Polar bears ‘running out of food’

By Paul Homewood

More “polar bears are all going to die” drivel from the BBC:

High-tech tracking collars on nine female polar bears have measured the animals’ efforts to find food on the diminishing Arctic ice.

Each bear wore a collar – recording video, location and activity levels – for 8-12 days, while metabolic tracers tracked the bears’ energy use.

This revealed that most of the animals were unable to catch enough prey to meet their energy needs.

The team says wild bears have higher metabolic rates than thought.

Moreover, climate change appears to be having dramatic effects on the Arctic sea-ice, forcing polar bears to move greater distances as they hunt, and making it harder for them to catch prey.

The vision of a polar bear plucking a vulnerable seal off an ice floe is something familiar to wildlife documentary fanatics. Earlier this winter, though, an image of an emaciated polar bear went viral, with many asking if this was the telltale image of climate change.

The authors of this study, published in the journal Science, point out that the animals do now need to travel further to find seals, and that this is likely to be an "important factor explaining declines in their body condition and survival" of polar bears.

In Spring of 2014, 2015, and 2016, Anthony Pagano, a researcher at the University of California Santa Cruz and his colleagues, set out to track the polar bears’ hunting and survival during this critical season. They captured nine females on the sea-ice of the Beaufort Sea and measured the metabolic rates of each bear using blood and urine samples.

They also fitted the bear with the GPS-camera collars, to record and film their activity.

"We found that polar bears actually have much higher energy demands than predicted. They need to be catching a lot of seals," Mr Pagano explained.

The extent of Arctic sea-ice, as measured at its minimum in September, is decreasing at a rate of about 14% per decade, which is likely reducing polar bears’ access to seals. And their plight could be exacerbated by the need to alter hunting strategies with the seasons.

In the spring, the researchers explained polar bears are mostly preying on juvenile seals. But later in the year, after the bears’ long summer fast, those young seals are older and wiser, meaning polar bears are not able to catch as many.

"It’s thought that bears might catch a couple per month in the fall, compared to five to 10 per month in the spring and early summer," Mr Pagano said.

"We now have the technology to learn how they are moving on the ice, their activity patterns, and their energy needs, so we can better understand the implications of these changes we are seeing in the sea-ice."

http://www.bbc.co.uk/news/science-environment-42909866

For a start, it is simply not true to say that the extent of Arctic sea-ice, as measured at its minimum in September, is decreasing at a rate of about 14% per decade.

It WAS falling between 1979 and 2007, but since then has actually been increasing again.

http://ocean.dmi.dk/arctic/icecover_30y.uk.php

My first reaction was – so what? Is there any evidence that anything unusual is going on here? Anything that has not happened regularly in the past?

It is a simple fact of nature that animals sometimes run out of food and starve, for all sorts of reasons.

Fortunately Susan Crockford has taken a look at Pagano’s paper, and revealed the flaws and misinterpretations it contains.

The really significant content of a new paper being heavily-hyped by the media¹ is what wasn’t said rather than what the authors discovered about metabolic rates and weight maintenance of a small sample of nine Southern Beaufort Sea bears in 2014 to 2016 (Pagano et al. 2018; Whiteman 2018).

This paper does not document starving or dying bears but merely found some (5/9) that lost weight when they should have been gaining, given that early April is the start of the ringed seal pupping season (Smith 1987) and the intensive spring feeding period for polar bears (Stirling et al. 1981).

The question is, why were Southern Beaufort Sea polar bears off Prudhoe Bay (see map of the study area below), still hunting and capturing only adult and subadult ringed seals from sea ice leads when newborn ringed seal pups and their mothers should have been plentiful and relatively easily available in their birth lairs on the sea ice (see below)?

“Using video collar data, we documented bears’ hunting behavior and foraging success. Bears used sit-and-wait tactics to hunt seals 90% of the time and stalking comprised the remaining 10% of hunts (movies S1 to S4) (19). Bears that successfully killed and ate adult or subadult ringed seals either gained or maintained body mass, whereas bears that only scavenged or showed no evidence of eating lost mass.”

There was no discussion in the paper of ringed seal birth lairs or sea ice conditions at the time of the study, but several mentions about what might happen in the future to sea ice and potential consequences for polar bears. The press release did the same.

However, as you’ll see by the sea ice thickness maps below, there may be good reason for the lack of ringed seal lairs, and a general lack of seals except at the nearshore lead that forms because of tidal action: the ice just a bit further offshore ice looks too thick for a good crop of ringed seals in all three years of the study.

This is reminiscent of conditions that occurred with devastating results in the mid-1970s and mid-2000s (Burns et al. 1975; Cherry et al. 2009; Harwood et al. 2012, 2015; Pilfold et al. 2012; Stirling 2002, Stirling et al. 1987).

Those events affected primarily bears in the eastern half of the Southern Beaufort and were almost certainly responsible for the recorded decline in SB bear numbers in the 2001-2010 survey (Bromaghin et al. 2015; Crockford 2017; Crockford and Geist 2018).

It seems very odd to me that Pagano and colleagues suggested no reasons for the unexpectedly poor showing of polar bear hunting success during their study except a bit of hand-waving about higher-than-we-thought metabolic rates in the bears.

For years, I’ve worried that the inevitable next episodes of thick Southern Beaufort spring ice would cause problems for polar bears and seals but we wouldn’t know it because whatever effects were documented would be blamed on reduced summer ice: I suspect that time may have come.

Figure 1 from Pagano et al. 2018 cropped to show only the study area off Alaska.

Here are some sea ice thickness maps from the US Navy for the three years of the study, with 2015 shown first.

Ice thickness map from the Naval Research Laboratory for 8 April 2015. Look at the thick band of 5 m thick ice along the exact stretch of Alaska coastline where the Pagano et al. 2018 study took place. Ringed seals cannot live, let alone give birth, in that kind of thick ice habitat and must move to areas of thinner ice with open leads (available to the east and west in 2015).

Below, ice thickness map for April 2016, showing slightly better conditions, especially to the east:

The band of thick multiyear ice (~5m thick) was less extensive in April 2016 but still present within the Pagano et al. 2018 study area. Thinner ice to the east and west would have been more attractive to pregnant ringed seals.

Below, ice thickness map for April 2014, showing ice thickness of 2 m or more across the region, which is probably too thick in most places for ringed seals:

Sea ice within the Pagano et al. 2018 study area in April 2014 was not quite as thick as 2015 and 2016 (only 3 – 4.5m thick) but this was probably enough to drive breeding ringed seals away. Note the lack of openings or areas of thinner ice that would attract seals except very close to shore.

From a 2015 post about Beaufort Sea polynyas, I discussed what marine mammal biologists Ian Stirling and colleagues had to say about polar bears and the effect of the Cape Bathurst polynya on the distribution of polar bears in spring (Stirling et al. 1981:49):

“Polar bears prey mainly upon ringed seals and, to a lesser degree, on bearded seals. Polar bears appear to be more abundant in polynya areas and along shore leads, probably because the densities of seals are greater and they are more assessable. For example, between March and June in the Beaufort Sea from 1971 through 1975, 87% of the sightings of polar bears were made adjacent to floe edges or in unstable areas of 9/10 or 10/10 ice cover with intermittent patches of young ice.” [my bold]

Later, the same authors discussed why these areas of open water can be so important in the Southern Beaufort area (Stirling et al. 1981:54):

“One useful approach is to ask what would happen if the polynya was not there? Obviously, this is impossible to evaluate on an experimental basis, but by examining the consequences or natural seasonal variation, some useful insights can be gained. For example, the influence of rapidly changing ice conditions on the availability of open water, and consequently on populations of seals and polar bears, has been observed in the western Arctic. Apparently, in response to severe ice conditions in the Beaufort Sea during winter 1973-74, and to a lesser degree in winter 1974-75, numbers of ringed and bearded seals dropped by about 50% and productivity by about 90%. Concomitantly, numbers and productivity of polar bears declined markedly because of the reduction in the abundance of their prey species. …If the shore leads of the western Arctic or Hudson Bay ceased opening during winter and spring, the effect on marine mammals would be devastating.”[my bold]

https://polarbearscience.com/2018/02/01/polar-bear-specialists-double-down-on-message-of-future-starving-bears/

DMI charts confirm what Susan is saying. If we compare April 2003 with April 2015, for instance, we can see that the ice is much thicker off the Alaskan coast.

Also there is little difference in ice extent in the Beaufort Sea.