High walrus numbers may explain why females and calves are hauling out in droves

Are the recent mass gatherings of females and calves on the beaches of western Alaska and the Russian Far East a sign that population are again close to the limit the habitat can support, as they were in the 1970s?

At the risk of belabouring the point that lack of sea ice does not seem to be a reasonable explanation for this phenomenon (see my last two posts, here and here, and Andrew Montford’s excellent additions here), I think this is a possibility that needs to be considered.

Walruses at Point Lay Sept 27 2014 NOAA CMDA0007_sm

2014 Point Lay, NOAA photo

Walrus are well protected in the US and Russia, and population numbers have risen again since the 1990s, after significant declines in the early 1900s and again in the 1980s.

A paper from the late 1980s suggests that a return to high population numbers may be a more plausible cause for mass gatherings of females and calves that lead to stampede deaths than does reduced sea ice due to global warming.

Have a look and see what you think. It may not be the only reason but it may be a major contributing factor.


From the 2011 NOAA Status Report on walrus (Garlich-Miller et al. 2011:):

“…there is a diverse network of international, Federal, State and local laws and regulations that provide protection to Pacific walruses and their habitats
.”

Later (Garlich-Miller et al. 2011:12), these authors state:

“Large-scale commercial harvests are believed to have reduced the population to 50,000-100,000 animals in the mid-1950s (Fay et al. 1997). The population appears to have increased rapidly in size during the 1960s and 1970s in response to harvest regulations that limited the take of females (Fay et al. 1989). Between 1975 and 1990, aerial surveys were carried out by the United States and Russia at 5-year intervals, producing population estimates ranging from 201,039 to 290,000.

Based on evidence of changes in abundance, distributions, condition indices, and life-history parameters, Fay et al. (1989; 1997) concluded that the Pacific walrus population increased rapidly in size during the 1960s and 1970s, and postulated that the population was approaching, or had exceeded, the carrying capacity of its environment in the early 1980s.

Changes in the size, composition and productivity of the sampled walrus harvest in the Bering Strait Region of Alaska over this time frame are consistent with this hypothesis (Garlich-Miller et al. 2006).” [my bold]

Figure 1. Walrus haulouts by season, from the 2011 status report (Garlich-Miller et al. 2011).

Figure 1. Walrus haulouts by season, from the 2011 status report (Garlich-Miller et al. 2011). Click to enlarge.

I had a look at exactly what Fay and colleagues had to say back in 1989 (pg. 4-6):

“… in the next two decades [after 1960] the walrus population recovered again, at least doubling in size. By 1980, it already was showing density-dependent signs of having approached or reached the carrying capacity of its environment. As productivity and calf survival declined sharply in the late 1970s and early 1980s the catches more than doubled. We believe that the combined effects of natural curtailment and human intervention may be bringing the population down again rather rapidly.

The steady increase was indicated also by the fact that the animals gradually reoccupied nearly all of their former range, including some areas that had been vacant for up to 100 yr (Fay et al. 1986).

The trend of increase appeared to be documented most reliably by the censuses in Soviet waters, where the numbers were estimated in the same way each time and were based mainly on counts from aerial photographs of herds on the coastal haulouts. In both the Soviet and the American
census data, the relative increase of the population appeared to be at least 100% between 1960 and 1975.

Already by the late 197Os, however, there were signs of other changes that could be interpreted as density-dependent responses of the population to the carrying capacity of its environment (Fay and Kelly 1980, Fay 1981, Fay et al. 1986). Walrus populations are presumed to be food-limited, and the events that transpired in the late 1970s and early 1980s appeared to confirm that presumption. In 1976, the Alaskan Eskimos began to report that the harvested walruses were much leaner than they had been a few years earlier. They also reported that the stomach contents increasingly included unusual items such as seal flesh, suggesting that the normal prey (bivalve mollusks) were becoming scarce. Our investigations of those claims disclosed that:

(a) The animals were significantly leaner in the 1980s than they had been in the late 1950s to early 1970s (Table 1; Kruskal-Wallis test, P < 0.02). This suggested that either the food supply had diminished or the animals were expending more energy than before to get it (or both).

(b) The occurrence of seal flesh in the stomach contents of animals taken in the Bering Strait region in spring was unusually high in the late 1970s (Lowry and Fay 1984), and we found that other alternate prey such as fishes, anemones, and polychaetes, also occurred more frequently than before (Fay and Stoker 1982a, 6; Fay et al. 1984). Seal flesh diminished in frequency of occurrence in the samples after 1979, but the presence of parasites acquired from eating fishes remained high. From 1975 to 1982, the bivalves found in the walruses’ stomachs were of diminishing size in each successive sample, suggesting that selective predation was changing the structure of the prey populations. That is, having expanded into essentially “virgin” range, some of which had been unoccupied for more than a century, the walrus population finally appeared to be exerting a detectable effect on its primary food supply in the central part of the range and was taking increasing amounts of alternate prey.

Further evidence of the walrus population having increased greatly was seen in the steadily rising use of traditional haulouts on islands in the Bering Strait region during the 1960s and 1970s. At first, this amounted to increased use during seasonal migrations, but it later changed to prolonged (up to 6-mo) occupancy from spring to fall (Fay et al. 1986). At St. Lawrence Island, native residents expressed great concern that the numbers of walruses using the Punuk Island haulout was becoming too large, for the amount of natural mortality there had risen markedly in the 1970s. Our investigations confirmed that the mortality was higher there in 1978 than it had been for at least the previous 30 yr (Fay and Kelly 1980). Data acquired since then indicate that it has
declined again.

The decreased fatness, change in diet, increased natural mortality, decreased productivity and calf survival, increased age at first pregnancy, and change in age composition appeared after the increase in population size had been underway for at least 15 yr. These conditions indicated that a significant change in status had taken place in the late 1970s probably related to the population having reached or approached K and begun to have an adverse impact on its food supply. Perhaps, the population may even have exceeded the carrying capacity of its environment by the early 1980s for the animals still are very lean, their reproductive performance still is highly erratic, and the survival of calves still seems very low. We believe that the probability of the population having overshot carrying capacity is very high, because its expansion was into a range that had been unused for as much as a century. That is, we suspect that the momentum gained by the walrus population in its expansion was comparable to that of an exotic species expanding into virgin habitat, where the population exceeds carrying capacity before its density-dependent regulators can come into full operation. A classic analogy is the sea otter’s (Enhydra lutris) increase, then decrease, following reinvasion of some parts of its former range (Kenyon 1969).” [my bold]

Conclusions
Protecting walrus from over-hunting is a good thing but that means the population then has to come to some kind of equilibrium with its environment and food supply. [See this post for a discussion of carrying capacity in relation to polar bear populations]

A very large population may behave differently than a smaller one: the propensity for females with young calves to gather together on land  (keeping together helps defend against polar bear attacks) may backfire when there are very large numbers of them.

When walrus female and calves haul out in large groups, a stampede can kill large numbers of calves and weaken adults  — it’s happened recently but it also happened in the late 1970s, when the population was also very large (Fay and Kelly 1980).

Walruses may not be food limited yet, I haven’t checked that literature, but they may still be exhibiting other signs of a very large population size, like hauling out together on remote beaches or moving from one area to another in late summer and fall to find more productive food sources.

[There has not yet been a sufficiently accurate population estimate done recently to allow officials to determine whether or not walrus should be considered threatened by future sea ice declines, since walrus are particularly difficult to count (Garlich-Miller et al. 2011)]

In short, recent huge gatherings of females and calves on Chukchi Sea beaches may have more to do with the fact that the population of walruses is now very large rather than because there has recently been no ice for a few months in late summer/fall.

Large population size now and in the late 1970s — not lack of sea ice — is a common denominator for the phenomenon of huge herds of females and calves hauling out on shore in late summer and fall. It may not be the only commonality, but large population size is almost certainly a significant factor contributing to these events.

Point Lay 2013 NOAA photo

Point Lay 2013 NOAA photo

References
Fay, F.H. and Kelly, B.P. 1980. Mass natural mortality of walruses (Odobenus rosmarus) at St. Lawrence Island, Bering Sea, autumn 1978. Arctic 33:226-245. http://www.aina.ucalgary.ca/scripts/minisa.dll/144/proe/proarc/se+arctic,+v.+33,+no.++2,+June+1980,*?COMMANDSEARCH [open access] PDF: https://polarbearscience.files.wordpress.com/2014/10/walrus-st-lawrence-island-mortality-1978-fay-and-kelly-1980-marked.pdf

Fay, F.H., Kelly, B.P. and Sease, J.L. 1989. Managing the exploitation of Pacific walruses: a tradegy of delayed response and poor communication. Marine Mammal Science 5:1-16. PDF HERE.

Garlich-Miller, J., MacCracken, J.G., Snyder, J., Meehan, R., Myers, M., Wilder, J.M., Lance, E. and Matz, A. 2011. Status review of the Pacific walrus (Odobenus rosmarus divergens). US Fish and Wildlife Service, Anchorage, Alaska. PDF HERE.

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