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

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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

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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

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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

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• 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

Low frequency (38 kHz)

Distance along trackline (nmi)

Depth

(m

)

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100

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Pollock

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• More recently, backscatter data at several frequencies and plankton trawls have been used to identify euphausiid aggregations.

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Low frequency (38 kHz)

50

100

150

Distance along trackline (nmi)

Depth

(m

) Pollock

High frequency (120 kHz)

50

100

150

Euphausiids

Frequency response of acoustic backscatter can be used for objective classification

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Empirical classification scheme based on multi-frequency acoustic data and plankton trawl catches

(De Robertis, McKelvey, and Ressler, AFSC, manuscript in prep.)

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Euphausiids

(De Robertis, McKelvey, and Ressler, AFSC, paper in prep.)

120 kHz38 kHz

18 kHz 200 kHz

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• Using these methods, the distribution and abundance of midwater pollock can be compared with those of one of its most important prey.

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Spatial information on pollock and euphausiids, 2004-2008

2008

Pollock Euphausiids

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0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

2003 2004 2005 2006 2007 2008 2009

Year

no

rmal

ized

to

200

4

Euphausiid backscatter

Pollock backscatter

Time series of pollock and euphausiids, 2004-2008

Ianelli et al.; http://www.afsc.noaa.gov/refm/stocks/assessments.htm

0.00

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1.00

1.50

2.00

2.50

3.00

3.50

2003 2004 2005 2006 2007 2008 2009

Year

no

rmal

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to

200

4

Euphausiid backscatter

Pollock backscatter

Pollock biomass from stockassessment model

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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?

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Depth Averaged Temperature (M2)

Depth Averaged Temperature Anomaly (M2)

Bering Sea temperatures, 2004 - 2008

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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)

0

200

400

600

800

1000

1200

1400

1600

1800

2003 2004 2005 2006 2007 2008

Year

Zo

op

lan

kto

n b

iom

ass,

mg

m--

3

Outer shelf

Middle shelf

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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.

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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

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Acknowledgments

Photo credit: Chris Wilson

• MACE Program scientists• Skipper and crew of NOAA Ships Miller

Freeman and Oscar Dyson• Funding: NPRB (BEST-BSIERP), AFSC

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Future directions

• Analysis of trawl samples2004-2006 2007 2008

• 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

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Model estimated age-1 recruits

0

10,000

20,000

30,000

40,000

50,000

60,000

70,000

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

Year

Mil

lio

ns

of

age-

1 p

oll

ock

Ianelli et al.; http://www.afsc.noaa.gov/refm/stocks/assessments.htm

Pollock age-1 recruitment time series

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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

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Empirical classification scheme based on multi-frequency acoustic data and trawl catches

(De Robertis, McKelvey, and Ressler, AFSC, paper in prep.)

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Bering Sea temperatures and ice extent, 2004-2008

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

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2004200620072008

Spatial information on pollock and euphausiids, 2004-2008

Pollock Euphausiids

Documents

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