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The Pribilof Pup Count: From Land & Air - Post 3

August 20, 2021

Join Alaska Fisheries Science Center staff as they test the viability of aerial surveys via uncrewed aircraft systems to advance methods used to estimate northern fur seal pup production on the Pribilof Islands.

Photo of northern fur seals in a rookery strewn with large rocks. Northern fur seal rookery on Zapadni Rookery. Credit: NOAA Fisheries.

The Mark-Recapture Method

Estimates of pup production have been used to assess the population size of northern fur seals on the Pribilof Islands since the early 1900s. This makes northern fur seals one of the most studied marine mammals on the planet. In fact, this dataset is one of the longest continuous datasets for any mammal population in the world. COVID canceled the 2020 census, which left a hole in this long record. This year, we followed carefully designed safety plans to get a smaller crew out to St. Paul Island to minimize a gap in the data.

As mentioned in the last blog entry, we are conducting a mark-recapture method to estimate the total number of pups born on St. Paul Island. This has been the method used since the early 1960s.

Photo of two fur seal pups on rocks with other fur seals in the background.
Fur Seal Pups. Credit: NOAA Fisheries.

The way it works is simple. You mark a certain number of animals, let those animals mix back into the population, and then count the number of marked to unmarked animals in predetermined sample sizes. From the ratio of marked to unmarked, and knowing how many total animals you marked, you can then calculate the total population. 

But you might be asking, how do you mark northern fur seal pups so you can see them in their very crowded groupings? Well, we give them a haircut.

Photo of fur seal pups on rocks next to sea with notations indicating which are marked.
Group of fur seal pups, some marked, some un-marked. Credit: NOAA Fisheries.

Northern fur seals have a thick fur coat and soft under fur. This keeps them warm in the cold water and climate of the North Pacific Ocean. Those same attributes also made them an important resource in the late 19th and early 20th centuries when they were heavily harvested to make things like fur coats. 

Cross section photo of northern fur seal fur showing light colored under fur with darker mottled outer layer.
Cross section of northern fur seal coat showing underfur and longer guard hairs. Credit: NOAA Fisheries.

Unlike older animals, the fur seal pups are black, but their under-fur is white. So, the haircut mark trims the coarse outer guard hair, and exposes the white under-fur, which is very easy for biologists to see on follow-up surveys. The marks are very short-lived, because the pups will molt in a few weeks, but they remain long enough for us to observe them. Within a few weeks the pups shed their black newborn coat, and in its place grow a beautiful silver coat. The latter is much thicker and warmer, and prepares them for exposure to the cold Bering Sea waters and their winter migration.

Photo of northern fur seal pups with lighter post molt fur.
Molted pups. Credit: NOAA Fisheries.

Marking the fur seal pups is very difficult work. Over the course of our four days of “pup shearing” last week, our group of eight marked more than 3,000 pups in five different rookeries using manual handheld sheep shears. We were sure glad for a break. Over the weekend, we turned our efforts to another round of aerial surveys, maintaining receiver stations that monitor animals with radio tags, and preparing for re-sighting the marked pups.

On our final day of pup shearing, at the Zapadni Rookery, we encountered two fur seals that were entangled in marine debris. Rod, Rolf, and Brian F were able to isolate the animals and successfully cut the ropes free. This likely helped save both animals' lives. One of these two animals was an adult female with a pup, so this likely saved the pup’s life too.

Photo of female fur seal entangled in green fishing line with a pup next to her, amid large rocks, and field crew approaching with rods.
Photo of field crew securing fur seal with rod and working to cut away entangled fishing line.
Photo of fur seal female and pup amid large rocks with sea in background.
Marine debris disentanglement effort. Credit: NOAA Fisheries.

Stay tuned for the next blog that will cover our second week of fur seal science. More coming soon!

Photo of field crew holding shears standing on green grass with a sea inlet and rocky coastline in the background.
Our crew after our last day of pup shearing. Crew from left to right: Paul, Rod, Burlyn, Brian F, Molly, Kilali, Brian B, and Rolf. Credit: NOAA Fisheries.


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