If you recall our blog back on May 9th, we directed you to some preliminary findings for a subset of 13 adult female Steller sea lions that were captured and tagged between 2011 and 2015 (marked =24 through =36) in the Aleutian Islands. To follow-up, here’s an update on some of the final findings for those animals.
So, what influences a sea lion’s decision to stay close to home or hit the highway, so to speak? Is it size, age, offspring dependency, geographic region, prey availability, or just individual preference? Actually, it’s hard to say because we couldn’t find many patterns in the data.
Overall, we found that 7 of 13 animals remained exclusively on the continental shelf and close to shore (and in most cases their capture locations). The remaining 6 animals used both shelf and offshore habitats, traveling as far as 420 km (261 miles) into the open ocean, often referred to as ‘pelagic waters’. In some cases, sea lions traveled south of the Aleutian Island chain into the North Pacific Ocean, either near or beyond the Aleutian Trench, whereas two sea lions visited off-shelf areas in the western Bering Sea.
So, what influences a sea lion’s decision to stay close to home or hit the highway, so to speak? Is it size, age, offspring dependency, geographic region, prey availability, or just individual preference? Actually, it’s hard to say because we couldn’t find many patterns in the data. For example, there did not appear to be a relationship between distance traveled from haul-out site and sea lion body weight. In fact, both the smallest and largest individuals we captured displayed similar movement behaviors. Remember, =34 was the largest sea lion we captured and =35 was the smallest sea lion we captured, yet both of them stayed close to shore (though =34 wasn’t strictly a homebody; see map below). We’re also uncertain if age influenced the movements of those animals because they weren’t permanently marked previously, nor did we use methods to age them with certainty. As a whole, however, the adult females did tend to venture into offshore areas more frequently than the juveniles we’ve tagged in the past.
Perhaps the age of the females’ offspring influenced their behaviors. For this study, we specifically targeted females that appeared to have a dependent pup and/or juvenile. For the most part, we were able to determine that most of the tagged females were lactating when they were captured and three females may have had a yearling (=32, =33, and =34). Again, there did not appear to be an obvious pattern in the data suggesting that adult females with offspring of different ages behave differently, but additional samples are needed to explore this idea. Similarly, our sample size was too small to detect any inter-annual or regional patterns in the data. However, it’s probably worth noting that some sea lions tagged at the same site during the same year even displayed different behaviors.
More than likely, females that traveled offshore were targeting oceanographic features known to concentrate prey items like meso-scale eddies, fronts, or currents, whereas females that remained close to shore may have been targeting prey items associated with benthic features. Although these habitat associations weren’t readily apparent for all individuals, the dive data tended to support this theory.
Overall, we found females primarily foraged at night. They used a combination of benthic (to the sea floor) and epipelagic (surface waters or top zone of the ocean where light still penetrates) foraging strategies. The satellite tags of 10 females were programmed such that dives were tallied into depth bins, which were received as histogram messages for six hour periods throughout the day. Together, these data indicated that average dive depths were shallow during night and deeper during day when they were in pelagic waters (open ocean), whereas the opposite occurred when they were in waters on the continental shelf, or nearshore. This pattern suggests the females were possibly feeding on vertically migrating prey species (e.g. Salmonidae, Myctophidae, and Gonatidae) while off-shelf, whereas when foraging closer to shore, they may have been feeding on Atka mackerel. Atka mackerel display surface directed vertical excursions during daylight hours and little to no vertical migration during night.
Interestingly, the remaining three sea lions (=33, =34, and =36) had tags that provided dive depths with a time stamp. This allowed us to interpolate their dives to their location data along with some bathymetry data. Those data indicated most of the dives for =34 and =36, which primarily remained on the shelf, were benthic dives (most of which occurred at night). In contrast, the majority of dives for =33, which used both shelf and non-shelf habitat, were epipelagic (or shallow surface) dives throughout the day. To that end, maybe the old adage “different strokes for different sea lions” goes without saying.
I am a wildlife biologist with MML’s Alaska Ecosystem Program, where I am responsible for designing, implementing, and reporting field research related to the foraging ecology and health of Alaskan Steller sea lions and northern fur seals. I received a B.S. in biology from SUNY Albany, a M.S. in marine science at Moss Landing Marine Laboratories, and earned my Ph.D. from the School of Aquatic and Fisheries Sciences at the University of Washington. Prior to my employment at MML in July of 2004, I held positions with North Pacific Wildlife Consultants and The Marine Mammal Center.
Understanding northern fur seal relationship with prey key to conservation
October 11, 2017
Carey Kuhn Biologist
This blog was featured on the Alaska Fisheries Science Center’s Dispatches from the Field. Since we have seen a sighting of a northern fur seal on Steller Watch we thought it would be great to share this incredible project with you all. Katie Sweeney of the Steller Watch team recently returned from the trip effort in September!
July 7, 2016—The northern fur seal population on the Pribilof Islands, Alaska has been experiencing an unexplained decline since the mid-1970s. This despite it being one of the most studied marine mammals.
Critical information is still lacking about the relationship between fur seals and their prey, which is mostly fish. That’s why this summer scientists will begin researching where the prey is located, how abundant it is and how that affects fur seals’ behavior and population trends.
In mid-July, we start tracking adult female northern fur seals in the Bering Sea near the Pribilof Islands using temporary tags glued onto the animals. The tags are removed after the animals make a few trips to sea.
At the same time, researchers will also be measuring the availability of fish that are the seal’s main food source. This part of the study is made possible by using two Saildrones. The Saildrones are unmanned, solar and wind powered boats that are collecting data across the Bering Sea this summer. Follow their movements here.
This project is an important step forward in our understanding of northern fur seal ecology and behavior. It’s vital for developing effective management and conservation strategies as the northern fur seal population continues to decline.
Check out the blog posted during the first year of this project conducted during the summer of 2016.
Tagging females at strategic breeding site on the northeast point of St. Paul Island
July 20, 2016—We’re half way through our field work capturing and tagging fur seals breeding on St. Paul Island, Alaska. We arrived on St. Paul on July 13 and after gathering our gear and doing basic upkeep to our equipment, we headed out to a northern fur seal rookery, or breeding site, last Thursday. We’re working at the northeast point of St. Paul Island, at the Vostochni rookery.
This site was chosen for a number of reasons. One consideration was the ability to maneuver around the terrain and groups of animals, which are called harems, with our capture gear.
The terrain also provides great cover to easily recapture the animals later in the season. The instruments we use to track the fur seals record and store all of the data so it’s necessary to recapture these animals to get a complete picture of their behavior over the summer breeding season.
But the most important reason we chose Vostochni rookery is that we have good historical data on the fur seals that breed here. Based on previous studies we know that fur seals from northeast point generally feed north and northwest of the island on the Bering Shelf. This information helps us know where to direct the unmanned Saildrones that will gather information about fur seals’ prey.
For this part of the study, there are three of us on the field team: Jeremy Sterling a colleague from the Marine Mammal Lab, John Skinner a volunteer who works for the Alaska Department of Fish and Game and me.
So far, we’ve captured and instrumented 17 females and we are aiming for 30. Each fur seal is equipped with a satellite-linked dive recorder that will measure dive behavior and provide at-sea location information. These data will be linked with the fish abundance data measured by the Saildrones to help us understand how prey availability influences fur seal behavior.
Met our tagging goal and already obtaining important at-sea data about northern fur seals
July 25, 2016—Today the team is heading home to Seattle after a very successful field session. But I’ll be coming back in September to complete the study. For this first leg, we reached our goal and captured 30 fur seal mother-pup pairs and deployed 30 satellite tracking instruments on the adult females. We were pretty excited about that 30th fur seal. As of today, all but four of the fur seals are out to sea on their first summer foraging trip of the year. The remaining four will likely leave in the next day or two.
We use what we call the “box” or “tank” to move into the rookery, between harems, and work on an animal. I can’t help but think of the Flintstone’s car as we pick up our box and drive it into the rookery each day.
I mentioned in the last post that I’d tell you how we catch the fur seals. It’s not easy during the early breeding season (July) since male fur seals are aggressive about holding territories and keeping females within their harem, or group of females.
We use what we call the “box” or “tank” to move into the rookery, between harems, and work on an animal. I can’t help but think of the Flintstone’s car as we pick up our box and drive it into the rookery each day. Often, we can position the box right next to a harem with minimal disturbance. Check out this blog to learn more about the box-capture technique.
We then select a female and pull her into our box leaving her head out, facing the harem. This year we also collected each female’s pup to get its weight measurement. This will help us track the pup’s growth over the season which is linked to mom’s success finding food. The more fish a female fur seal can find, the more milk she can give her pup.
Now that most of the females are out at sea, the tags are collecting detailed diving data which are stored on the device until I can recover them. The tags also send location information through satellites so we know where the fur seals are and we can watch the fur seals’ movements in relation to the two Saildrones that are measuring fish densities.
Currently, each Saildrone is following a grid pattern that we had established before tagging the females. But as the season progresses, we can adjust the pattern to make sure the Saildrones are sampling the feeding areas that our tagged fur seals are using.
As I said, I head back to St. Paul Island in September but I’ll be with a different team of researchers. We’ll recover all of the satellite tracking instruments and see how much the pups have grown.
In the coming weeks, I’ll share the latest information we’re getting from the fur seals, the Saildrones (click to follow the Saildrones’ movements), and any new discoveries we come across. This is crucial information that will help us in our efforts to conserve northern fur seals.
Early results from Saildrone research mission, one fur seal traveled 165 miles for food
August 22, 2016—We’re quickly approaching the final days of the northern fur seal portion of the Saildrone 2016 mission. The two Saildrones have already surveyed more than 1700 miles within the fur seal foraging area. Devices attached to the unmanned boats are measuring and locating walleye pollock, northern fur seals’ main food source.
As for the fur seals, I’ve been closely monitoring limited real-time data coming in from the tags glued onto the animals. All tracking instruments continue to send useful information about the fur seals’ movements and dive behavior. Each fur seal has made between three and five foraging trips, alternating time at sea with time on land nursing their growing pup and resting too.
I’m really looking forward to September when I head back to St. Paul Island to weigh the pups, measure their growth and recover the tracking instruments, obtaining a wealth of information stored directly on the tags.
Meanwhile, there is still that smaller group of fur seals not feeding within the grid pattern the Saildrones have been following. We want to learn more about what they are feeding on too. That’s why in the next couple of days we’ll have the Saildrones move farther north and east, where the other animals are traveling.
After that, one Saildrone will make a quick trip east to listen for critically endangered North Pacific Right whales. There are just an estimated 30 left. Then both boats head back to Dutch Harbor, Alaska to end the mission.
The next step will be starting the process of analyzing all the data from the fur seal tags and the devices on the Saildrone. It will take a couple of months for my colleague, fisheries biologist Alex De Robertis and me to process all the information. We are excited to get a better idea of the prey available to the fur seals during these summer months which may help us unravel why this population continues to decline.
Recovering instruments and collecting blood samples to gain a wealth of new information
September 29, 2016—I’m back on St. Paul Island and fur seal recaptures are well underway. We started the work on Thursday and have already recovered 22 instruments! It’s been an intense couple of days but we are happy to be ahead of schedule.
Catching fur seals this time of year is much different than catching them in July. The box has been put back into storage. Now we are spending much of our time on the ground, crawling among the fur seals. Read how seasons affect fur seal capturing techniques.
Once an animal is caught, her tracking instruments are removed by cutting the top layer of hair just under the instrument. This layer is called the guard hair and it will regrow after the fur seal goes through her annual molt in October. We also collect external or morphometric measurements, including the animals’ weight and length, and take a blood sample. The blood will be used in a variety of studies, including: an assessment of general health, a study investigating mercury levels, and research measuring stable isotopes as a method for identifying foraging locations.
While mother fur seals are out at sea feeding, their pups are spending time sleeping, playing, and roaming the rookery in little pup packs. This makes the pups a little harder to catch. And since pups are not marked in any way, it’s also difficult to match mothers with their rightful pups. An instrumented mom and her pup are often surrounded by a number of other pups awaiting their moms’ return. But despite this challenge, we have been quite successful and captured 12 mother-pup pairs so far.
The ten other recaptured females were not with their pups when we caught them. So, as we wait for the remaining instrumented females to return to the island, we will try to catch as many of those pups as we can. Pups are an important part of the study because we can use their weight gain over the summer to determine how successful their mothers were during foraging trips. The largest pup to date was 34 pounds, more than double what he weighed in July.
Since the majority of the instruments have been recovered, we expect the remaining animals to trickle in over the next few days. Check back next week for more instrument recovery updates. Will we meet our goal? I’ll also share a glimpse of the data that we collected this year!
New data from satellite instruments shows stark differences in fur seal feeding behavior
October 13, 2016— After a very busy couple of weeks, I am happy to report that I am back in Seattle and ready to start the next leg of our 2016 Saildrone mission: data analysis!
We were able to recapture 29 of the 30 instrumented females and remove their tags, resulting in one of our highest instrument recovery rates in years. Eighteen of those females were caught with their pups, giving us the ability to link the females’ foraging behavior and their reproductive success.
I wanted to share with you what our fur seal dive records look like. Below are two females’ dive patterns. They’re good examples of how different dive behavior can be between individuals. The graphs show dive behavior over the same 12 hours window – but on different days. The top plot is for July 28. The second plot is from July 30. Notice how both the number of dives and dive depths differ between these two fur seals.
The first female (top graph in image above) was regularly diving more than 70 meters which is about 200 feet. The second female (bottom graph) never went past about 65 meters and only went to that depth twice. Yet, the foraging grounds used by these two females were relatively close, just northeast of St. Paul Island. We don’t know why there are such differences but we hope to find out.
The data collected from the Saildrone will be able to tell us more about the fish at these fur seals’ foraging sites. Perhaps we’ll find that where the first seal was feeding the pollock were larger in deeper water.
The data analysis stage will take several months and the process is a collaborative effort. I’ll be analyzing all of the fur seal data and will work with my colleague Alex De Robertis who is examining the fish abundance data. That information was collected from acoustic devices attached to the Saildrone. We’ll merge the data sets to get a clearer picture of fur seals and their pollock prey in the Pribilof Islands.
This field season was incredibly successful overall and it wouldn’t have been possible without the hard work of my field teams in both July and September. Their enthusiasm and commitment, even in some awful weather conditions, made it possible to recover a wealth of data that will be vital for helping us understand fur seal declines.
Carey Kuhn is an ecologist at the Alaska Fisheries Science Center’s Marine Mammal Laboratory. Carey joined the Lab’s Alaska Ecosystems program in 2007 after completing her Ph.D. at the University of California Santa Cruz. Her research focuses on the at-sea behavior of northern fur seals.
*Notes: Research conducted and photos collected under the authority of MMPA Permit No. 14327. All data presented here are preliminary analyses and subject to change.
Our featured sea lion for October is ~100 and he is a bit of a mystery to biologists but not to those of you who are helping us classify images on Steller Watch! Many of you have reported sightings of this male sea lion at Cape Wrangell on Attu Island. We have only seen him in person in 2013, the year he was marked.
On June 24, 2013 we captured ~100 and he weighed about 78 Ibs (35 kg), measured almost 4 ft (117 cm) long, and was just over 2.5 ft (78 cm) around his body (or girth, measured just below the front flippers). He was born on Gillon Point on Agattu Island, which is why he has the ~ symbol. We observed him staying at this in the remote camera images until the fall of 2013.
We didn’t see him again until he showed up in remote camera images from Cape Wrangell on Attu Island in the spring of 2014. Perhaps most interesting, he was seen multiple times suckling from a female next to another suckling juvenile that was much larger, likely born a year prior to ~100—this is a rare sight! Since we saw this multiple times, we assume that this female birthed both of these juvenile sea lion since females don’t usually happily nurse sea lions that are not their offspring.
Curious about other pinnipeds (seals and sea lions)? Check out our neighbors in the Pacific Islands to the south, the monk seals of the Northwest Hawaiian Islands. The Pacific Island Fisheries Science Center features their own Monk Seal of the Month!
We will share the story of one marked sea lion each month. Be sure to check our Sea Lion of the Month page on the 1st of every month to learn about our featured Steller sear lion. You may nominate a sea lion by submitting their full mark on the Sea Lion of the Month nomination forum. Thank you all for your nominations!