Birthing season for harp seals in Labrador Sea just in time to feed hungry polar bears

From polarbearscience

The main birthing period for NW Atlantic harp seals has arrived. Local populations of ringed and bearded seal pups will soon follow but in the Gulf of St. Lawrence in eastern Canada, the pupping season for harp seals that is usually in progress by this time has likely been redirected north due to lack of suitable ice conditions. Sea ice off Labrador and Newfoundland is in good condition and this is where the vast majority of the global population go to give birth (ca. 7.6 million vs. 1.5 million in the White Sea and 434,000 in east Greenland).

Timing of births

The timing sequence for births is White Sea, Russia (late February to mid-March); Gulf of St. Lawrence, Canada, aka the “Gulf” (traditionally early March, now 26 February, see below); Labrador/Newfoundland, Canada aka the “Front” (mid to late March, mean 12 March); East Greenland around Jan Mayen Island, aka the “West Ice” (late March to early April), see the map below. The Jan Mayen/West Ice region is the furthest north that harp seals pup, breed and moult, and thus the last in the season for this species (Stenson et al. 2020). Current sea ice map here.

However, it appears that this traditional timing has changed in the Gulf to coincide with changing ice conditions. According to the report by Mike Hammill and colleagues in 2021 (pg. 21, my bold), the time when the first seals give birth in the season advanced by one week between 2009-2019:

In the meantime, as ice cover declines, it is predicted that the timing and duration of peak ice will also change with a shift in peak ice to later dates and earlier breakup. In the Gulf the appearance of first births has advanced by one week, from 28 to 21 February, over the last decade (Hammill unpublished). It will be interesting to see if pupping at the Front also advances as the ice declines in that area as well.

Consequently, the mean date of birth in the Gulf has also shifted forward from March 5 to February 26 (Stenson et al. 2020:3). However, I have not heard any seal specialist mention this evidence of evolutionary adaptation to the media when discussing this population.

This is what the IUCN Red List (Kovacs 2015) says about the newborn seals that polar bears feed upon (references removed for easier reading, see original here):

“Pups are born on the open, free-floating, pack ice and nursed for 12 days, during which time they gain weight at about 2.2 kg per day. To minimize energy expenditure, most pups are sedentary for the first month; some are so immobile that they melt into the ice beneath them, forming ice “cradles”.

Pups are referred to as “whitecoats” because they are born with an insulating coat of white lanugo. Lactating females spend up to 85% of their time in the water, depending on the weather. Towards the end of lactation, females come into oestrus and mate.

Weaned pups remain on the ice for up to six weeks, losing up to 50% of their weight before entering the water to feed.”

This means that harp seal pups in each discrete area of the eastern Arctic are available on the sea ice as easy prey for polar bears for about 8 weeks, although during the first few days they would provide little in the way of fat calories. Distracted adults of both sexes may also be at risk of becoming a polar bear meal during this short period.

Contrary to a report in January by the CBC (19 January 2023, ‘It’s the season for seal pups, and they might show up in weird places’) which implies that harp seals will have their pups on beaches if sea ice is not available, biologists say this is not the case.

In poor ice years, like 2020 and 2021, harp seals move east to the Front to give birth on the pack ice off Newfoundland and Labrador; in 2017 (a poor ice year in the Gulf), there were no reports of births on land (Hammill et al. 2021:18). However, a few pups born on the ice that have been prematurely dumped into the sea, by storms or disintegration of ice, may be washed onto beaches while still alive.

Harp seal numbers in eastern Canada are still increasing (from 7.6 million in 2017) and are predicted to continue doing so (up to > 9 million) until about 2035 (Hammill et al. 2021). A decline in abundance after that point is based on an assumption, which may or may not turn out to be correct, that sea ice in the Labrador Sea (i.e. the Front) will decline continuously after 2040 or so, see graph below from the Hammill report.

Sea ice conditions in the Gulf

About as much ice as there was last year about this time but it’s still not very thick. Thin first year ice (ca. 50cm thick, light green on the chart below for the week of 27 February) is best for whelping females, so virtually all harps that may once have given birth in the Gulf will have moved north to the Front:

Sea ice conditions at the Front

For the week of 6 March 2023, the ice has set up nicely at the Front for seal births: perfect conditions in the south around Newfoundland (light green), the northern Gulf around the Strait of Belle Isle, and along the Labrador coast, especially for those individuals that give birth early in the season:

If we look at daily ice charts for this week, the dynamic pack ice at the Front has spread out and has compacted along the coast due to winds from the north, which is not unusual:

With seals come bears

And of course, with the seals come the polar bears, which began making regular appearances in coastal communities in the Front last week and continued this week.

St. Carol, northern Newfoundland 7 March 2023.

Photo below from the small community of St. Lewis on the southern Labrador coast, 13 March.

References

Hammill, M.O., Stenson, G.B., Mosnier, A. and Doniol-Valcroze, T. 2021. Trends in abundance of harp seals, Pagophilus groenlandicus, in the Northwest Atlantic, 1952-2019. Department of Fisheries and Oceans, Canadian Science Advisory Secretariat (CSAS). Research Document 2021/006.

Kovacs, K.M. 2015. Pagophilus groenlandicus. The IUCN Red List of Threatened Species 2015: e.T41671A45231087. http://dx.doi.org/10.2305/IUCN.UK.2015-4.RLTS.T41671A45231087.en

Stenson, G.B., Haug, T., and Hammill, M.O. 2020. Harp seals: Monitors of change in differing ecosystems. Frontiers in Marine Science 7:569258. Open access.