New research details northern fur seal foraging patterns along ocean fronts

Each summer, the female northern fur seals on Alaska’s remote Pribilof Islands go hunting. Leaving the safety of the land for days at a time, they must find enough food to give them the energy to continue nursing their newborn pups. If they are lucky, they’ll find a large patch of prey in one place. If not, they’ll graze along the way until their instincts call them home.

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Pups and adult female northern fur seals look on as a bull makes threatening calls.

What causes this pattern of behavior? Where do northern fur seals go when they are at sea, and how do they know where to find food?

These questions are at the core of two new studies led by Chad Nordstrom (UBC Marine Mammal Research Unit) and published in Deep-Sea Research II.

Working with 87 lactating female fur seals fitted with high-tech tracking tags, Nordstrom analyzed thousands of dive profiles logged over dozens of foraging trips, some of which lasted over one week. The data from the tags gave him detailed insights on how far and how deep the seals travel, and where they stop to eat.

Animation showing two northern fur seals originating from St. Paul Island foraging along a submesoscale surface front over the central shelf-break and basin of the eastern Bering Sea. Individual fur seals are represented by colored lines. Fronts are depicted as dark bands and are 4-day snapshots; and time of day is shown at the bottom in GMT.

“I wanted to know why the fur seals were foraging in those locations,” Nordstrom says. “In order to answer that question I needed to collect more advanced oceanographic data, and I was lucky enough to collaborate with a group of researchers who were doing just that.”

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Fur seal researchers (from left to right) John Gibbens, Chad Nordstrom, Andrew Trites, and Al Baylis

Fine-scale Ocean Fronts
Nordstrom joined the Patch Dynamics Team of the Bering Sea Integrated Ecosystem Research Program, a team of oceanographers and biologists who were mapping the intricate interplay between the physical ocean and the life within it. The collaboration gave Nordstrom a chance to compare the oceanographic data from his tags—produced by foraging fur seals—with measurements taken aboard a ship.

“It’s a little like tracing the outline of a picture and then coloring it in,” Nordstrom explains. “When environmental variables like ocean temperatures are collected by a ship or a satellite, it paints a broad picture that misses the

A daily diary tag carried by northern fur seals to record water temperature, depth, light levels, acceleration, and directional movement as frequently as 16 times per second.

nitty-gritty details. By tracking tagged animals as they move around and collect data about their environment, we could fill in the information gaps by looking at the data in finer detail than anyone had before.” 

Some of Nordstrom’s tagged fur seals would linger in particular spots and then head home directly. Others would run a predictable circuit of the ocean. But the patterns were inconsistent, and Nordstrom’s data couldn’t explain this behavior—until he began to overlay fine-scale maps of ocean currents generated by an oceanographer.

“That’s when everything started to make sense,” Nordstrom says. “It turns out the seals were eating along narrow ribbons of current, which acted like an invisible fence to concentrate prey in one place.”

“These small-scale anomalies—ocean fronts—are like currents within a current,” Nordstrom explains, “which channel zooplankton into corridors. A concentration of zooplankton at the face of an ocean front attracts fish and squid, which in turn attract larger predators like fur seals.”

Water temperatures (ranging from 0-10C) of the eastern Bering Sea at 1m (left panel) and 50m (right panel). Data were collected from ships and northern fur seals in 2009. Anomalously cold and warm eddies are circled in the right panel.

Location Matters
Nordstrom’s studies compared two cohorts of fur seals—one on St. Paul Island and one on Bogoslof Island. He was surprised to find that fur seals from St. Paul Island ventured three times farther and stayed at sea twice as long as fur seals from Bogoslof Island. This suggests that prey may be less concentrated near St. Paul Island, requiring lengthier foraging trips.The take-home message, Nordstrom says, is that location matters for northern fur seals.

“Female northern fur seals are tied to one rookery when they are raising their pups,” Nordstrom says. “So the availability of high-quality prey around them has a huge impact on their choices when foraging. The availability and proximity of prey to nursing females, which is largely determined by the location of ocean fronts, could determine the fate of an entire population.”

PublicationsPUBLICATIONS

Foraging habitats of lactating northern fur seals are structured by thermocline depths and submesoscale fronts in the eastern Bering Sea.
Nordstrom, C. A., B.C. Battaile, C. Cotté and A. W. Trites. 2013.
In Deep-Sea Research II: Topical Studies in Oceanography.  88-89:78-96.
abstract
The relationships between fine-scale oceanographic features, prey aggregations, and the foraging behavior of top predators are poorly understood. We investigated whether foraging patterns of lactating northern fur seals (Callorhinus ursinus) from two breeding colonies located in different oceanographic domains of the eastern Bering Sea (St. Paul Island˜shelf; Bogoslof Island˜oceanic) were a function of submesoscale oceanographic features. We tested this by tracking 87 lactating fur seals instrumented with bio-logging tags (44 St. Paul Island, 43 Bogoslof Island) during JulyˆSeptember, 2009. We identified probable foraging hotspots using first-passage time analysis and statistically linked individual areas of high-use to fine-scale oceanographic features using mixed-effects Cox-proportional hazard models. We found no overlap in foraging areas used by fur seals from the two islands, but a difference in the duration of their foraging trips˜trips from St. Paul Island were twice as long (7.9 d average) and covered 3-times the distance (600 km average) compared to trips from Bogoslof Island. St. Paul fur seals also foraged at twice the scale (mean radius = 12 km) of Bogoslof fur seals (6 km), which suggests that prey were more diffuse near St. Paul Island than prey near Bogoslof Island. Comparing first passage times with oceanographic covariates revealed that foraging hotspots were linked to thermocline depth and occurred near submesoscale surface fronts (eddies and filaments). St. Paul fur seals that mixed epipelagic (night) and benthic (day) dives primarily foraged on-shelf in areas with deeper thermoclines that may have concentrated prey closer to the ocean floor, while strictly epipelagic (night) foragers tended to use waters with shallower thermoclines that may have aggregated prey closer to the surface. Fur seals from Bogoslof Island foraged almost exclusively over the Bering Sea basin and appeared to hunt intensively along submesoscale fronts that may have converged prey within narrow bands near the surface. Bogoslof fur seals also foraged closer to their island which was surrounded by strong surface fronts, while fur seals from St. Paul Island traveled4100 km and extended some trips off-shelf to the basin to forage at similar oceanographic features. The relative distribution and accessibility of prey-concentrating oceano- graphic features can account for the observed inter-island foraging patterns, which may in turn have population level consequences for the two fur seal colonies.

keywords     Habitat selection, First-passage time, Submesoscale features, Finite-size Lyapunov exponent,Cox proportional hazard model, Alaska, Eastern Bering Sea
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Northern fur seals augment ship-derived ocean temperatures with higher temporal and spatial resolution data in the eastern Bering Sea.
Nordstrom, C.A., K. J. Benoit-Bird, B.C. Battaile and A.W. Trites. 2013.
Deep Sea Research II 94:257-273.
abstract
Oceanographic data collected by marine vertebrates are increasingly being used in biological and physical studies under the assumption that data recorded by free-ranging animals are comparable to those from traditional vertical sampling. We tested this premise by comparing the water temperatures measured during a 2009 oceanographic cruise with those measured during 82 foraging trips by instrumented northern fur seals (Callorhinus ursinus) in the eastern Bering Sea. The animal-borne data loggers were equipped with a fast-response temperature sensor and recorded 6,492 vertical profiles to depths ≥ 50 m during long distance (up to 600 km) foraging trips. Concurrent sampling during the oceanographic cruise collected 247 CTD casts in the same 5-week period. Average temperature differences between ship casts and seal dives (0.60 ± 0.61 °C), when the two were within 1 day and 10 km of each other (n = 32 stations), were comparable to mean differences between adjacent 10 km ship casts (0.46 ± 0.44 °C). Isosurfaces were evaluated at region wide scales at depths of 1 m and 50 m while the entire upper 100 m of the water column was analyzed at finer-scales in highly sampled areas. Similar trends were noted in the temperature fields produced by ships or seals despite the differences in sampling frequency and distribution. However, the fur seal dataset was of higher temporal and spatial resolution and was thereby able to visualize finer-detail with less error than ship-derived data, particularly in dynamic areas. Integrating the ship and seal datasets provided temperature maps with an unprecedented combination of resolution and coverage allowing fine-scale processes on-shelf and over the basin to be described simultaneously. Fur seals (n = 65 trips) also collected 4,700 additional profiles post ship cruise which allowed ≥1 °C warming of the upper 100 m to be documented through mid-September, including regions where ship sampling has traditionally been sparse. Our data show that hydrographic information collected by wide-ranging, diving animals such as fur seals can contribute physical data comparable to, or exceeding those, of traditional sampling methods at regional or finer scales when the questions of interest coincide with the ecology of the species.
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