1. 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; mike.litzow@noaa.gov * Current address: Sigma Plus, Fairbanks; fmueter@alaska.net Franz Mueter 1 * and Mike Litzow 2

2. Biological effects of retreating sea ice

3. Winter sea ice drives summer bottom temperatures Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 0 20 40 60 0 20 40 60 Depth (m) High-ice year (1997) Low-ice year (1998) Redrawn from Stabeno et al. 2001. Fisheries Oceanography 10:81-98 -2 0 2 4 6 8 10 12 14 °C

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

5. Objectives Use NMFS bottom trawl survey (1982-2006) to describe community response to warming Identify outstanding questions for predicting response to future warming

6. Northward shift in biomass Change in total CPUE 1982-1986 to 2002-2006 -2-2 0 1 2 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

7. Shifts in North-South gradients, averaged among 51 taxa CPUE-where-present Probability of occurrence 19851990199520002005 -0.20 -0.10 0.00 0.10 19851990199520002005 -0.03 -0.01 0.01 0.03 Latitudinal gradient North South P < 0.001 P = 0.0095 Year

8. Shifts in center of distribution for 45 taxa, 1982-2006 Snow crab Pollock Rock sole Halibut Mean shift = 31 km One-sample t = 3.50 P = 0.0005 Rate similar to North Sea (Perry et al. 2005) 2-3 times faster than terrestrial mean (Parmesan and Yohe 2003)

9. Retreat of the cold pool 1982-1986 0 2 4 6 8 Summer bottom temperature (°C) °N 55 57 59 61 °W 175 170 165 160

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

11. Changing community structure in the cold pool 0.30 0.25 0.20 0.15 0.05 1985 1990 1995 2000 2005 Arctic CPUE:subarctic CPUE Ratio Arctic:subarctic biomass Year 0.10 P < 0.01

12. Changing community structure in the cold pool 16 15 14 13 12 1985 1990 1995 2000 2005 Species / haul Diversity (species richness) Year P < 0.01

13. Changing community structure in the cold pool 3.76 3.74 3.72 3.70 3.66 1985 1990 1995 2000 2005 Trophic level Mean trophic level Year 3.68 P < 0.01

14. Climate – biogeography links

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

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

17. Climate – commercial fisheries links 0 4 8 12 16 Catch (10 4 t) Ice cover index (3-yr running mean) Commercial snow crab catch, 1982-2005 R 2 = 0.59

18. Climate – commercial fisheries links Trophic level Ice cover index (3-yr running mean) Mean trophic level of total commercial catch, 1982-2004 R 2 = 0.36

19. 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?

20. 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) 0.80 0.83 0.12 0.67 Change in center of distribution for 45 taxa, 1982-2006

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

22. 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

23. 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