Differential responses of seabirds to environmental variability over 2 years in the continental shelf and oceanic habitats of southeastern Bering Sea

Abstract

Abstract. Seasonal sea-ice cover has been decreasing in the southeastern Bering Sea shelf, which might affect ecosystem dynamics and availability of food resources to marine top predators breeding in the region. In this study, we investigated the foraging responses of two seabird species, surface-foraging red-legged kittiwakes Rissa brevirostris (hereafter, RLKI) and pursuit-diving foraging thick-billed murres Uria lomvia (TBMU) to different marine environmental conditions over 2 years. At-sea distributions of RLKI and TBMU breeding on St. George Island, the largest seabird colony in the region, were recorded using GPS loggers, and blood samples were taken to examine their physiological condition and isotopic foraging niche in a given year. Between the study years, winter ice retreated earlier and summer water temperatures were relatively warmer in 2014 compared to those in 2013. RLKI foraging occurred mostly over the oceanic basin in both years. TBMU, however, foraged mostly over the shelf but showed a relatively higher use of the shelf break and oceanic basin in 2013. The foraging distances from the colony peaked at 250–300 km in 2013 and bimodally at 150–250 and 300–350 km in 2014 for RLKI and tended to be farther in 2013 compared to those in 2014 for TBMU. Plasma levels of corticosterone did not differ between the years in RLKI but differed in TBMU, showing higher levels of physiological stress incurred by murres in 2013, the year of relatively cooler sea surface temperatures with later sea-ice retreat. δ13N (a proxy of trophic level of prey) did not differ between the years in either RLKI or TBMU. These results suggest that the response of ecosystem dynamics to climate variability in the southeastern Bering Sea may differ between the ocean basin and continental shelf regions, which, in turn, may generate differential responses in seabirds relying on those habitats for foraging.

Taking a trip to the shelf: Behavioral decisions are mediated by the proximity to foraging habitats in the black‐legged kittiwake

“…Flexibility in foraging behavior is pivotal to buffer spatial and temporal variations in food availability and abundance (Pettex et al., ). Hence, to implement proper long‐term conservation measures intended to protect marine top predators, it is crucial to understand their foraging flexibility and how this is related to the oceanographic (e.g., Daunt et al., ; Pettex et al., ; Votier et al., ) and environmental conditions (Lescroël et al., ; Lewis, Phillips, Burthe, Wanless, & Daunt, ; Yamamoto et al., ) that surround them.…”

Timing of sea-ice retreat affects the distribution of seabirds and their prey in the southeastern Bering Sea

“…In the southeastern Bering Sea, interannual variability in the availability of both large, lipid-rich zooplankton and age-0 pollock affects the productivity and physiology of seabirds nesting on the Pribilof Islands. Evidence is accumulating that in years with early sea-ice retreat and warm water, black-legged kittiwakes have lower levels of stress hormones, which have been associated with higher reproductive performance, than they do in years characterized by cold water (Satterthwaite et al 2012, Yamamoto et al 2016. There is also evidence that some seabird species nesting on the Pribilof Islands shift the region in which they forage between years with early and late sea-ice retreat.…”

Foraging segregation of two congeneric diving seabird species breeding on St. George Island, Bering Sea

“…We used a regression between time spent in flight (h) and maximum distance from the colony (km) during foraging trips, obtained from GPS-tracked TBMUs with time-depth recorders attached to their leg (n = 17 foraging trips; maximum distance from the colony (km) = 27.284 (regression coefficient) × total flight duration (h); R 2 = 0.787). The GPS-tracked birds did not carry accelerometers, the GPS data were collected concurrently to this study, and the detailed results are reported in Yamamoto et al (2016).…”

“…First, we did not sample the sources in the study year, and so used source samples caught in 2009 instead (methods of SI analyses of prey previously reported in Barger and Kitaysky, 2012). Second, there were no available source samples of age-1 walleye pollock (Gadus chalcogrammus) within 100 km from the colony, a distance in which birds are more likely to forage (Yamamoto et al, 2016). Because both murres are known to deliver walleye pollock to their offspring (and thus may consume them as well) we used data from outside the 100 km range (133 to 161 km distant, n = 6 source samples, located on the shelf, northwest of the study colony).…”

“…The maximum distance from the colony during foraging trips estimated by total flight duration was 42.6 ± 21.1 km (ranging 12.8-81.2 km) for COMUs and 38.1 ± 21.9 km (ranging 4.4-76.4 km) for TBMUs, respectively. With these small foraging ranges, both COMUs and TBMUs probably foraged on the continental shelf (bottom depth < 200 m: Yamamoto et al, 2016).…”