Oyster population biomass variance estimates for the maryland portion of chesapeake bay 1994 2006
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Oyster Population/Biomass/Variance Estimates for the Maryland Portion of Chesapeake Bay 1994 – 2006. Linda Barker Maryland DNR Fisheries Service November 29, 2007 AFS Conference. Chesapeake Bay 2000 Agreement. Baseline: 1994 level of abundance Goal: 10-fold increase by 2010

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Oyster population biomass variance estimates for the maryland portion of chesapeake bay 1994 2006

Oyster Population/Biomass/Variance Estimates for theMaryland Portion of Chesapeake Bay 1994 – 2006

Linda Barker

Maryland DNR Fisheries Service

November 29, 2007

AFS Conference


Chesapeake bay 2000 agreement
Chesapeake Bay 2000 Agreement

  • Baseline: 1994 level of abundance

  • Goal: 10-fold increase by 2010

  • Indicator: Annual biomass estimate of small & market size oysters


Indicator timeline
Indicator Timeline

  • Developed in: 2000

  • Original estimates: 1994-2000

  • Updated: 2001, 2002

  • Documented: 2007

  • Variance estimate: 2007

  • 2007 update: 1994-2006 w/variance


Methods population biomass calculations
Methods: Population/Biomass Calculations

Area x Density = Oysters

Oysters x Weight = Biomass


Methods area
Methods: Area

Spatial BasisTemporal Basis

8 basins values constant 1994-2006

Habitat Surveys

Yates (1906-1911)

MBBS (1976-1983)

Md DNR COL (1999-2000)

High Quality HabitatLow Quality Habitat

71% decline from MBBS 73% decline from MBBS

91% decline from Yates 89% decline from Yates

Shell plantings < 5 years old

60 to 1,800 acres/basin 1,000 to 12,000 acres/basin


Methods density
Methods: Density

On High Quality Habitat

From DNR Fall Survey (annual values)

  • Observed value: oysters/bushel

  • Transformed to: oysters/acre ( ~750x )

    Hatchery seed plantings not included

    On Low Quality Habitat

    Basis is undocumented

    Value changed in 2002 (to reflect drought?)

  • 1994-2001 = 2.02 oysters/m2

  • 2002-2006 = 0.36 oysters/m2


Methods biomass
Methods: Biomass

Biomass

Biomass = Population x Weight (by size class)

Total = sum for all size classes/basins

Population by Size Group

For 5-mm size classes

Total population x rel. abundance

From Md DNR Fall Survey

Conversion to Biomass

Jordan et al., 2002

log weight = -3.8 + 2.1 * log size


Methods variance
Methods: Variance

For Population on HQ Habitat

Density variance (oysters/bu)2 x (730 bu/ac)2 x (acres) 2

For Population on LQ Habitat

none calculated


Results ope time series as an unintelligible table
Results: OPE Time Series(as an unintelligible table)




Conclusions
Conclusions

  • OPEs based on many critical assumptions

    • many contain error.

  • The data are not the problem

    • values at a very small spatial scale are inflated to create values at a much greater spatial scale

  • High variance

    • even so, these are underestimates!

    • insufficient precision to show change over time.


Recommendation
Recommendation

  • The use of an absolute estimate of abundance that has sufficient precision to show real trends in the bay-wide oyster population will require entirely different stock assessment methods,

    at orders of magnitude more cost.

  • A relative index of abundance based on observed densities may be useful.


Many thanks to kelly greenhawk dnr fisheries col
MANY THANKS to Kelly GreenhawkDNR Fisheries, COL


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