina richardii)) on the east and west coast of the United States of America. We discuss possible drivers of this emerging behaviour, including bald eagle population increases, reductions in other prey abundance, and changes in prey choice (for harbour porpoises). Further research is needed to determine if this behaviour is opportunistic in nature, or a more common foraging strategy.
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