Warming climate alters the biogeography of the southeast Bering Sea 1 Joint Institute for the Study of the Atmosphere and the Oceans, University of Washington 2 Alaska Fisheries Science Center, Kodiak; * Current address: Sigma Plus, Fairbanks; Franz Mueter 1 * and Mike Litzow 2

Biological effects of retreating sea ice

Winter sea ice drives summer bottom temperatures Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Depth (m) High-ice year (1997) Low-ice year (1998) Redrawn from Stabeno et al Fisheries Oceanography 10: °C

Summer bottom temperature P = 0.04 The problem – persistent climate forcing Ice cover index Winter ice extent P = 0.01

Objectives Use NMFS bottom trawl survey ( ) to describe community response to warming Identify outstanding questions for predicting response to future warming

Northward shift in biomass Change in total CPUE to Change in biomass (CPUE 0.25, tons / km 2 ) °W °N 51 taxa consistently identified through time series: 9 Arctic taxa / 42 subarctic taxa, 10 crustaceans / 41 fishes

Shifts in North-South gradients, averaged among 51 taxa CPUE-where-present Probability of occurrence Latitudinal gradient North South P < P = Year

Shifts in center of distribution for 45 taxa, Snow crab Pollock Rock sole Halibut Mean shift = 31 km One-sample t = 3.50 P = Rate similar to North Sea (Perry et al. 2005) 2-3 times faster than terrestrial mean (Parmesan and Yohe 2003)

Retreat of the cold pool Summer bottom temperature (°C) °N °W

Predictions: Increased subarctic:Arctic community biomass Increased diversity Increased average trophic level Retreat of the cold pool

Changing community structure in the cold pool Arctic CPUE:subarctic CPUE Ratio Arctic:subarctic biomass Year 0.10 P < 0.01

Changing community structure in the cold pool Species / haul Diversity (species richness) Year P < 0.01

Changing community structure in the cold pool Trophic level Mean trophic level Year 3.68 P < 0.01

Climate – biogeography links

Mean bottom temperature (° C) Arctic and subarctic biomass in cold pool area R 2 = 0.70 R 2 = 0.08 Subarctic taxa Arctic taxa

Climate – biogeography links Mean bottom temperature (° C) Mean trophic level of survey catches in cold pool area R 2 = 0.39

Climate – commercial fisheries links Catch (10 4 t) Ice cover index (3-yr running mean) Commercial snow crab catch, R 2 = 0.59

Climate – commercial fisheries links Trophic level Ice cover index (3-yr running mean) Mean trophic level of total commercial catch, R 2 = 0.36

Even after temperature effects removed, significant northward displacement remains Center of distribution averaged over 45 taxa What we don’t know, #1: what else besides direct temperature effects is driving distribution shifts?

What we don’t know, #2: what explains variability in distribution shifts? General Linear Model to explain variability among taxa: EffectP Commercial status (fished vs. non-fished) Temperature preference (from survey) Trophic level Maximum length (for fish) Change in center of distribution for 45 taxa,

Conclusions Warming of the Bering Sea since 1982: Community-wide northward shift Community reorganization in cold pool area Change in fisheries

Conclusions Continued warming: Loss of Arctic species, increase in subarctic species Variability in shifts = potential for new community state Need to understand: How biotic interactions constrain response to warming Emergent effects

Acknowledgements Everyone who has participated in the annual Bering Sea survey for the last 25+ years Claire Armistead, Bern Megrey and Jeff Napp for their assistance