October 24, 2017
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.