Kathy J. Kuletz

Prey Patch Patterns Predict Habitat Use by Top Marine Predators with Diverse Foraging Strategies

Spatial coherence between predators and prey has rarely been observed in pelagic marine ecosystems. We used measures of the environment, prey abundance, prey quality, and prey distribution to explain the observed distributions of three co-occurring predator species breeding on islands in the southeastern Bering Sea: black-legged kittiwakes (Rissa tridactyla), thick-billed murres (Uria lomvia), and northern fur seals (Callorhinus ursinus). Predictions of statistical models were tested using movement patterns obtained from satellite-tracked individual animals. With the most commonly used measures to quantify prey distributions - areal biomass, density, and numerical abundance - we were unable to find a spatial relationship between predators and their prey. We instead found that habitat use by all three predators was predicted most strongly by prey patch characteristics such as depth and local density within spatial aggregations. Additional prey patch characteristics and physical habitat also contributed significantly to characterizing predator patterns. Our results indicate that the small-scale prey patch characteristics are critical to how predators perceive the quality of their food supply and the mechanisms they use to exploit it, regardless of time of day, sampling year, or source colony. The three focal predator species had different constraints and employed different foraging strategies – a shallow diver that makes trips of moderate distance (kittiwakes), a deep diver that makes trip of short distances (murres), and a deep diver that makes extensive trips (fur seals). However, all three were similarly linked by patchiness of prey rather than by the distribution of overall biomass. This supports the hypothesis that patchiness may be critical for understanding predator-prey relationships in pelagic marine systems more generally.

Active acoustic examination of the diving behavior of murres foraging on patchy prey

During the 2008 and 2009 breeding seasons of murres Uria spp., we combined visual observations of these predators with active acoustics (sonar), fish trawls, zooplankton net tows, and hydrographic measurements in the area surrounding breeding colonies in the southeastern Bering Sea. We acoustically detected thousands of bubble trails that were strongly correlated with the number of visually detected murres, providing a new tool for quantitatively studying the foraging ecology of diving birds. At the regional scale, the number of acoustically detected bubble trails, which served as a proxy for diving murre abundance, was related to the combined availability and vertical accessibility of squid, krill, and pollock. There were, however, no clear relationships at this scale between diving murres and any individual prey taxon, highlighting the importance of prey diversity to these animals. Individual krill patches targeted by murres had higher krill density and were located shallower than the mean depth of krill patches, but were similar in total krill abundance and overall size. The diving depth of murres within krill patches was highly correlated to the depth of the upper edge of these patches, whereas murres found outside of krill patches showed a depth distribution similar to that of juvenile pollock. Throughout the study area, murres showed strong diel patterns in their diving behavior in response to the diel migrations of their prey. These results suggest that murres select prey with specific patch characteristics implying effective information gathering about prey by murres. The high proportion of diving murres in aggregations and their consistent inter-individual spacing support the hypothesis that intraspecific local enhancement may facilitate foraging in these predators.

Marine Fishes, Birds and Mammals as Sentinels of Ecosystem Variability and Reorganization in the Pacific Arctic Region

Extreme reductions in sea ice extent and thickness in the Pacific Arctic Region (PAR) have become a hallmark of climate change over the past decade, but their impact on the marine ecosystem is poorly understood. As top predators, marine fishes, birds and mammals (collectively, upper trophic level species, or UTL) must adapt via biological responses to physical forcing and thereby become sentinels to ecosystem variability and reorganization. Although there have been no coordinated long-term studies of UTL species in the PAR, we provide a compilation of information for each taxa as an ecological foundation from which future investigations can benefit. Subsequently, we suggest a novel UTL-focused research framework focused on measurable responses of UTL species to environmental variability as one way to ascertain shifts in the PAR marine ecosystem. In the PAR, indigenous people rely on UTL species for subsistence and cultural foundation. As such, marine fishes, birds and mammals represent a fundamental link to local communities while simultaneously providing a nexus for science, policy, education and outreach for people living within and outside the PAR.

Timing of ice retreat alters seabird abundances and distributions in the southeast Bering Sea.

Timing of spring sea-ice retreat shapes the southeast Bering Sea food web. We compared summer seabird densities and average bathymetry depth distributions between years with early (typically warm) and late (typically cold) ice retreat. Averaged over all seabird species, densities in early-ice-retreat-years were 10.1% (95% CI: 1.1–47.9%) of that in late-ice-retreat-years. In early-ice-retreat-years, surface-foraging species had increased numbers over the middle shelf (50–150 m) and reduced numbers over the shelf slope (200–500 m). Pursuit-diving seabirds showed a less clear trend. Euphausiids and the copepod Calanus marshallae/glacialis were 2.4 and 18.1 times less abundant in early-ice-retreat-years, respectively, whereas age-0 walleye pollock Gadus chalcogrammus near-surface densities were 51× higher in early-ice-retreat-years. Our results suggest a mechanistic understanding of how present and future changes in sea-ice-retreat timing may affect top predators like seabirds in the southeastern Bering Sea.

A spatial–seasonal analysis of the oiling risk from shipping traffic to seabirds in the Aleutian Archipelago

Some of the largest seabird concentrations in the northern hemisphere are intersected by major shipping routes in the Aleutian Archipelago. Risk is the product of the probability and the severity incidents in an area. We build a seasonally explicit model of seabird distribution and combine the densities of seabirds with an oil vulnerability index. We use shipping density, as a proxy for the probability of oil spills from shipping accident (or the intensity chronic oil pollution). We find high-risk (above-average seabird and vessel density) areas around Unimak Pass, south of the Alaska Peninsula, near Buldir Island, and north of Attu Island. Risk to seabirds is greater during summer than during winter, but the month of peak risk (May/July) varies depending on how data is analyzed. The area around Unimak Pass stands out for being at high-risk year-round, whereas passes in the western Aleutians are at high risk mostly during summer.

Using Ships of Opportunity to Assess Winter Habitat Associations of Seabirds in Subarctic Coastal Alaska

Abstract In subarctic waters winter may be the period during which seabirds face the greatest environmental and physiological pressures, yet seabird distribution during this time is poorly understood. Using at-sea surveys conducted in Prince William Sound, Alaska on research ‘ships of opportunity’ from November 2007 to March 2009, we investigated how seabird abundance and distribution vary within and between winters for three common seabird species with extensive ranges: common murre (Uria aalge), marbled murrelet (Brachyramphus marmoratus), and black-legged kittiwake (Rissa tridactyla). Due to a large proportion of zeros in the survey data, hurdle models were performed using generalized additive mixed models. Across the two winters, consistent temporal patterns in density and distribution were observed for all species. Common murre and marbled murrelet both increased in number in midwinter, while black-legged kittiwake decreased to very low numbers. Habitat association models revealed that common murre favored relatively protected waters while marbled murrelet favored inside bays and passages (which make up 45% of semi-protected waters) and areas of higher sea surface temperatures. Our results suggest that winter storms influenced seabird distribution, particularly in midwinter when temperatures were lowest and storms more frequent. This influence was greater than variables providing proxies for foraging opportunities, which were absent from selected models. Our study highlights the importance of considering species-specific temporal patterns throughout the non-breeding season to guide marine spatial planning that will fully address seabird conservation issues.