Paper in a Nutshell
Bishop AM, Brown CB, Rehberg M, Torres L, & Horning M. 2018. Juvenile Steller sea lion (Eumetopias jubatus) utilization distributions in the Gulf of Alaska. Movement Ecology, 6:6 Open Access
It is a Saturday morning. You wake up, walk to your kitchen, and make breakfast. Maybe you then head out into town to run errands, and then go for a long hike into the mountains. Maybe you take your breakfast to the couch and then remain there for roughly 8 hours binge-watching the latest new show on Netflix.
If we were to map out all of these different patterns in space, we’d be able to characterize what is called your “home-range”. Taking it a step further, if we focus on those places on the map where you spent a lot of concentrated time, we would call that our core space (in the case of the Netflix binge–the couch).
This concept is the same for animals. Their home range is the total space they use to fulfill all of their biological needs: where they eat, where they sleep, where they breed. How big the space is and what shape it takes is different across species (think of the total space a whale might use compared to a robin) but it can also differ across individuals of a single species.
Why do we care about characterizing home ranges and space use?
Many conservation issues stem from human activities and animal activities relying on the same space. Take for example the case of the critically endangered North Atlantic Right Whales. According to NOAA, there are only an estimated 450 individuals remaining and the two biggest threats to the population are entanglement with fishing gear and collisions with ships. To help mitigate these threats—scientists and managers conducted surveys to find out where and when whales and ships were using the same space, and thus had an increased risk of strike or entanglement. That information enabled them to establish a ship strike reduction rule in 2008 which required ships to slow their speed in areas of overlap. In 2009, they also shifted shipping lanes in and out of Boston Harbor to avoid areas of high whale density. By understanding space use of animals, these efforts resulted in reduced ship strikes, and enabled managers to facilitate commerce while also protecting whales
Juvenile Steller Sea Lions
As we’ve talked about in many blogs here on 60⁰N, Steller sea lions are endangered in parts of their range in Alaska. While many of our studies are investigating adult female physiology, behavior, and breeding, researchers have also been interested in understanding juvenile survival and risk of predation. Questions include:
“How many juveniles die due to predation?”
“Where is predation more likely?”
“Does predation risk influence animal movement?”
“What predators are targeting juveniles in this region?”
One way to think about these questions is to treat them like the case of the whales and ships—“where are juveniles and predators overlapping in space?” To answer that, we first needed to know where juveniles spent their time and to identify what areas are biologically important.
So, we developed what are called “utilization distributions”— or the probability of an animal using a particular space. If the exhibited concentrated use in a certain space, relative to their total wanderings, then we considered that to be ‘core’ space.
As following animals in the marine environment can be tricky at best, we utilized satellite tracking data collected by the Alaska SeaLife Center and the Alaska Department of Fish and Game to quantify the movements of 84 juveniles in the Gulf of Alaska from 2002-2014 and generate the utilization distributions. (For more information on tracking and satellite tags check out this previous blog: Animals on the Move).
What did we find?
- Juvenile Steller sea lions have core space use within their home range. Most of the core spaces were around haulouts are would be expected for an animal that spends part of its time on land, but some core spaces were identified at sea as well like in the glacial fjords (e.g. Northwestern) or narrow passages (e.g. Culross Pass).
- Males tended to have larger home ranges and core spaces than females. It is fairly common for males to have larger space use and to be more dispersive than females. This pattern may be driven by males having greater energy requirements and experiencing greater competition for mates later in life—especially for sexually size dimorphic species like the Steller sea lion: where males are considerably larger than females.
- There were no differences in size of space use for animals from Prince William Sound compared to Kenai Fjords; however, animals in Kenai Fjords exhibited seasonal differences in their space use—the size of their core space and distance of core space to a haulout were both larger in winter than in summer. Both regions are part of the endangered Western Distinct Population Segment of the Steller sea lion, but we wanted to see if there were local differences in animal movements. In the Kenai Fjords region, haulouts are on average slightly closer together—which may facilitate animals moving to respond to seasonal shifts in prey resources better than in Prince William Sound. Further exploration of how the habitat, prey and environmental factors shape movements will help us understand this pattern.
This study has provided the first look into the population and individual utilization distributions for this vulnerable age class of Steller sea lions in the eastern Gulf of Alaska. These findings will allow for future assessments of ecological dynamics, such as predator-prey interactions, and inform the management of a listed population.
Written by: Amy Bishop