We define “patches” as significant spatial variation in any feature of prey that is important for exploitation by predators.
Prey patches may occur at scales of <1 meter to several kilometers, and may last anywhere from minutes to months.
Patches also vary in species composition, biomass, energy content of prey, and distribution (size of patch, density within a patch, density of patches, and distance from colony/rookery).
We don't know how top predators respond to variability in prey patches (patch dynamics) and the consequence this has on population dynamics of top predators in the Bering Sea.
We need this fundamental information to predict how the Bering Sea ecosystem will respond to global warming.
The M/V Tiglax (TEKH-lah, Aleut for eagle) provides critical support for biological work and management programs on the far-flung lands of the Alaska Maritime National Wildlife Refuge.
The Tiglax is the research and transportation support vessel for the Refuge. In a season, Tiglax may sail to islands in Southeast Alaska, to the far western end of the Aleutian Chain, and into the Bering Sea, typically traveling 15,000 to 20,000 nautical miles.
The BEST-BSIERP team appreciated the opportunity to travel on Tiglax out to Bogoslof Island. (Tiglax photo by Carrie Eischens)
The Pribilof Islands, rocky outposts in the central Bering Sea more than three hundred miles west of mainland Alaska, are a spectacular collection of steep cliffs, winding beaches, verdant hills, and wetlands.
About 700 people live here year-round, plus animal species found nowhere else on earth, such as the Pribilof Arctic fox (Alopex lagopus pribilofensis) and the Pribilof Arctic shrew (Sorex pribilofensis).
These islands are also home for 12 species of seabirds and a host of marine mammals, such as Steller sea lions (Eumetopias jubatus), northern fur seals (Callorhinus ursinus, and harbor seals (Phoca vitulina), as well as several species of dolphins, porpoises, and whales.
More than 2.5 million seabirds travel to the Pribilofs each summer to hatch and raise their young. Although many species of seabirds flock to each island, most prefer St. George Island because it has eight times more cliff area than St. Paul.
Most numerous -- and the focal points of BEST-BSIERP patch dynamics studies in the Pribilofs -- are thick-billed murres and black-legged kittiwakes.
Murres (above, Carrie Eischens) and kittiwakes (right, Francis Wiese) were chosen for study because they are numerous and use different foraging strategies: kittiwakes feed on the sea surface, but murres dive as deep as 95 m in their search for food. Their diets include fish, krill, squid, and euphausiids.
Understanding more about where and what these birds are eating will help reveal more about so-called “prey patches” and how the process of finding enough to eat affects the reproduction and overall health of these seabird populations.
Seabird work has been taking place at Zapadni and Tolstoi Points on St. Paul Island, and on the rugged cliffs that ring the edges of St. George Island.
In July, NPRB Science Director Francis Wiese (left) and BSIERP Lead Principal Investigator Mike Sigler (right) joined researcher Vernon Byrd (center) for a few days of "bird immersion" and site scouting at St. George Island in the Pribilofs. (Auklets in flight, Francis Wiese; group picture, Nikolai Konyukhov)
Researchers want to know where kittiwakes go and the amount of time they spend on the water versus the time they spend flying. On St. Paul Island, they captured 21 birds and fitted them with GPS trackers, as well as wet/dry sensors, which recorded flying and floating periods.
All but one were recaptured and sampled for stress hormone levels; researchers then sent the tracking data to others aboard the F/V Frosti, who were searching for forage fish so they could guide crews to where the kittiwakes were feeding.
Early data have revealed that kittiwakes were traveling 100-200 km offshore -- much farther than expected -- to find food.
See a map of Patch Dynamics at Sea, as sampled by the Frosti, including vessel transects and the locations of tagged kittiwakes and murres. Each transect was comprised of acoustic sampling at four frequencies, bird and mammal observations, a net tow for fish, and 2 CTD casts. Much of the ship track also has underway acoustics and bird and mammal observations.
Right: The St. Paul field team. Back row, left to right: Chad Nordstrom, Rachael Orben, John Warzybok, Brian Battaile, Sarah Youngren. Front row, left to right: Ana Santos, BEST-BSIERP researcher Andrew Trites, Alexis Will, Ine Dorresteijn. (Carrie Eischens)
We also want to learn how murres make a living from the sea. Carefully plucking murres from their cliffside nests (left, Carrie Eischens) is challenging, but researchers can capture birds and fit them with GPS transmitters as well as TDRs (time-depth recorders) that log the amount of time the birds spent underwater and how deeply they dive. After two days they attempt to recapture the birds to learn where they've been.
Remarkably, researchers aboard the Frosti, which was nearby using hydroacoustic equipment to locate forage fish patches, reported “seeing” the birds dive into these patches, which were 75-90 m beneath the water’s surface!
Northern fur seals range across the North Pacific from the west coast of the US and across the Aleutian Chain to the waters off Russia and Japan. Fifty percent of the world northern fur seal population lives on the Pribilof Islands. Once lucratively harvested for their dense, waterproof underfur, fur seals have been protected from hunting since 1911, when the International North Pacific Fur Seal Treaty ended pelagic sealing. Right: Male and female fur seals.
At that time, Pribilof Islands seal numbers were severely depleted, down from an estimated two to five million animals at the time of Russian discovery, to just over 215,000. The Pribilof population now numbers about 800,000, but has been declining for several years for reasons not fully understood. The species has been considered Depleted since 1988 under the Marine Mammal Protection Act. The NOAA National Marine Fisheries Service conducts regular fur seal surveys to look at population trends.
Other northern fur seal work is taking place at Reef Point rookery on St. Paul. Females arrive on the island in June to birth and raise pups, and leave the island with their pups by November.
Right: Wooden catwalks constructed above the beach cobbles and grassy meadows allow observation of the seals without disturbing the animals or potentially being injured by the territorial and much larger males.
Below: The fur seals are fitted with GPS tags, TDRs, and “dead reckoning” devices that track the seals’ travel through different depths of water. Seals are recaptured when they return to the beach after foraging.
Only females are tagged because they will reliably return to the island to nurse their pup left on the beach.
Right: Brian Battaile listens for seals with a VHF transmitter to learn -- without entering the colony and potentially disturbing seals -- when the seals return to the beach. The goal is to instrument and recapture 25 seals, which should become easier to meet once the male fur seals leave the rookery later in the summer. (Bird and fur seal photos by Carrie Eischens except where noted)
From July 11 to August 9, six scientists cruised the Bering Sea aboard the F/V Frosti to study the food of fur seals and seabirds around the Pribilofs.
Nathan Jones (left), a graduate student from Moss Landing Marine Laboratory working with BEST-BSIERP researcher Kathy Kuletz, watched birds -- he tallied the numbers of each species, observed their behavior, and collected their gut contents for comparison with the direct sampling.
BEST-BSIERP researcher Scott Heppell led the team in sampling the food available to seabirds and fur seals.
Right: Research Assistant Chad Waluk works with the Frosti’s crew to collect the last of a sample from the ship’s deck. Much of the red trawl net is spooled on the reel in the foreground with the remainder stretching out along the back deck of the 120-foot vessel.
We use acoustics to locate krill in the water column, and to “see” the size of the patch.
At the same time, BEST-BSIERP researcher Kelly Benoit-Bird used a multi-frequency scientific echosounder -- a fancy sonar -- to let us “see” prey beneath the water’s surface.
The image at right shows what we observed. Each panel shows a different frequency, or researchertch of sound, with the lowest frequency in the upper left and the highest in the lower right. The seafloor appears as dark red, at a depth of about 135 m. A very dense patch of krill shows just above the bottom. We can tell because of the intense scattering, shown by the red and yellow in the high frequencies but weak scattering, in blue at the lower frequencies. See a larger version
Now knowing the depth of the krill patch, Scott Heppell then directed the trawl to confirm the contents of the patch.
At left, Scott shows off part of the catch. The 20-minute trawl from the patch shown in the acoustic image above yielded 200,000 krill! This patch also had lots of foraging murres and fin whales.
It was a very successful cruise. In 28 days, we collected data from thousands of linear survey miles in the 100 miles surrounding the Pribilof Islands. We traveled to random locations chosen before the trip, or to find the animals tagged by the land-based crews.
We sampled in places where we saw interesting patches of fish, or followed groups of birds observed from the ship to sample where they were feeding. Now that we’re back on land, we’ve got a lot of work left to do to understand all of the information we collected on the food and the predators.
The acoustics team, left to right: Graduate Student Greg Kowalke, Principal Investigator Kelly Benoit-Bird, Research Assistant Chad Waluk, Graduate Student Luke Whitman, and Principal Investigator Scott Heppell. (Acoustic survey photos by Kelly Benoit-Bird)
Bogoslof, a 173-acre, uninhabited island (right), is located about 50 miles west of Dutch Harbor. It has been designated a sanctuary since 1909.
Boglof is now part of the Alaska Maritime National Wildlife Refuge, and is also a National Natural Landmark. (photo courtesy Wikipedia)
In July, NPRB Assistant Program Manager Carrie Eischens joined BEST-BSIERP researcher Sasha Kitaysky and field team members Sergei Drovetski, Dave Gummerson, and Ann Harding on the Tiglax, the research ship of Alaska Maritime National Wildlife Refuge, for colony-based seabird and fur seal fieldwork at Bogoslof Island.
This reconaissance trip to Bogoslof went well, and coordinated sampling work between predator tagging and prey samplers. Prey sampling primarily covered kittiwake work, but mostly missed murre and fur seal, as expected; something to examine for next year. Above: Dave, Ann and Carrie. (Carrie Eischens)
This work is part of the Patch Dynamics Focal Area and involves a fine-scale predator-prey study on thick-billed murres, black-legged kittiwakes, northern fur seals, and forage fishes to examine controlling mechanisms that determine whether populations of top predators grow or decline.
Right: Sasha and Sergei. (Carrie Eischens)