Trends in walleye pollock and euphausiid abundance on the Bering Sea shelf since 2004

1. Trends in walleye pollock and euphausiid abundance on the Bering Sea shelf since 2004 Patrick H. Ressler, Alex De Robertis, and Christopher D. Wilson NOAA Fisheries-Alaska Fisheries Science Center Phyllis J. Stabeno NOAA OAR-Pacific Marine Environmental Lab

2. Outline • Acoustic surveys in the Bering Sea: walleye pollock (old), euphausiids (new) • Trends in pollock and euphausiid biomass since 2004 • Possible interpretations of those trends

3. Acoustic surveys in the eastern Bering Sea • Acoustic backscatter has been used to estimate the biomass of midwater walleye pollock in the eastern Bering Sea since 1979

4. Low frequency (38 kHz) Depth (m) Pollock Distance along trackline (nmi) Acoustic surveys in the eastern Bering Sea • Standard practice: 38 kHz backscatter data is visually examined and classified by trained analysts, while midwater trawling confirms species ID and provides length and age information 50 100 150

5. Acoustic surveys in the eastern Bering Sea • More recently, backscatter data at several frequencies and plankton trawls have been used to identify euphausiid aggregations.

6. Frequency response of acoustic backscatter can be used for objective classification Low frequency (38 kHz) High frequency (120 kHz) 50 50 Euphausiids Depth (m) Pollock 100 100 150 150 Distance along trackline (nmi)

7. Empirical classification scheme based on multi-frequency acoustic data and plankton trawl catches (De Robertis, McKelvey, and Ressler, AFSC, manuscript in prep.)

8. 38 kHz 120 kHz Euphausiids 200 kHz 18 kHz (De Robertis, McKelvey, and Ressler, AFSC, paper in prep.)

9. Acoustic surveys in the eastern Bering Sea • Using these methods, the distribution and abundance of midwater pollock can be compared with those of one of its most important prey.

10. Pollock Euphausiids 2008 Spatial information on pollock and euphausiids, 2004-2008

11. Ianelli et al.; http://www.afsc.noaa.gov/refm/stocks/assessments.htm Time series of pollock and euphausiids, 2004-2008

12. Interpretation of interannual patterns in pollock and euphausiid time series since 2004 • Independent responses to environmental forcing and other factors? • Euphausiids affect pollock: improved feeding conditions for age-1+ pollock in 2007 and 2008, relative to 2004? • Pollock affect euphausiids: euphausiid biomass has increased in part because of reduced predation from pollock?

13. Depth Averaged Temperature (M2) Depth Averaged Temperature Anomaly (M2) Bering Sea temperatures, 2004 - 2008

14. Other studies of large zooplankton suggest a temporal trend similar to euphausiid index • Coyle and Pinchuk, Hunt et al., 2008 , DSR II Vol. 55(16-7): in 2004, a warm year with early ice retreat, the abundance of euphausiids (Thysanoessa sp.) and large copepods (Calanus marshallae) was quite low • Some evidence for increase in the biomass of zooplankton on the middle and outer shelf since 2004 (Jeff Napp, AFSC, unpublished data)

15. Consumption of euphausiids by pollock • Food habits data for age-1+ pollock (Aydin and Buckley, AFSC, unpublished data) suggest that in 2004, pollock consumed much less euphausiid and copepod biomass than was typical in prior years. • Euphausiid consumption (% weight) by pollock increased in 2006 and 2007, but analysis of all stomach samples is not yet complete. • Could pollock consumption have a large effect upon the euphausiid standing stock? Maybe.

16. Summary • A new analysis of acoustic data allows us to index summertime euphausiid biomass in the Bering Sea since 2004. • Walleye pollock biomass has declined by half since 2004, while euphausiid biomass has apparently undergone a three-fold increase. • We are working on a quantitative estimate of euphausiid biomass and abundance (see also posters by Warren et al. and Smith et al.) • BEST-BSIERP data collection will continue in 2009 and 2010

17. Acknowledgments • MACE Program scientists • Skipper and crew of NOAA Ships Miller Freeman and Oscar Dyson • Funding: NPRB (BEST-BSIERP), AFSC Photo credit: Chris Wilson

19. 2007 2008 2004-2006 Future directions • Analysis of trawl samples • Target strength modeling to allow quantitative estimates of numerical abundance and biomass (see posters by Warren et al. and Smith et al.) • 2009, 2010 summer surveys will continue this work as part of BEST-BSIERP • Compare results and collaborate with other elements of BEST-BSIERP

20. Pollock age-1 recruitment time series Ianelli et al.; http://www.afsc.noaa.gov/refm/stocks/assessments.htm

21. Benefits of new method • Solid empirical basis • Four frequency (18, 38, 120, 200 kHz) classification based on all possible pairwise frequency differences • Performance metric (z-score) allows evaluation of results

22. (De Robertis, McKelvey, and Ressler, AFSC, paper in prep.) Empirical classification scheme based on multi-frequency acoustic data and trawl catches

23. Bering Sea temperatures and ice extent, 2004-2008 • Phyllis: temperature and ice extent in Bering Sea, 2004-2008

24. 2006 2008 2004 2007 Spatial information on pollock and euphausiids, 2004-2008 Pollock Euphausiids