Map of study area from 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 <i>Calanus marshallae/glacialis</i> were 2.4 and 18.1 times less abundant in early-ice-retreat-years, respectively, whereas age-0 walleye pollock <i>Gadus chalcogrammus</i> 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

The distribution of juvenile walleye pollock in 2009 based on three different metrics.

<p>A. biomass density, the most commonly used measure, B. the mean volumetric density of pollock within aggregations, a measure of local density within a patch, and C. the maximum volumetric density of pollock per sampling transect. Map surfaces were generated using minimum curvature interpolations (N = 165).</p

Predicted and observed predator habitat use in 2009.

<p>A. The predicted density classes for each predator species using the full multiple-regression model based on transect data and B. the kernel densities for tagged individual predators at each sampled transect. C. The difference between the model category and the kernel category. Positive, cool colored values indicate that fewer predators used an area than predicted by the model while negative, warm colored values indicate the opposite. On each plot, the center of each transect that was visually surveyed for birds and mammals and thus was used to create the regression model is shown with a +. The center of each transect for which environmental and prey data were available but could not be used to create the regression model is shown by o. Map surfaces were generated using minimum curvature interpolation that did not allow values plotted at sampled points to differ from their actual values.</p

Summary of best subsets multiple regression models for densities each of three focal predators visually surveyed in the Southeastern Bering Sea in 2008 and 2009.

<p>Explanatory variables are listed in descending order of importance for each species’ model. The slope of the relationship for each explanatory variable is shown along with its regression coefficient. The R² for each model adjusted for the number of variables in the model is also shown.</p

Predicted and observed predator densities.

<p>The observed density of each predator versus the density predicted by the full multiple regression model for each species.</p