Q&A: Estimates of Abundance for Pacific Walrus Active
The Pacific walrus ranges across the Bering and Chukchi seas during the year. This wide geographic distribution makes it difficult to estimate Pacific walrus population abundance. However, recent technological advances in genetics and drone imagery have generated precise regional and range-wide abundance estimates.
Return to USGS Alaska Q&A Series
Pacific walruses breed in winter in the Bering Sea, and in spring most adult females and juveniles migrate north to the Chukchi Sea as sea ice melts. Most adult males remain in the Bering Sea during spring and spend summers foraging and resting at coastal haulouts. Most adult females and juveniles spend summer in the Chukchi Sea (see map and photo below). Walruses rest on sea ice between foraging trips when sea ice is available. When sea ice is not available, walruses rest on land.
How many walruses are there? It’s difficult to estimate Pacific walrus abundance for several reasons. First, their wide geographic distribution and use of onshore and offshore habitats makes it difficult to access the population. Second, walruses spend a lot of time in the water where they cannot be seen by observers. However, recent technological advances have generated precise regional abundance estimates, and a recently completed multi-agency effort has provided the first Pacific walrus abundance estimate since 2011.
Abundance estimates of Pacific walrus are necessary for federal and Indigenous co-management decisions, yet several past efforts to obtain abundance estimates have generated imprecise estimates because of the clumped and dynamic distribution of walruses at sea. The most recent range-wide survey was conducted in 2006 (Speckman et al. 2011) and despite improvements compared to all previous efforts, the survey yielded an estimate somewhere between 55,000 and 507,000 walruses.
In this Q&A, we talk with USGS research biologists Tony Fischbach and William Beatty about two new approaches that they each recently took towards estimating the number of walruses in the Chukchi Sea region. The first approach (Fischbach et al. 2022) used a drone or unoccupied aerial system (UAS) to fly surveys over walruses resting on a beach in northwestern Alaska (see left photo below). The second approach (Beatty et al. 2022) was based on a five-year genetic mark-recapture study based on samples from biopsy darting of walruses to estimate survival rates and abundance (see right cartoon below).
Q: The two abundance estimates that your papers describe yield very similar numbers: about 175,000 to 200,000 walrus in the Chukchi Sea region. Were these estimates surprising or expected for the number of females and juveniles likely to be in this area?
William: We were surprised that estimates were similar in both the point estimate and precision. We arrived at both estimates using different datasets, so estimates from these two studies should be interpreted differently.
Tony: The drone work provided estimates for the number of Pacific walruses that used the coastal haulout at Point Lay in 2018 and 2019. The Point Lay Haulout is nearly entirely composed of adult females and juveniles, though some adult male walruses use the haulout, too. In 2018, we estimated 166,000 animals were using the haulout and in 2019 we estimated 189,000 walruses were using the haulout with coefficients of variation of 0.11 and 0.16, respectively. These surveys estimate the number of walruses in the entire US Chukchi Sea region during the surveys, which lasted 9–10 days each year. We were surprised to find that far more walruses were using this haulout than previously thought, and we were pleased by the precision of the results. The estimates from drone surveys represent a big step forward to develop an approach to estimate the total number of walruses in the Chukchi Sea. The next step for this method is to apply it to coastal haulouts throughout the Chukchi Sea simultaneously to estimate abundance in both the US and Russian waters.
William: Right, and on the other hand, the estimate from the genetic mark-recapture study represented the total number of calves, juveniles and adult females in the entire Pacific walrus population, which was about 188,000 with a range of 119,000 to 287,000 for 2014–2017. The estimate for the genetic mark-recapture estimate is higher than the estimate from the drone surveys in 2018, which we would expect given the drone work is a regional abundance estimate and the mark-recapture project is a total abundance estimate. Nevertheless, these two estimates are not different from a statistical perspective. A reasonable conclusion could be made that the Point Lay haulout represented the majority of calves, juveniles, and adult females in the population in 2018, but probably not all of them.
We also provided an estimate of total population abundance from the genetic mark-recapture study, which was around 257,000 animals with a range of 171,000 to 366,000. To get this estimate, we assumed that there were an equal number of adult males in the population as adult females.
Q: Is it correct that these numbers don’t provide any information on the population trend of walrus over time?
William: Correct. Neither estimate provides information on population trend alone. However, both projects will contribute information to larger efforts that could provide valuable estimates of population trend in the future.
Tony: The genetic mark-recapture project and results from the drone work could provide information on population trend with existing data, but that requires more complex modeling that uses other types of data. The drone work at the Point Lay haulout could also be expanded to all large coastal haulouts along the Chukchi Sea in Russia and the U.S. simultaneously.
Q: Are there plans to repeat these studies in the future?
William: The genetic mark-recapture project from 2013–2017 was led by the USFWS Marine Mammals Management office in Anchorage, Alaska. USGS is working closely with them and the Eskimo Walrus Commission to conduct another set of walrus research cruises in 2023. On these cruises, USFWS will start a second generation of the mark-recapture project and USGS will collect information on walrus age structure. Both datasets will generate information on walrus population dynamics to aid co-management decisions.
The Pacific walrus ranges across the Bering and Chukchi seas during the year. This wide geographic distribution makes it difficult to estimate Pacific walrus population abundance. However, recent technological advances in genetics and drone imagery have generated precise regional and range-wide abundance estimates.
Return to USGS Alaska Q&A Series
Pacific walruses breed in winter in the Bering Sea, and in spring most adult females and juveniles migrate north to the Chukchi Sea as sea ice melts. Most adult males remain in the Bering Sea during spring and spend summers foraging and resting at coastal haulouts. Most adult females and juveniles spend summer in the Chukchi Sea (see map and photo below). Walruses rest on sea ice between foraging trips when sea ice is available. When sea ice is not available, walruses rest on land.
How many walruses are there? It’s difficult to estimate Pacific walrus abundance for several reasons. First, their wide geographic distribution and use of onshore and offshore habitats makes it difficult to access the population. Second, walruses spend a lot of time in the water where they cannot be seen by observers. However, recent technological advances have generated precise regional abundance estimates, and a recently completed multi-agency effort has provided the first Pacific walrus abundance estimate since 2011.
Abundance estimates of Pacific walrus are necessary for federal and Indigenous co-management decisions, yet several past efforts to obtain abundance estimates have generated imprecise estimates because of the clumped and dynamic distribution of walruses at sea. The most recent range-wide survey was conducted in 2006 (Speckman et al. 2011) and despite improvements compared to all previous efforts, the survey yielded an estimate somewhere between 55,000 and 507,000 walruses.
In this Q&A, we talk with USGS research biologists Tony Fischbach and William Beatty about two new approaches that they each recently took towards estimating the number of walruses in the Chukchi Sea region. The first approach (Fischbach et al. 2022) used a drone or unoccupied aerial system (UAS) to fly surveys over walruses resting on a beach in northwestern Alaska (see left photo below). The second approach (Beatty et al. 2022) was based on a five-year genetic mark-recapture study based on samples from biopsy darting of walruses to estimate survival rates and abundance (see right cartoon below).
Q: The two abundance estimates that your papers describe yield very similar numbers: about 175,000 to 200,000 walrus in the Chukchi Sea region. Were these estimates surprising or expected for the number of females and juveniles likely to be in this area?
William: We were surprised that estimates were similar in both the point estimate and precision. We arrived at both estimates using different datasets, so estimates from these two studies should be interpreted differently.
Tony: The drone work provided estimates for the number of Pacific walruses that used the coastal haulout at Point Lay in 2018 and 2019. The Point Lay Haulout is nearly entirely composed of adult females and juveniles, though some adult male walruses use the haulout, too. In 2018, we estimated 166,000 animals were using the haulout and in 2019 we estimated 189,000 walruses were using the haulout with coefficients of variation of 0.11 and 0.16, respectively. These surveys estimate the number of walruses in the entire US Chukchi Sea region during the surveys, which lasted 9–10 days each year. We were surprised to find that far more walruses were using this haulout than previously thought, and we were pleased by the precision of the results. The estimates from drone surveys represent a big step forward to develop an approach to estimate the total number of walruses in the Chukchi Sea. The next step for this method is to apply it to coastal haulouts throughout the Chukchi Sea simultaneously to estimate abundance in both the US and Russian waters.
William: Right, and on the other hand, the estimate from the genetic mark-recapture study represented the total number of calves, juveniles and adult females in the entire Pacific walrus population, which was about 188,000 with a range of 119,000 to 287,000 for 2014–2017. The estimate for the genetic mark-recapture estimate is higher than the estimate from the drone surveys in 2018, which we would expect given the drone work is a regional abundance estimate and the mark-recapture project is a total abundance estimate. Nevertheless, these two estimates are not different from a statistical perspective. A reasonable conclusion could be made that the Point Lay haulout represented the majority of calves, juveniles, and adult females in the population in 2018, but probably not all of them.
We also provided an estimate of total population abundance from the genetic mark-recapture study, which was around 257,000 animals with a range of 171,000 to 366,000. To get this estimate, we assumed that there were an equal number of adult males in the population as adult females.
Q: Is it correct that these numbers don’t provide any information on the population trend of walrus over time?
William: Correct. Neither estimate provides information on population trend alone. However, both projects will contribute information to larger efforts that could provide valuable estimates of population trend in the future.
Tony: The genetic mark-recapture project and results from the drone work could provide information on population trend with existing data, but that requires more complex modeling that uses other types of data. The drone work at the Point Lay haulout could also be expanded to all large coastal haulouts along the Chukchi Sea in Russia and the U.S. simultaneously.
Q: Are there plans to repeat these studies in the future?
William: The genetic mark-recapture project from 2013–2017 was led by the USFWS Marine Mammals Management office in Anchorage, Alaska. USGS is working closely with them and the Eskimo Walrus Commission to conduct another set of walrus research cruises in 2023. On these cruises, USFWS will start a second generation of the mark-recapture project and USGS will collect information on walrus age structure. Both datasets will generate information on walrus population dynamics to aid co-management decisions.