This post has been reblogged from PolarBearScience.
The harp seal is the most abundant seal species in the northern hemisphere (estimated to number more than 9 million animals – that’s more harps than ringed seals) but are found only in the North Atlantic. Partly because they give birth on mobile pack ice, harps have their pups earlier in the season than all other Arctic seals, which means that in some regions, they are a critical food source for polar bears that have eaten little over the winter months.
Although young ringed seals are considered the primary prey of polar bears throughout the Arctic, young harp seals undoubtedly represent an increasingly important resource for populations of Davis Strait, East Greenland and Kara Sea bears.
Most of the harp seals in the NW Atlantic/Atlantic Canada (about 80% of them) have their pups off Newfoundland and Labrador, an area known as the “Front” (the location of my polar bear attack novel, EATEN). Harps seals at the Front now provide a huge prey base for polar bears of the large (and possibly still growing) Davis Strait subpopulation (photo below courtesy DFO Canada).
There are an estimated 7.4 million harps in Atlantic Canada today (range 6.5-8.3m), an exponential increase over the early 1980s, when perhaps only half a million so remained. Pagophilus groenlandicus was assigned a conservation status of ‘Least Concern’ by the IUCN Red List in June last year (Kovacs 2015), when it was estimated that the global population size of the harp seal was greater than 9 million animals and probably growing1, 2 due to reduced human hunting:
“…harp seals have been harvested for thousands of years but currently the population is large and the number of animals harvested is declining.” [my bold]
Photographers and animial rights activists love cute, fluffy harp seal pups and rarely mention the carnage that goes on in spring as polar bears devour the naive youngsters. See the video below (from 2008), for an example of the cuteness factor.
Range and breeding grounds of harp seals
While adults and fully independent young range more widely across the Eastern Arctic (Fig.1), the likelihood of them being consumed by polar bears decreases as they move away from these three pupping grounds over the summer and fall.
Harp seals are also known as “saddlebacks” because of the distinctive markings on the males (Fig. 2, below):
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):
“Birthing takes place in vast herds from late February to April. There is some variation across the range in the precise timing but herds are highly synchronized. 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. During this post-weaning fast they complete moulting of the lanugo. After this coat is shed, the black and silver-gray pelage is exposed and the juveniles are known as “beaters” until they are about a year old.” [my bold]
The reference to late February pupping in the Red List summary applies only to White Sea females: the timing sequence is White Sea (late February); Gulf of St. Lawrence, aka the “Gulf” (early March, mean 5 March); Labrador/Newfoundland, 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). The Jan Mayen/West Ice region is the furthest north that harp seals pup, breed and moult.
This means that harp seal pups 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 (Fig. 3 below).
From early April to early May every year, seals one year of age and older spend most of their time on the ice as they moult their hair coat, during which they may be more vulnerable to predation by polar bears (Fig. 4 below). The spring/summer sequence goes like this for harps and indeed all Arctic seals (although the timing is slightly different for each species and each stage): pups born and nursed, pups weaned, mating, moulting.
In contrast to ringed seals (which do most of their feeding in late summer and early fall, in open water), harp seals feed most heavily in winter and summer and less in spring and autumn. They feed on a variety of fish species, but mostly capelin and/or sandlance and Arctic cod.
Population numbers and trends
While solid numbers are not available for Arctic ringed seals, regional estimates suggest the global total is more than 4 million: how much more is uncertain but ringed seals globally may now be less numerous than the global population of harps (9.4 m +/- 1 million or more at last estimate). 2
Things have changed dramatically since the early 1980s, when harp seal numbers in the NW Atlantic hit their lowest level since the early 1700s (generally considered the “pre-commercial harvest” population), see Fig. 5. below.
Would polar bear biologists in 2007 ever have imagined that the bottom would totally drop out of the harp seal hunt by 2011 (Fig. 5), leaving virtually all harp seal pups as prey for Davis Strait polar bears? I doubt it.
Potential impacts of climate change
There have been concerns about recent declines in late winter sea ice in Atlantic Canada and what that might mean for harp seals on the pupping grounds.However, most of this “decline” has occurred in the Gulf region, not the Front – although there is year to year variation in both regions – and low ice years in the Gulf are not a new phenomenon.
According to Sergeant (1991: 116) :
“…it is possible to categorize the type of ice used by harp seals for whelping (Fig. 127). This is medium winter ice in late February with 6 to 8/10 ice cover, i.e. it must be strong enough but have enough open leads for the seals to penetrate it.” [my bold]
Garry Stenson (2014) used the sea ice coverage at 5 March generated by the Canadian Ice Service to compare harp seal pupping habitat over time (up until 2012): below I extend that graph to 2015.
Historically, however, ice at the Front has been less variable for the week of March 12 (when most harps are born at the Front), especially in the area off southern Labrador (Fig. 7):
Whelping females spread out over the ice off Newfoundland when conditions permit (Fig. 8):
So far this year, ice at the Front is about average but in the Gulf the ice coverage is way below average (see yesterday’s post).
Sergeant (1976:98, 38) pointed out that ice in the Gulf is formed in situ and usually only gets about 40-50cm thick, whereas the ice at the Front is thick first year ice with origins in the far north. This makes Gulf of St. Lawrence ice formation much more susceptible to local conditions (warm OR cold) and thus, the highly variable sea ice conditions are not new (Johnston et al. 2005), a fact corroborated by reports by early 20th century sealers from 1924-1941 (Ryan 2014).
Sergeant (1991: 31) made this point:
“The effect of climatic change in the 20th century on the distribution of whelping sites of this species has been small. In the west, this is because the Gulf Stream – Labrador Current system is stable. Greatest year-to-year changes are seen in the Gulf of St. Lawrence at the southern margin of the range, where ice in any one season may be thick or almost absent.” [my bold]
What do harp seals do when ice conditions are poor in the Gulf of St. Lawrence, as has happened often in the past? Sergeant (1991:56) said this:
“In 1981, with almost no ice in the Gulf, mortality of at least several hundred young and tens of adults was seen on the north shore beaches of Prince Edward Island….
Sergeant (1982) found that ice conditions affected whelping patterns markedly in the Gulf of St. Lawrence in 1953, 1969 and 1981, or about one year in ten. In 1969 at least, ice conditions at the Front and in the Strait of Belle Isle were light also and would have allowed adult females which had not whelped in the Gulf to search northward for ice; some may have done so.
[in 1969] There was no ice in the Gulf except in Northumberland Strait and shore ice on the north coast of Prince Edward I. It was generally agreed that no more than 40 000 animals whelped here… The number at that time expected to whelp in the southern Gulf was ca. 100 000.
Probably, the remainder searched for ice, and finding none in the northern Gulf, passed through the Strait of Belle Isle and whelped together with the Front herd on the coast of Labrador at Hamilton Inlet (But see p. 130).
In 1981, however, although ice was of minimal extent and thickness in the Gulf, harp seals whelped off the west coast of the Magdalen Is. and drifted to the north coast of Prince Edward I. Here storms destroyed the small amount of ice and young harp seals died due to starvation and loss of body reserves (Dr. J.R. Geraci, in litt.).
Under unusually heavy ice conditions, it is not possible for harp seals to move from the Front to the Gulf before whelping, since the Strait of Belle Isle is then blocked by ice. Whelping merely occurs further south along the east coast of Newfoundland.” [my bold]
See ice map below from 2013 of the Gulf for locations of Magdalen and Prince Edward Island (original here).
No difference has been found between harp seals at the Gulf and those at the Front, so together they are treated as one population for management purposes (Sergeant (1991; Stenson 2014), which also means that projections into the future based on worst-case scenarios for the Gulf may be erroneously assumed to apply to the Front also (Stenson et al. 2016).
As ice conditions this year demonstrate (Fig. 10), there can be plenty of ice off southern Labrador and Newfoundland but little in the Gulf:
More on the history of polar bears feeding on harp seals in a future post.
1. Regarding the taxonomy of the harp seal (from the IUCN Red List)
“The scientific name for the Harp Seal has shifted frequently in the past with generic names of Pagophoca, Pagophilis and Phoca, and specific names of groenlandicus and groenlandica. Berta and Churchill (2011) used the name Pagophilis groenlandicus and that is the name recognized by the Society of Marine Mammalogy (Committee on Taxonomy 2014). Rice (1998) recognized two subspecies, P. g. groenlandicus and P. g. oceanicus, the former breeding in the western North Atlantic off North America and also around Jan Mayen in the Greenland Sea, and the latter breeding in the White Sea. There certainly are three distinct populations (Lavigne 2002) centred on the breeding localities, with some associated morphological, genetic and behavioural differences. Heptner et al. (1996) provided evidence for two distinct groups, but included the Jan Mayen breeding group with the White Sea group. Analysis of DNA sequence variation, and also comparisons of fingerprint band-sharing coefficients, revealed that the breeding groups in the Northwest Atlantic (Gulf of St. Lawrence and the Front off Labrador and Newfoundland) were one group and that the animals that breed in the White Sea and those in the Greenland Sea (north of Jan Mayen) were another group (Meisfjord and Sundt 1996, Perry et al. 2000). Given the uncertainties in the level of genetic differentiation between the breeding populations (i.e., whether subspecies should be recognized) and their actual affinities, they will not be differentiated in this assessment.”
2. According to the Red List assessment of 2015, there were an estimated 7.4 million (range 6.5 – 8.3m) harp seals that had their pups in Atlantic Canada, another 627,000 or so harps bred off NE Greenland (near Jan Mayen Island, around what is called the “West Ice” polynya) and 1.4 million or so in the White Sea east of Finland (Fig. 1).
Department of Fisheries and Oceans Canada (DFO) 2012. Current status of northwest Atlantic harp seals (Pagophilus groenlandicus). Science Advisory Report 2011/070.
From the summary:
A model assuming density-dependent population growth, carrying capacity of 12 million and annual reproductive rate data was fitted to the survey data. The model estimated a total population of 8,300,000 (95% CI=7,500,000-8,900,000) in 2008.
– The projected trend in the population between 2008 and 2012 is difficult to predict because of uncertainty associated with reproductive rates and how density dependence is expressed in the model. The 2010 assessment assumed that reproductive rates would remain high and predicted a 2010 population that would lie between 8.61- 9.55 million (95% CI 7.80 to 10.80 million) animals. However, reproductive rates have declined since 2008, and the estimated 2012 population is now estimated to be 7,700,000 (95% CI=6,900,000-8,400,000)
From pg 13
With survey intervals of 4-5 years it is not possible to determine environmental carrying capacity for this population (K), or how rapidly the population is approaching K. Reconstruction of historic population size provides an indication of the preexploitation population size and an indication of potential K, but it assumes that environmental conditions today are similar to those observed in the 18th century. Uncertainty in K and future trends in reproductive rates and population response to changing environmental conditions (e.g. ice conditions) limits our ability to predict harvest impacts. Harp seals are pelagic and undertake seasonal migrations between an Arctic ecosystem and a north Atlantic ecosystem. Availability of food resources and carrying capacity in these two systems are not known and likely vary.
Department of Fisheries and Oceans Canada DFO. 2014. Status of Northwest Atlantic harp seals, Pagophilus groenlandicus. DFO Can. Sci. Advis. Sec. Sci. Advis. Rep. 2014/011.
From pg 7: The mean age at which females become sexually mature was 5.8 years in the mid-1950s, declining to ~ 4.5 years of age from the late 1970s through the mid-1980s, then increasing to 5.7 years by the mid-1990s. It has remained high since then. Annual age-specific pregnancy rates were estimated by smoothing the raw pregnancy-rate data over the period 1954–2013 (Fig. 4). Pregnancy rates among 4 to 6 year olds were low during the 1950s and early 1960s, after which they increased to reach maximum levels in the 1980s, and then declined. Animals aged 8 + years account for over 70 % of the total pup production. Reproductive rates of these animals were high until the 1980s, but have generally declined since then, although there is considerable variability among years (Fig. 4). The general decline in fecundity is thought to be a reflection of density dependent processes associated with increased population size. The large inter-annual variability is influenced by the occurrence of late term abortions in some years.The occurrence of abortions appear to be related to prey availability and mid-winter ice conditions, either directly as an indicator of breeding habitat or as a proxy for ecosystem productivity. Climate change models predict that ice conditions will continue to deteriorate suggesting that reproductive rates may, on average, remain low. [2008 count recorded virtually double the number of pups at the Front compared to the 2004 and 2012 surveys]
From pg 8: At the 2011 assessment, catch information going back to the 18th Century were used to reconstruct the trajectory of the population back to the period when commercial harvesting began. This approach assumed that the population was stable and at its environmental carrying capacity just prior to commercial exploitation. This reconstruction resulted in an estimated pre-exploitation population of 10.8 million (range=7.6-15.4 million) animals….Projecting to 2014, results in an estimated pup production of 853,000 (SE=202,000) and a total population of 7,411,000 (SE=656,000) animals. The population appears to be relatively stable, showing little change in abundance since 2004 (Figs. 6 and 7). [i.e., the pre-exploitation population size is assumed to be the carry capacity, 10.8 million animals, range 7.6-15.4 m]
Johnston, D.W., Friedlaender, A.S., Torres, L.G., Lavigne, D.M. 2005. Variation in sea ice cover on the east coast of Canada from 1969-2002: climate variability and implications for harp and hooded seals. Climate Research 29:209-222.
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
Ryan, S. 2014. Appendix 3: Chafe’s “Notes of the Voyages” 1924-1941, In: The Last of the Ice Hunters: An Oral History of the Newfoundland Seal Hunt, pg. 445-457. Flanker Press, St. John’s. [Contains critical notes about ice conditions between 1924 and 1941 and where harp seals were found in those years]
Sergeant, D.E. 1976. History and present status of populations of harp and hooded seals. Biological Conservation 10:95-118.
Sergeant, D.E. 1991. Harp Seals, Man and Ice. Canadian Special Publication of Fisheries and Aquatic Sciences 114. Fisheries and Oceans Canada, Ottawa.
Stenson, G.B. 2014. The status of harp and hooded seals in the North Atlantic. Report presented at the Scientific Council Meeting, June 2014. Northwest Atlantic Fisheries Organization SCR Doc. 14/026, Serial No. N6321.
Stenson, G.B., Buren, A.D. and Koen-Alonso, M. 2016. The impact of changing climate and abundance on reproduction in an ice-dependent species, the Northwest Atlantic harp seal, Pagophilus groenlandicus. ICES Journal of Marine Science 73(2):250-262. http://icesjms.oxfordjournals.org/content/73/2/250