by Jim Steele
NOAA’s Arctic report card made claims that hinge on the unproven hypothesis that a reduction in sea ice is detrimental by denying walruses access to foraging habitat.
In the Pacific the number of calves per cow increased as has calf survival, both indicators of a growing population, contradicting NOAA’s claim. As detailed in Hijacking Successful Walrus Conservation, historical records for the Pacific walrus (Fay 1982, 1989) observed an overall increase in the use of land haulouts coinciding with increasing populations of recovering walrus. In the Barents Sea’s Svalbard archipelago, despite the greatest decline of sea ice, recent research has also observed an increased use of land haulouts coinciding with an exponential population growth, a 48% increase in abundance between 2006 and 20012 (Kovacs 2014). Yet despite all the positive indicators, NOAA downplays growing populations and makes the empty assertion, “the overall carrying capacity of the region for walruses is almost certainly declining because of sea ice declines.”
The full weight of evidence suggests an alternative hypothesis is more likely. Less sea ice allows more access to larger areas of bountiful foraging habitat that had been previously covered by heavy ice. The carrying capacity of walrus habitat – its ability to nourish and sustain a population – will only decline if the following are true but perusal of the evidence suggests the carrying capacity has increased.
1) Carrying capacity will decline if the population becomes so abundant it reduces the prey base and competition for dwindling food creates nutritional stress
2) Carrying capacity will decline if there is a general decline in marine productivity
3) Carrying capacity will decline if the areal extent of potential foraging habitat is reduced, and/or
4) Carrying capacity will decline if access to foraging habitat is reduced.
Food Competition, Density-dependent Regulation, and Healthy Vital Rates
Populations are naturally regulated by “density-dependent” factors. As a growing population adds more individuals to a given area, the density increases. As the density approaches the carrying capacity of that habitat, competition for a limited food supply increases nutritional stress. Marine mammals such as polar bears, ringed seals and walruses respond to nutritional stress by reducing their reproductive output, which ultimately reduces population growth. The ratio of calves to cows decreases because pregnancy rates decline, young cows defer their first year of pregnancy to an older age, and calf survival rates decrease. Conversely when the food supply is abundant, walruses’ pregnancy rates increase, cows give birth at an earlier age, and calf survival rates increase. When those critical factors raise the ratio of calves to cows the population increases.
Based on 20th century surveys, researchers believed the Pacific walrus had rebounded from an overhunted population reduced to ~ 50,000 in the 1950s which then grew to ~250,000 to 300,000 walrus by 1980 (Fay 1989). Consistent with density-dependent theory, when the population was below the carrying capacity in the 1950s and 60s, researchers observed the highest ratios of calves per cows. As the population grew subsistence hunters reported increasing numbers of leaner individuals and a steady decline in the ratio of calves to females suggesting walruses were reaching or exceeding the region’s carrying capacity. The resulting decline in reproductive output caused the population growth rate to stop and the population peaked around 1980. Researchers then calculated a brief population decline during 1980s exacerbated by an uptick in Russian walrus harvests (Fay 1997). But the calves:cows ratio then began to increase throughout the 1990s and some researchers believed population growth had resumed. The calves:cows ratio is now as high as it was in the 1960s when the recovering population was rapidly growing (McCracken 2014). Presently calf survival rates have nearly doubled (Taylor 2015) and cow’s age of first pregnancy has been increasingly younger (Garlich-Miller 2006). All those vital signs usually suggest a well fed, growing population, supporting early research but contradicting NOAA’s current argument that the carrying capacity is “certainly declining”.
Marine Productivity is Improving
The shallow shelves of the Bering and Chukchi seas prevent nutrients from sinking to a dark abyss far from the reach of photosynthesizing plankton. Shallow seas more readily upwell nutrients enabling high rates of productivity. Furthermore ocean currents bathe large sections of those shallow shelves with nutrient rich subtropical waters further enhancing productivity. And because surface productivity more rapidly reaches the floor of those shallow shelves, bottom dwelling organisms collectively called the “benthos,” receive over 70% of the energy sequestered at the surface. As a result the Bering and Chukchi seas sustain some of the earth’s richest bounty of bottom dwelling prey sought by walrus, gray whales and bearded seals (Sirenko 2007). Contrary to earlier suggestions that global warming may possibly decrease productivity (Grebmeier 2006), satellite observations have determined marine productivity has increased by 30% since the 1990s (Arrigo 2015). The reason for this increase is elementary. Less sea ice allows more photosynthesis. Grebmeier 2015 has now reported that the Bering and Chukchi Sea “hotspots” she has studied have sustained high levels of biomass over the past 4 decades.
From a marine productivity perspective, the evidence does not support NOAA’s claim of a declining carrying capacity; just the opposite. Increased productivity has increased the carrying capacity.
Areal Extent of Foraging Habitat Has Increased
The key variable that determines walrus foraging habitat is depth. Telemetry studies found walrus spent nearly 98% of their time foraging in shallow water no deeper than 60 meters (Jay 2005) and other observations suggest foraging at depths deeper than 80 meters is unlikely. As seen in Figure 1, much of the Arctic is not suitable for walruses. The darkest blue regions represent inaccessible regions of great depth. The 3 lightest shades of blue-gray outline the only depths with potential walrus foraging habitat.
The white mass in the upper right of Figure 1 represents the summer minimum of the 2007 ice pack. The average historic summer minimum (the yellow line in Fig.1) indicates large portions of the Chukchi Sea’s foraging habitat have been covered with summer ice concentrations of 50% and greater for much of the 20th century. Because walrus avoid ice-covered waters where sea ice concentration is 80% or greater, any heavy ice concentrations reduce the areal extent of walrus foraging habitat.
Notice that along the northern coast of Alaska in the Beaufort Sea, sea ice historically retreated over deep waters every year. Thus the most recent retreat of sea ice further northward did not impact the areal extent of foraging habitat in that region. Likewise once the Chukchi summer sea ice retreated over the deep Arctic Ocean, any additional retreat had little consequence. In contrast, the initial reduction in summer sea ice over the western Chukchi Sea opened vast regions of potential foraging habitat.
It is believed that 70 to 80% of the total Pacific walrus population exploits the western Chukchi habitat especially during the autumn when reduced sea ice exposes the most habitat. Russian researchers surveying the western Chukchi in September of 1980, estimated approximately 150,000 walrus had hauled out in roughly equal numbers on sea ice and on land. A repeat of that survey in October as freezing conditions increased, revealed the number of walrus hauled out on ice had been greatly reduced but walrus on land remained unchanged (Fedoseev 1981). Clearly 75,000 walrus were not forced onto the Russian coast due to the lack of ice. Although the lack of sea ice in 2007 very likely increased the numbers of walrus hauling out on land, media hyperbole that sensationalized terrestrial haulouts are solely due to global warming, inexcusably ignores all historical observations of natural land haulouts. Based on observations that roughly 50% of the walruses use land haulouts despite plentiful potential resting platforms of sea ice, any occupation of land haulouts serves as an indicator of where walrus accessed Chukchi habitat as sea ice cover waxed and waned.
In Figure 3 below (from Garlich-Miller 2011) the numbers locate known land haulouts. The red arrow I added points to Cape Serdse-Kamen (#50) that has always been occupied in September and October during past surveys. The numbers to the west of Cape Serdse-Kamen and to the north around Wrangel Island represent traditional haulouts that are used only in years of light sea ice but unoccupied in years of heavy ice (Fay 1984). For example despite the shallow foraging habitat north of Wrangel Island, walruses were not observed there in the 1980s (Fedoseev 1981). When sub-freezing winds removed much of the thick Arctic ice from this region in the 1990s when Arctic Oscillation shifted, walrus rapidly exploited the region’s resources and over 120,000 walruses hauled around Wrangel Island. Such observations support the hypothesis that reduced ice increases available foraging habitat and consequently the western Arctic’s carrying capacity.
Due to heavy sea ice cover, access to rich foraging habitat on shallow shelves naturally fluctuates between seasons, years, decades and millennia. The heavy ice of the last Ice Age must have been the nadir for walrus populations. Not only was there maximum sea ice coverage, but also the drop in sea level left the shallow shelves of the Arctic Seas high and dry. Although this allowed humans to enter North America, it relegated walrus populations to narrower shelf waters as far south as central California. Eventually Holocene warmth raised sea level and reduced sea ice allowing walrus populations to once again flourish in the Arctic. Flexible migratory patterns are likely an adaptation to the constant changes in sea ice even during the warm Holocene. Proxy data covering the past 9000 years from Point Barrow revealed annual sea ice covering the eastern Chukchi Sea varied from only 5.5 to 9 months, and summer sea surface temperatures ranged from 3 to 7.5 °C, much higher than today (McKay 2008).
Seasonally, winter ice forces walrus to abandon the Chukchi. They re-enter after the warmth of spring reduces sea ice cover. Whether caused by CO2-driven global warming, observed natural changes in atmospheric circulation due to the Arctic Oscillation, or changes in the volume of intruding waters associated with the Pacific Decadal Oscillation, the extent of summer sea ice summer has fluctuated greatly over decades as seen in Figure 5 (from Jay 2012.)
Accessing Foraging Habitat
NOAA began their report card by arguing, “Sea ice deterioration due to global climate change is thought to be the most pervasive threat to ice-associated marine mammals in the Arctic, including walruses.” But that threat has yet to be substantiated. The perceived threat to walruses is solely based on a hypothesis that walruses “require” sea ice as a platform from which they dive to suction clams, worms, etc. from the ocean floor. Based on that belief, some researchers argue that declining sea ice denies access to habitat and forces them to forage closer to their land haulouts. Expanding on that assumption NOAA argues Arctic’s carrying capacity “must be in decline.”
But several lines of evidence clearly demonstrate walruses do not “require” sea ice as a resting platform in order to hunt. A resting platform of sea ice is likely an opportunistic and beneficial convenience – not a requirement. For example after breeding a large proportion of male walruses abandon the sea ice and migrate south to dwell in land haul outs in ice free waters along the Russian and Alaskan coast (represented by red dots in Figure 3). From those traditional land haulouts they embark on foraging trips that last for 4 to 10 days and range as much as 130 kilometers away (Jay 2005). In addition satellite radiotelemetry determined walruses throughout the Bering and Chukchi spend over 80% of their time swimming, and the amount of time in the water was the same whether walrus used sea ice or land for a resting platform. Swimming at a relaxed speed of 10 km/hour, a walrus easily range over 200 km while foraging along the way (Jay 2010, Udevitz 2009).
Some researchers suggest that the lack of resting platforms of sea-ice will restrict walrus to hunting only along the coast and hypothesizing they will more quickly deplete more limited accessible resources. However the opposite scenario is more likely. Heavy sea ice restricts hunting grounds and the most extreme example would occur if heavy ice remained all summer in the Chukchi forcing herds to remain in the Bering Sea throughout the year. Certainly the Bering Sea’s prey base would be rapidly depleted. The migratory behavior of females and their calves into the shallow waters of the Chukchi each summer is most likely a behavior that evolved to reduce resource competition and exploit temporary access to rich foraging habitat. With a greater reduction of Chukchi summer ice, migrating herds can spread out and reduce localized foraging pressure.
NOAA Expert Opinion Claims Pacific Walrus have declined by 50%. Seriously?
Finally NOAA’s report card suggested that “expert opinion” calculated a 50% decline in Pacific Walrus populations between 1980 and 2000. The experts did agree the population had decreased during the early 1980s due to density-dependent effects when population abundance increased and exceeded the region’s carrying capacity. But the expert consensus ended there. Fay 1986 suggested after a relatively brief decline in the 80s, population growth subsequently resumed. A growing population would be in agreement with recent observations of increased marine productivity, greater access to habitat due to decreased heavy ice, higher calves:cows ratios and higher survival rates.
Estimating walrus abundance is extremely difficult and all experts agree that abundance estimates have extremely wide error bars and are totally unreliable. Russian and American biologists jointly surveyed walrus populations in the autumn every 5 years between 1975 and 1990, but survey efforts were suspended because experts could not agree on how to interpret limited data and the tremendous resulting uncertainty (Speckman 2010). The major problem revolves around estimating how many walrus are in the water and escape detection. Furthermore due walrus movements, it was impossible to replicate survey transects and constrain error estimates. A repeated transect just one week later often resulted in observed numbers differing by 2 or 3 orders of magnitude.
To circumvent survey uncertainties there have been attempts to model abundance based on observed age structure of the population (Taylor 2015), and those model results disagree with earlier calculations of a growing population. They suggested populations continued to decline from 1980 to 2000, but admit their results after 2003 were equivocal. They also acknowledged that information provided by age structure data cannot mitigate uncertainties in the population size, admitting the absolute size of the Pacific walrus population will “continue to be speculative until accurate empirical estimation of the population size becomes feasible”
Thus experts would likely agree that NOAA’s claim of a 50% reduction due to “expert opinion” is likewise speculative and rather meaningless. NOAA failed to express that extreme uncertainty and failed to report the tremendous wide range in abundance estimates. For example in the most recent survey (Speckman 2010) of wintering walrus in the Bering Sea, researchers used heat detectors calibrated by high-resolution photographic evidence to estimate abundance. Unfortunately swimming walruses were undetectable. For the region surveyed, they estimated 129,000 walrus that would support a estimated 50% decline. However their 95% confidence ranged from 55,000 to 507,000 walrus. Furthermore due to time and weather constraints, the survey covered less than 50% of the Bering Sea habitat known to contain walrus. A complete survey may well have increased the estimate to well over 200,000 individuals. A midrange estimate would be similar to peak estimates of the 1980s, and high-end estimates would support hypotheses of a growing population in the Pacific; a growth that parallels observed growth in the Atlantic walrus.
Curiouser and curiouser, NOAA cited McCracken 2014 who used Speckman’s knowingly biased underestimate of 129,000 to suggest the increasing ratio of calves per cow supported a declining walrus population. Biologically such an assertion contradicts density-dependent mechanisms. Increased reproduction increases a population, unless survival rates drastically declined, but rates had increased.
McCracken 2014 argued that calves:cows ratios are inversely correlated with population abundance as illustrated in Figure 4. However that correlation is partly speculative and unsupported and depends on using Speckman’s unrealistic estimate of half the population. No one disagrees that overhunting reduced the population in the 1950s so that more food became available for the survivors stimulating walruses to increase reproductive output as evidenced by high calves:cows ratios; a high ratio that approached the theoretical maximum. Density increased as walruses recovered from overhunting (and increasing sea ice was coincidentally recovering from its minimal in the late 1930s) so that the carrying capacity declined and walrus responded with declining calves:cows ratios that bottomed out in the 1980s. But the consensus on any population trends stops in the 1980s.
McCracken 2014 acknowledged that the validity of their inverse correlation is totally dependent upon the assumption that 300,000 walrus was the maximum population that could be sustained by the region. However they did not explore the possibility that the carrying capacity could possibly increase due to less sea ice and higher marine productivity. So they assumed that any observations of higher calves:cows ratios that would normally indicate a growing population, were only possible if the population had declined by such an extent that more food again became available.
The only dynamic that could have possibly offset increased ocean productivity and cause a population decline in an era of regulated hunting, and conservation efforts that are now protecting haulouts, was a strictly hypothetical dynamic that less sea ice prevents access to foraging habitat and was reducing the Arctic’s carrying capacity. But all reported evidence discussed above contradicts that hypothesis and McCracken’s suggestion the population had declined by 50% is untenable.
NOAAs claim that the “carrying capacity is almost certainly declining because of sea ice declines” is advocated by USGS and US Fish and Wildlife researchers who believe that CO2 warming and declining sea ice must be bad. That belief is advocated in the opening paragraphs of nearly every publication. Wedded to that belief their interpretations ignore robust evidence suggesting less has been beneficial. So one must wonder how politicized those agencies have become and if political pressure has biased their publications. Researchers in those agencies likewise ignored their own observations that it was cycles of thick springtime ice in the Beaufort Sea that caused declines in ringed seals and polar bear body condition. Instead without evidence, they only advocated that reduced summer ice, consistent with CO2 warming, has negatively impacted polar bear populations and walrus Such unsupported biased interpretations are most likely the result of the politicization of science, and I fear this decade will be viewed as the darkest days of environmental science.
JC note: As with all guest posts, please keep your comments civil and relevant.