Open water grouping behaviour in harbour seals (Phoca vitulina richardii) of the Salish Sea

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.