VIDEO CONNECT

Follow female northern fur seals as they leave Bogoslof Island in the southern Bering Sea and forage along ocean fronts. Videos such as these help scientists understand feeding behavior of seals and how they interact with fine-scale oceanography.

Click on map to start animation of fur seals foraging.

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This animation helps visualize the interaction between fur seal movements and the immediate physical ocean conditions. It is an example of northern fur seals originating from Bogoslov Island and foraging over the southern basin of the eastern Bering Sea. Ocean fronts are depicted as black/grey bands and are 4-day snapshots of a dynamic system (time at the bottom is in GMT). Individual fur seals leaving the island to forage are represented by coloured lines. There are yellow points along the fur seal tracks at regular intervals, and at each point there is a ‘halo’ representing how much time they spent in that general area (larger halos equal greater residence time). Such visualization software allows scientists to better understand the complicated interactions between animals and their environment.

Influences of fine-scale oceanography on northern fur seal foraging

The world's largest population of northern fur seals breeds on St. Paul Island, Alaska and has been declining by ~6% a year for the past 15 years.  The decline is even more mysterious since a small population 400 km south on Bogoslof Island has been increasing over the same time period.  How can animals at one location thrive while the other wanes? The answer may lie in how far female fur seals need to forage to raise a pup that will be strong enough to survive and ultimately breed.

Chad Nordstrom (UBC) is studying how oceanographic conditions shape fur seal foraging trips from these two rookeries with different population trajectories in the Bering Sea. Specifically, Chad's study examined how foraging trips differed in relation to fine-scale physical oceanographic conditions for each population. Field crews tagged 90 lactating fur seals in the summer of 2009 with instruments that recorded GPS locations at the surface as well as depth and water temperature during dives. This was compared to additional data provided by ships sampling the physical properties of the water column and satellites that mapped the ocean’s surface currents.

Chad wanted to know whether fur seal foraging was targeting surface fronts (areas where physical conditions change rapidly) and thermoclines (dramatic changes in temperature at depth). Both of these features are thought to make their prey denser and easier for fur seals to capture.

Chad found no overlap in foraging areas between islands, but a difference in foraging duration-St. Paul trips were twice as long (7 d average) and covered 3-times greater distance (600 km average) compared to trips from Bogoslof. The fur seals used a combination of thermoclines and fronts when foraging.

Thermoclines (vertical barriers) were prevalent around both islands. However, fronts (horizontal barriers) were everywhere around Bogoslof Island, but over 100 km from St. Paul. This difference in oceanography may explain why Bogoslof fur seals primarily targeted fronts while St. Paul fur seals primarily used thermoclines during foraging.

Chad suspects that surface fronts are more effective at concentrating prey. He further thinks that the differing foraging patterns observed for seals from each island is tied to the unequal distribution of fronts, which in turn may ultimately affect population trends.

Chad Nordstrom is a Masters student at U.B.C.