My maiden voyage to Alaska

The 2018 Steller sea lion research cruise


December 11, 2018
Burlyn Birkemeier



This year, a team of NOAA scientists completed a re-sight trip by boat from the eastern Aleutian Islands to the Gulf of Alaska. The purpose of a re-sight trip was to survey Steller sea lion rookeries and haulouts for previously marked animals, similar to Steller Watch citizen scientists who look for marked animals in remote camera images. This year, after many years of working at the Marine Mammal Laboratory, looking through more than 200,000 remote camera images, I finally had the opportunity to go into the field and see Steller sea lions in person, for the first time.


On July 12, I met the other three women who would compose our team at the airport, on our way to Dutch Harbor via Anchorage. I had heard many horror stories of canceled flights and stranded researchers trying to make their way to Dutch Harbor, so I braced myself for the worst case scenario when we arrived to Anchorage. My first surprise was that the only gate for passengers travelling to Dutch Harbor had no TSA, and upon checking in, every passenger was weighed with their carry-ons before getting on the plane. As we waited for our flight, we watched the crew load luggage onto the plane, wondering if ours would make it. Lucky enough, it was smooth sailing and we arrived in Dutch Harbor on our first attempt, luggage and all, just in time for dinner. We stayed the night in the only hotel in Dutch Harbor. The next morning, we got up bright and early to get the gear ready and hauled over to the M/V Pŭk-ŭk, our research vessel and home for the next two weeks.


Once aboard the Pŭk-ŭk, we wasted no time and set off for our first site. I took in the first views of the beautiful green cliffs and incredible wildlife—as well as the onset of nausea (sea sickness!)—as we entered the Bering Sea. The weather was too bad for us to survey that day, so it wasn’t until the second day that I saw my first Steller sea lion. The first skiff survey was at Tanginak, a lower priority site home to a mere 12 sea lions. Our team donned large mustang suits for warmth and flotation, climbed down into the skiff from the Pŭk-ŭk and observed the sea lions while the skiff driver navigated our boat back-and-forth in front of the site. We used stabilizing binoculars to look for marked animals and conducted counts while we slowly moved closer to the site in a zig-zag fashion, so as not to disturb the sea lions. Once we were sure we had a thorough look at each sea lion, we returned to the Pŭk-ŭk.


DSC01639It was on our second day of surveying when I was the first person to spot a marked animal, an exciting and personal feat. After that, finding them became much easier. On day three, we visited seven sites but only needed to get in the skiff for a closer look at three of the sites. Cape Morgan (Akutan Island) was the first large rookery I had visited, with over 1,000 sea lions. Larger sites, such as this one, took a couple hours to survey, and despite the cold and rain, I enjoyed every moment. My favorite site was Billings Head (Akun Island), a large rookery tucked into a cove at the base of a massive bluff. The water was so clear you could see to the bottom. Some curious sea lions would come and investigate the skiff, while we rapidly called out the brands of the many marked animals we were seeing to the data recorder. The remainder of the two weeks continued the same fashion, doing multiple surveys some days, and other days, remaining on the boat and not surveying because of bad weather or because we were travelling between sites.

DSC01591.JPGBy the end of the two weeks, we had surveyed 29 sites. All but five had sea lions, and four we surveyed multiple times. Multiple trips to larger sites like Cape Morgan or Billings Head gave us chances to see new sea lions that were out foraging during our previous visit. In total, we observed 98 different marked individuals, many of which we saw more than once. One male was even present at Billings Head and then the next day we saw him at Cape Sarichef (Unimak Island), which are 50 kilometers apart! We also saw six dead sea lions (unknown causes) and, sadly, two entangled animals (packing bands). In addition to Steller sea lions, we saw sea otters, harbor seals, northern fur seals, a pod of humpback whales feeding, and even heard the calls of one very lost California sea lion. While we could have had better weather—we were unable to work due to poor weather a total of 5 days—it was still a productive trip.

The data we collected will help us better understand Steller sea lion survival, movement, and reproductive success of this endangered population. The information we collected, in addition to the data collected from Steller Watch, will help us understand the potential causes for decline of this fragile population.

I began working at the Marine Mammal laboratory for an internship in 2014 analyzing Steller sea lion remote camera images. I completed my senior thesis project on sea lion pup birth timing for my Aquatic and Fisheries Sciences degree at the University of Washington, where I also earned a degree in Biology. After graduating, I was contracted to continue my work at the Marine Mammal Laboratory analyzing remote camera images. I am currently a research biologist with the University of Washington’s Joint Institute for the Study of the Atmosphere and Ocean (JISAO) at the Marine Mammal Laboratory where I analyze high-definition videos from camera tags placed on Northern fur seals to study feeding rates, prey availability, and foraging success. 

The Aleutian Island Struggle

The 2018 Steller sea lion aerial survey struggle is real


November 14, 2018
Katie Luxa


Earlier this summer, NOAA scientists partnered with NOAA’s Aircraft Operations Center to conduct our annual aerial survey of Steller sea lions in Alaska. You can check out this post to read more about our aerial surveys.

Overhead view of the camera system installed in the NOAA Twin Otter. Photo credit: Katie Luxa

Our trip began in Anchorage on June 19, where my co-workers and I met the pilots, mechanic, and – most importantly – the Twin Otter airplane that would be our office for the next three and a half weeks. The first order of business was to install our survey equipment: an array of 3 digital cameras mounted in the belly of the plane and a computer that controls the mount system.

The next task was to plan the flight to our first destination. Despite our ability to cover a lot of ground in the Twin Otter, Steller sea lion sites are spread out over more than 2,500 miles of coastline and weather conditions aren’t always favorable enough to fly safely and capture high-quality images. For these reasons, we split sites into two survey areas: Southeast Alaska and the Gulf of Alaska are surveyed in odd-numbered years and the Aleutian Islands are surveyed in even-numbered years. This year, an Aleutian Island survey year, flights would be based out of Dutch Harbor (Unalaska Island) and Adak Island.


Dutch Harbor is approximately 800 miles southwest of Anchorage; it is the #1 commercial fishing port in the United States and was featured in the TV show “Deadliest Catch”. Adak is another 440 miles west of Dutch Harbor, and is the site of a US naval air facility that closed in 1997. At its peak, there were up to 5,000 troops and their families living in Adak – they even had a McDonald’s! Our area of highest priority was around Adak and so we hoped to start the survey on a high note and visit those sites first. Unfortunately, the weather forecast didn’t look great, so we headed to Dutch Harbor instead.

Aerial view of Adak (most of the homes in the image are empty). Photo credit: Katie Luxa

Once we settled in Dutch Harbor, it was time to start planning survey flights. We use an app called ForeFlight to upload a list of GPS locations for known Steller sea lion sites and draw a route from point to point; the route can then be shared with the pilots, who use the same app in the plane. Depending on how close the sites are to one another and how many passes are needed to photograph the sea lions, it’s possible to visit more than 30 sites in a single flight. We try to come up with a few potential flights each day, because being flexible is key to a successful survey.

Discussing that day’s flight options in Adak. Photo credit: Ben Hou

And, boy, this year was all about flexibility! We had unusually poor weather, including many days with low “ceilings” (height of the cloud layer) or gusting winds that made it impossible for us to fly. And when we were in the air, we often spent our survey flights “hunting and pecking” for sites that weren’t covered by fog. Despite our weather difficulties, though, we were still able to survey over 130 sites and take over 13,000 photos!

Bogoslof Island is a breeding site for both Steller sea lions and northern fur seals. The steam rising from the island is due to recent volcanic activity. Photo credit: Morgan Lynn

There were also some true highlights on this year’s survey. As Katie mentioned in an earlier post, we surveyed Bogoslof Island, which was completely transformed by a series of volcanic eruptions starting in December 2016. Compared to our aerial survey in 2016, the island had more than tripled in size and was still steaming! Bogoslof is one of the few places in Alaska where both Steller sea lions and northern fur seals breed, and it was amazing to see them there again, after such dramatic changes to the landscape.

Through the fall and winter, we’ll continue the important task of counting sea lions in this year’s images – and keep our fingers crossed for fair weather in 2019!

I’m a research scientist with the University of Washington’s Joint Institute for the Study of the Atmosphere and Ocean. I study food habits of Steller sea lions and northern fur seals, as well as Steller sea lion population abundance and survival, at the Alaska Fisheries Science Center. I’ve worked at the Center since 2008. I received my undergraduate degree in Marine Science from the Southampton College of Long Island University and my Master’s degree from Western Washington University.

And we’re back!

The Steller Watch team is back from our 2018 summer field season


October 5, 2018
Katie Sweeney



The Steller Watch team is back from a busy field season and we’re hard at work processing and analyzing data that we have collected over the summer. We had a couple of research cruises and an aerial survey to study Steller sea lions in Alaska this past summer.


There was also field work to study northern fur seals! Over the next few months me and some of my colleagues will be sharing more in-depth stories about these trips and results. But, for now, I’ll just give you a brief summary of what we were able to accomplish to study Steller sea lions.

Western Aleutian Island Research Cruise:


During our annual research cruise on board the U.S. Fish and Wildlife Service R/V Tiglax  we focused our efforts in the western Aleutian Islands, boarding and disembarking from Adak, Alaska. Weather was quite a challenge this year but we did quite well despite the fog and wind.

Pup work

We were able to visit the highest priority sea lions sites to look for marked animals, conduct counts, and do drone surveys. We were also able to access all of our remote cameras to collect over 330,000 images! There were three sites where we went to shore to work up pups in order to help assess pup condition in this area of concern. We handled almost 150 pups–phew! They were heavy…

NOAA Twin Otter Aerial Survey:


During the time we were on the R/V Tiglax in the western Aleutian Islands, our aerial survey team, including NOAA’s Aircraft Operations Center pilots and mechanic were surveying the area from the Delarof Islands to the east, along the Aleutian Islands. During our cruise, we had challenging weather and the fog and wind caused a lot of problems for our aerial survey team, as well! They didn’t get as many days of flights completed as is possible during a ‘good’ weather survey year, but despite the spotting nature of surveying, they were able to cover a lot of ground!


They even got to check out Bogoslof Island, which is an island that had been erupting pretty consistently in the previous year! Check out all of those fur seals, sea lions, and marine birds.


Eastern Aleutian Islands & Gulf of Alaska Resight trip:

Like the other two Steller sea lion projects, we encountered some difficult weather during our resight trip, as well. It didn’t keep us down! This was the first trip I’ve been on where we had a scientific crew that was all women! We were able to accomplish a lot of work in the Eastern Aleutian Islands and spotted many marked sea lions. Once we moved into the Gulf of Alaska, currents and winds slowed us down a lot which meant we had to focus our efforts on getting back to Homer in time to get home.


Steller Sea Lion Field Camps (Cancelled):

Unfortunately, this year the Marine Mammal Laboratory were unable to run the two annual Steller sea lion field camps. This is the first year since early 2000 when the field camps have not been conducted.

What’s next?

We returned with over 330,000 images and guess what? We still need your help to get through these images on Steller Watch. While we were away, you all did a great job completing 55% of the classifications in the Presence of Marked Animals workflow! We have a new set of images that are now live in or Presence or Absence workflow. Thank you Steller Watch team for your continued help and support!

I have been a biologist in NOAA Fisheries Alaska Fisheries Science Center studying Steller sea lion population abundance and life history for over 10 years. I am an FAA certified remote pilot and have been flying marine mammal surveys with our hexacopter since 2014. I earned my B.S. in Aquatic and Fishery Sciences at the University of Washington and my Master in Coastal Environmental Management at Duke University. 

It’s that time of year!

We’re heading out for our 2018 summer field season


June 19, 2018
Katie Sweeney



Well, Steller Watch team, it’s that time of year again! We are gearing up to head out for our summer field season to Alaska to study Steller sea lions. While we are away, we will not be present on our Project Blog or the Talk Forum. Our current workflow will still be live while we are away! We are hoping to be almost complete with this current set of images very soon since we plan on coming back in the fall with a whole new set of images!


We have several Steller sea lion trips happening this summer, very similar to last year: a research cruise to the western Aleutian Islands, a traditional aerial survey, and a resight cruise to the eastern Aleutian Islands and Gulf of Alaska. Unfortunately, this year we are not able to do our field camps. This will be the first time since our field camp effort began a couple decades ago that we will be unable to do field camps (except for in 2006 when field camps were on hold due to a law suit). Other science groups from the Alaska Fisheries Science Center are heading out this summer for field work, as well.

Western Aleutian Island Research Cruise:

This year’s cruise is very similar to last year. We will be on board the U.S. Fish and Wildlife Service’s R/V Tiglax for about two weeks surveying between Attu and Adak Islands. During this trip we will be conducting count surveys by boat, land, and air with our drone. We will also be looking for marked animals at all the sites we visit and visit those sites with remote cameras to collect more images for Steller Watch! We will be doing some work with pups to collect data to help figure out more about pup health in the Aleutian Islands. Finally, there will be a couple whale biologists on board with us to help look for whales in the area, including killer whales.

NOAA Twin Otter Aerial Survey:

Since 2006, NOAA’s Aircraft Operations Center has operated a NOAA Twin Otter for the aerial survey that will go from the Delarof Islands to the western Gulf of Alaska. This means they mostly operate out of Adak Island and Dutch Harbor. We even hope they’ll be able to check out Bogoslof Island, a volcano that erupted for over a year and has more than doubled in size. Will we see Steller sea lions, northern fur seals, and sea birds?

Eastern Aleutian Islands & Gulf of Alaska Resight trip:

We are not able to do field camps this year but luckily we are able to do a resight trip to look for animals that were marked on Ugamak Island, just last year. During this trip, we will just be visiting sites to look for those newly marked one year olds and marked adults beginning around Dutch Harbor and ending in Homer, AK.

A HUGE thank you to those of you who have contributed to Steller Watch! We’ll be back in the fall with many, many more images to share! 

I have been a biologist in NOAA Fisheries Alaska Fisheries Science Center studying Steller sea lion population abundance and life history for over 10 years. I am an FAA certified remote pilot and have been flying marine mammal surveys with our hexacopter since 2014. I earned my B.S. in Aquatic and Fishery Sciences at the University of Washington and my Master in Coastal Environmental Management at Duke University. 

Part II: Is that a healthy pup?

With a few measures we can check on the health of pup and find out about mom too


April 24, 2018
Brian Fadely


In my last post, I shared how we use pup weights and lengths to calculate a condition index to better understand the health of the pups. When we handle Steller sea lion pups that will be marked, we also collect blood, tissue, and fur samples. Collecting blood and other tissue samples allows us to evaluate health status in another way involving work in a lab. We look at blood chemistry and hematology parameters, to test for signs of disease, contaminant exposure, or other systemic concerns.

Some degree of clinical issues or disease is normal to find in any wild population; we’re interested in determining whether there is evidence of clusters of disease, contaminant exposure, or other concerns at a rookery or greater area. This can provide insight into local conditions that may help explain population declines or lack of recovery. Samples are collected while the pup is gently but firmly restrained by hand.

Collecting a blood sample from a restrained pup. The restraint board helps prevent wriggling so the procedure is safe for the pup and handlers.

The board that we place the pup on helps prevent wriggling so the procedure is safe for the pup and handlers. We looked at blood chemistry and hematology profiles of 1,231 pups sampled during 1998-2011 throughout Alaska. We found no indications that pup condition was compromised during their first month after being born, including pups within the declining parts of the Aleutian Islands (Lander et al. 2013).

Exposure to heavy metal contaminants (like mercury) is a concern since Steller sea lions are apex predators, or predators that feed at highest trophic level. In other words, Steller sea lions eat prey that are high up in the food web. That means, if there are contaminants in an environment, the contaminants can bioaccumulate and biomagnify through the food chain. Exposure to high levels of mercury can cause neurological disruption that may impact health and consequently survival and reproduction. Pups accumulate mercury during gestation in utero (while they are a fetus in their mothers), and again once they are born and suckling milk from their mothers. In a project led by collaborators at the University of Alaska Fairbanks and Alaska Department of Fish and Game, we’re investigating the mercury burden of pups throughout their range in Alaska and Russia. We shave off a small patch of hair from the pups when we handle them and are then able to measure the mercury content. Specifically, we can figure out the mercury concentration the pup was exposed to from its mother over a period of several months during gestation.

The patch where hair was removed for a sample to measure mercury content is evident on this pup chilling with mom at Agattu/Gillon Point. 

We found that pups in some areas of the endangered western population had a higher mercury exposure than pups from Southeast Alaska (Castellini et al. 2012). The greatest exposure is shown by pups from the Gillon Point rookery on Agattu Island, with three pups showing exposure levels known to cause neurological effects in other fish-eating wildlife (Rea et al. 2013). If you look at the figure below, you can see the difference in mercury exposure (median values are shown by colored lines and average values by black lines) between pups from Agattu Island and other rookeries can be seen in this boxplot that was published in Rea et al. (2017).


We do not have direct evidence that this exposure to mercury during gestation leads to health consequences for the pups and their subsequent survival, nor that it impacts adult reproduction. But, these levels of mercury exposure do indicate that further research is necessary to better understand the role of contaminants in the ecology and biology of Steller sea lions.

I am a research wildlife biologist with NOAA Fisheries Alaska Fisheries Science Center in Seattle, in the Alaska Ecosystems Program where I’ve studied Steller sea lions and northern fur seals since 2000. My primary research interest is vertebrate physiological ecology, which at NOAA Fisheries translates into studying sea lion foraging behavior, health status, and body condition to help address conservation questions and wildlife management issues.

Part I: Is that a healthy pup?

Part 1: Studying the condition of sea lion pups


April 10, 2018
Brian Fadely


When we handle Steller sea lion pups that will be marked, we also check their condition and health status, similar to when you take your pets to the veterinarian for a check-up.  Collecting health data can give an indication of local environmental conditions, and allows testing of some hypotheses for the population decline.

Pups are weighed by holding them in a small hoop net and measuring with a digital scale suspended from a tripod. Photo by Kristen Campbell.

While we are handling the pups, we weigh them and measure their length and girth as indicators of condition. We look at these measurements relative to the weighing date (since we don’t know a pups birth date), as well as, their weight relative to their length. Both are used as indices of body condition and help us explore trends among pup measured across regions or over years.

Weighing and measuring pups is straightforward, as simple as suspending them from digital scale while nestled in a hoop net. Length is measured from the tip of nose to the tip of their tail, and girth is measured around the body just behind the front flippers.

A pup that fell asleep in the net while being weighed

Pups are born between late May and early July but half of the pups are born by June 10th. For consistency, we try to sample pups between June 20th and July 7th, which means we’re sampling them when they are 12-25 days old, but possibly 5-37 days old. At this young age, the size and health of the pup largely reflects the mother’s condition while she was carrying the pup, since about April. Pup condition can vary with many factors including age and size of the mother and the local foraging conditions she encounters, which we typically don’t have any way to directly assess.

Looking at pup measurements collected throughout the Aleutian Islands from 1990 to 2017, the weight of female pups (a total of 1,958 measured) has ranged between 33 and 97 Ibs (15 to 44 kg), or an average of 62 Ibs (28 kg). The weight of male pups (a total of 2,234 measured) ranged between 29 and 115 Ibs (13 to 52 kg), with an average of 75 Ibs (34 kg). Male pups tend to weigh about 11 Ibs (5 kg) more than females. Generally, pups grow just under a pound (over a third of a kg) per day.

Just as with human infants, we can compare the size of any pup against all others to determine whether they are relatively large, small, or about average. In the figure below, the sizes of pups from Hasgox Point on Ulak Island (white squares) and Gillon Point on Agattu Island (black circles) are compared to all other Aleutian Island pups (light gray circles) for females (F, left figure) and males (M, right figure). It’s evident that while some individuals are small or large compared to others, the size ranges of pups from these islands are similar to all others.

In these plots, each dot represents the weight of a single pup. The left plot shows females and the right, males. The two sites you may be familiar with are Hasgox Point on Ulak Island (white squares) and Gillon Point on Agattu Island (black circles). The light gray circles are all other pups in the Aleutian Islands.

Since we don’t weigh the pups on the same day and they put on weight each day as they grow, to compare pup condition over years or between rookeries, we create a condition index. The condition index compares the weight we collect to the weight we would expect to see on the weighing date, or to the weight expected for their length. This condition index is a ratio of the measured weight to the expected weight which is calculated from doing a regression of all pup masses by weighing date.

In the figure below is called a box plot (also called a box and whisker plot). This is a great way to visualize data. The condition index ratio we described above is plotted in the following two figures. Median values (black lines) are shown within the 25th and 75th data percentiles (boxes), and outlier values (black dots) are plotted outside of the whiskers (1.5 times the percentile range, showing data dispersion). This box plot above shows the data collected from female pups measured from 1994 to 2017 at rookery sites within the area we have remote cameras deployed in the Aleutian Islands. Essentially, if the observed and expected weights are the same, then the condition index ratio is 1.0 (the horizontal dashed line).


Values above that are interpreted as ‘better’ condition (they weigh more than expected for their length), and ratios less than 1 are ‘poorer’. Pups from Agattu Island rookeries tended to weigh less for a given length than did pups at Kiska or Ulak Islands, though overall there is not a great difference among these sites.


Alternatively, we can look at differences in pup condition over the years at specific sites or region. The box plot above shows the condition indices for female pups at Hasgox Point (Ulak Island) collected from 1994 to 2017. This data suggest that the pup cohort of 1994 was in apparently relatively poorer condition compared to later years, while cohorts since 2013 have been in relatively better condition.

All of this information are valuable pieces in the puzzle towards figuring out why Steller sea lions have not recovered in the Aleutian Islands. In the next blog, I will be sharing what we can learn from the different samples that we collect from pups along with weight and length measurements. Be sure to sign up for blog notifications by filling in your email and clicking the “Follow” button!

I am a research wildlife biologist with NOAA Fisheries Alaska Fisheries Science Center in Seattle, in the Alaska Ecosystems Program where I’ve studied Steller sea lions and northern fur seals since 2000. My primary research interest is vertebrate physiological ecology, which at NOAA Fisheries translates into studying sea lion foraging behavior, health status, and body condition to help address conservation questions and wildlife management issues.


Why do we permanently mark Steller sea lions?


December 12, 2017
Lowell Fritz


We permanently mark Steller sea lions to estimate vital rates of the population, which are:

  • Survival (from year to year)
  • Reproduction (how often females give birth to a pup)
  • Dispersal (where marked sea lions are observed at each age)

Why is estimating vital rates important?

By seeing marked animals through time, we can determine which vital rate is most likely responsible for this decline.

For a population that’s declining, like Steller sea lions in the Aleutian Islands, estimating survival, reproduction and dispersal can help us determine what factors might be affecting the population. For instance, we know Steller sea lions in the western Aleutian Islands are declining at an alarming rate of about 7% per year. If they continue to decline at this rate, they could be go extinct in this region within the next 50 years. Which is why we need your help to classify images on Steller Watch.

Because the number of Steller sea lions (or abundance) is going down in the western Aleutian Islands we know that either they are dying faster than new pups are being born or they are abandoning this area and settling elsewhere.

Biologists look for marked Steller sea lions.

By seeing marked animals through time, we can determine which vital rate is most likely responsible for this decline. For example, suppose we discover that survival during the first 2 years in the western Aleutians is similar to areas where the species is currently increasing. This would suggest that factors that directly kill young sea lions, such as entanglement in fishing nets or predation by killer whales, are likely not affecting the western Aleutian population any more than in parts of the range where the population is increasing.  If we knew this, then we could focus our research and management attention on other pieces of the puzzle, such as factors that would affect reproduction (e.g., disease, nutritional stress) and adult survival (e.g., illegal shooting). In addition, because we know a lot about each of the pups that were marked, we can determine whether males and females are affected differently, or whether the weight of the pup (which is an indication of the health and age of its mother) was a factor.

A Steller sea lion pup that has been marked and a hair sample collected is being monitored in the pup recovery area.

We began marking Steller sea lion pups in the western Aleutian Islands in 2011, so as of December 2017, the oldest marked animals from this region are only about 6½ years old. Given that female Steller sea lions can live to be about 30 years old and don’t start having pups until they are 4-6 years old, this means we don’t yet have enough years of sightings to estimate reproduction or adult survival.  However, we are closer to being able to estimate juvenile survival.

I’m going to provide a short introduction into how we estimate survival, in this case, of juveniles. This will get a little messy and into the muddy math so skip ahead to the last two paragraphs if you want to skip this part. To set the stage, let’s look at a table of a simplified version of our experiment with ‘pretend’ data:


Imagine we marked 100 pups in 2011 and set them free. In each of the following years, there are really only 2 options for us as researchers: we either see them alive that year or we don’t. Let’s say in the 2nd year (in 2012) we observed only 50 of these 100 marked individuals. In the 3rd year (2013), we only saw 30. We want to try to estimate the percent of animals that survived to the 2nd and 3rd years (and beyond) which we call survival. In the most simplistic terms, survival is 50% to year 2 and 30% to year 3.

20150626_ULAK REMOTE CAMS_2.JPGBut it’s not that simple! What complicates this is sighting probability, or the chance that we will actually observe a live marked animal. While we have remote cameras at several locations and visit the Aleutian Islands at least once a year, we know that we do not see every single marked sea lion that is alive in the population. This means we have to account for the probability of observing a live marked animal, and how that might change over time, for instance, as the animals age or with different levels of sighting effort.  Another reason we might not see a marked animal is that it completely left our study area never to be seen by us again. We collaborate with researchers in Russia and look for marked animals in other parts of Alaska, but we still try to account for this possibility, however slim. For these reasons, we use the term “apparent” survival to describe what we are actually estimating since we can’t distinguish death from permanent emigration. But for this blog, we’ll just call it survival.

So, how do we account for sighting probability and how it might vary between years so we can estimate survival? This is where some math comes into play and why collecting data over many years is so valuable.

Capture history of marked sea lion sightings up to Year 2.

The table to the right is what we call a capture history of how many marked sea lions were seen (Y) or not seen (N) in Year 2. Of course, all of the 100 sea lions marked in 2011 were “seen” in the first year, which is why they have a “Y” listed for the first year. Then in year 2 (2012) there were 50 marked sea lions seen so their capture history is “YY”, and the other 50 were not seen which means their capture history is “YN”.

Pretty simple for year 2, right?  They were either seen or not seen.

Let’s add sightings collected during Year 3 (2013), and you can see that this is when it starts to get complicated. In the first table, you can see we saw only 30 marked animals in Year 3. Of those 30 marked animals seen, 10 were seen all three years so they have a capture history of YYY. The other 20 were not observed in year 2, so their capture history is: YNY.

Capture history of marked sea lions to year 3.

Seventy of the original 100 marked sea lions were not observed in year 3 but 10 of these were seen in year 2 so they have a capture history of YYN. That leaves the remaining 60 who were not seen in year 2 and 3, and these have a capture history of YNN.

How is this sighting data by year used to estimate sighting probability (P) and survival (S) in years 2 and 3? We use a mathematical model that finds the values of P and S that best fit the following equations. Let’s start from the top by examining the number of sea lions that had each type of capture history in year 3 and equations that express the probabilities for each one.

In our data, 10% of the original marked group of 100 pups has a capture history of “YYY” in year 3. This can also be expressed as:

Pr[YYY] = 0.1 = [P2 * S2] * [P3 * S3]

Our data indicate that the probabilities of both being seen (P2) and surviving (S2) to year 2 multiplied by the probabilities of both being seen (P3) and surviving (S3) to year 3 is equal to 0.1 or 10%.


That tells us a little bit but not too much about the individual values of each of the 4 parameters. Some more information will come from examining the equations associated with the other capture histories.

We not only have sighting probability and survival in our model, but we also have their opposites: the probability of NOT surviving (or dying) and of NOT being seen. Let’s say that we estimated that S = 0.6 for a particular year. The opposite of that, or the probability that an animal did NOT survive that year, would be (1 – S) = 0.4. In other words, if an animal had a 60% chance of surviving, it also had a 40% chance of dying. Similarly, if a marked animal had a 70% chance of being observed (P = 0.7), it also had a (1 – P) = 0.3, or 30% chance of NOT being observed. So for the capture history of “YNY” we would use the equation below:

Pr[YNY] = 0.2 = [(1-P2) * S2] * [P3 * S3]

For these 20 animals, we know they survived through year 2 because they were observed alive in year 3. Therefore, during year 2, the probability of being NOT seen (1-P2) is multiplied by the probability of surviving (S2), while for year 3, the terms are exactly the same as for the animals with capture histories of “YYY” since they were seen alive in year 3.

Pr[YYN] = 0.1 = [P2 * S2] * [(1-S3) + (S3 * (1-P3))]

OK, now it’s starting to look ugly, right?  Let’s just break it down term by term.  Since these 10 animals were all seen alive in Year 2, the equation has the same terms for year 2 as the “YYY”s. But year 3 is where it really starts to change, and this is because we don’t know if they didn’t survive to year 3 or they were alive but just not observed that year.  Data obtained in year 4 and beyond will help us untangle this, but at this point in the analysis of these example data, we do not know. Therefore, the year 3 term takes into account both possibilities: the probability that these 10 animals did NOT survive to year 3 (1-S3) and the probability that they survived to year 3 (S3) but were NOT observed (1-P3).

And now the messiest of all is the equation for the probability of having a capture history of “YNN”.

Pr[Y N N] = 0.6 = [(1-S2) + (S2 * (1-P2))] * [(1-S3) + (S3 * (1-P3))]

These 60 animals were marked in year 1 and never seen again, but we don’t know if they survived to year 3 (S2 and S3) but were just not observed either year [(1 – P2) and (1 – P3)]; if they didn’t survive to year 2 (1 – S2) and were not available to be seen in year 3; or if they survived to year 2 (S2) and were not observed (1 – P2) and then died in year 3 (1 – S3).

Without going into the gory detail, finding the values of sighting probability (P) and survival (S) for each year that best fit the data is quite a process, and luckily there’s a program called MARK that performs this task (and many more!) with remarkable speed.

For this example, survival to year 2 (S2) is estimated to be 0.82.  In other words, we estimate that 82% of the marked sea lion pups survived to celebrate their first birthday. Sighting probability during year 2 (P2) was estimated to be pretty low, only 0.19.  In other words, there was a 19% chance of seeing a marked animal during year 2.  You can see how adding sighting probability significantly changed our perception of survival, given that our first ‘guess’ for survival during year 2 was 50% when we only considered how many we actually saw alive in year 2. At this point in the data collection, P3 and S3 are not estimable with much precision because it is the last year of data in the analysis and we don’t have enough information to know whether a marked animal that was not seen in year 3 was alive or not. For each additional year of sightings, the number of years for which survival can be estimated usually increases, and the number of unique capture histories doubles. So you can see that the equations expressing the probabilities get very complicated very quickly! Add some other variables (also called co-variates) to the mix, such as sex, cohort (different island rookeries, different birth years), and weight at the time of marking, and you’ve got yourself quite a sophisticated model.

And that’s Survival 101!

I have been studying Steller sea lions since 1990 with NOAA Fisheries Alaska Fisheries Science Center in Seattle.  My primary research interests are sea lion population dynamics, demographics, and interactions with commercial fisheries.  I’ve also worked on fish during my career with NOAA, particularly species eaten by sea lions, like Atka mackerel, walleye pollock (you may know them as fish sticks and imitation “krab”), and Pacific cod.   I graduated from Bucknell University (B.A. Biology, 1976) and College of William and Mary (M.S. Marine Science, 1982), and started my science career in 1982 at Rutgers University as a Research Associate.  At Rutgers, I worked at the Haskin Shellfish Research Laboratory in Bivalve, NJ (down the road from Shellpile… you can’t make this up) studying the shells of mollusks living in habitats ranging from freshwater lakes and streams to deep-sea hydrothermal vents. I even had the opportunity to go down in the Alvin submersible!