Blue Crab Abundance

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Chesapeake Bay Program Indicator Framework

Reporting Level Indicators

Indicator and Data Survey

A.  Category/Name/Source/Contact

(1) Category of Indicator

___ Factors Impacting Bay and Watershed Health

___ Restoration and Protection Efforts

___ Watershed Health

_x__ Bay Health

(2) Name of Indicator: Blue Crab Abundance (age 1 and older)

(3) Description of Dataset used to calculate percent of goal achieved:  Abundance of overwintering blue crabs 1 year of age and older in Chesapeake Bay

For what purpose(s) were the data collected? (e.g., tracking, research, or long-term monitoring.) Tracking progress towards a goal and long-term monitoring.

Which parameters were measured directly? Which were obtained by calculation? Classification of maturity obtained from field collection and abundance estimate determined from winter dredge survey (direct enumeration methodology).  The abundance of over wintering blue crabs in the Chesapeake Bay was estimated from the winter dredge survey (see Section 3.2.4 of the stock assessment and Sharov et al. 2003). It is assumed that the estimated mean density of blue crabs in any year is representative of the entire distribution area for blue crabs in Chesapeake Bay. Absolute abundance is estimated by expanding crab density for every year to the total bay area, estimated at 9,812 km2 by GIS. It should be noted that the dredge survey does not sample waters less than 1.5 m depth, that account for approximately 10% of the total bay area. These shoal waters were sampled with a limited number of stations in 1992 and 1993 using a small, modified dredge. Density estimates derived from these shallow water sites were not significantly different than those derived for the area deeper than 1.5 m (Rothschild et al. 1992).

(4) Source(s) of Data:

Is the complete data set accessible, including metadata, data-dictionaries and embedded definitions?  If yes, please indicate where complete dataset can be obtained.  Yes.  All data is accessible.  Currently, data is accessible via the UMCES website with all documentation and full blue crab stock assessment report.  http://hjort.cbl.umces.edu/crabs/Assessment05.html

Annual updates are available through CBSAC – contact Derek Orner

(5) Custodian of Source Data (and Indicator, if different):Derek Orner, NCBO; Glenn Davis, MDNR.

(6) CBPO Contact:Derek Orner

B.  Communication Questions

(complete either part 1, 2, or 3)

1.  Restoration and Protection Efforts indicators only

(7a) How much has been completed since 1985 (or baseline year)?  How much has been completed since 2000?

(8a) How much was done last year?

(9a) What is the current status in relation to a goal?

(10a) What is the key story told by this indicator?

(11a) Why is it important to report this information?

(12a) What detail and/or diagnostic indicators are related to this reporting level indicator? (Detail and diagnostic indicators can be spatially-specific, parameter-specific, temporally-specific information, etc.)

2.  Bay Health or Watershed Health indicators only

(7b) What is the long-term trend?  (Since start of data collection)  Declining trend through early 2000, then a leveling off at reduced abundance levels. 

Note: The historic data featured in this indicator changed from that reported in the 2008 assessment due to adjustments in data used to calculate abundance levels.  Gear efficiencies are calculated as the average gear efficiency for a given boat over all the years that it conducts the winter dredge survey in Maryland.  The current captain has been the dredge captain since 2004.  Therefore, the efficiency estimates back to 2004 are adjusted with each additional year that this specific captain participates.  This retrospective adjustment of density, results in an adjustment to estimates of exploitation and are converted to abundance and adjusted for overwinter (OW) mortality. Both gear efficiency and OW mortality are measured annually in MD, but are applied to Baywide results.  The most recent years worth of data will continually be updated as efficiencies are updated each year with the current captain.

(8b) What is the short-term trend? (3-5 year trend)  

Recent levels of abundance have decreased to very low levels.  Regulatory actions were implemented in 2008 and 2009, primarily focused on the female portion of the blue crab stock.  Based on the control rule definitions for blue crab, the blue crab stock in 2008 was neither overfished, nor was it experiencing overfishing.  The estimated abundance (223 million age 1+ crabs) in 2009 was considerably higher than the abundance that defines the overfished condition and exceeded the interim target level for the first time since 1993.  This value (223 million age 1+ crabs) represents a 70% increase over the 2008 value of 131 million crabs.  This increase was primarily due to an increase in the number of females – due to regulations.  An interim target abundance level of 200 million crabs has been recommended by CBSAC for management adoption.  Results from the 2010 winter dredge survey are not currently available and will be reported in the annual Blue Crab Advisory Report in June 2010.  

(9b) What is the current status in relation to a goal?  Currently, the abundance of age1+ blue crabs is above the goal (target) considered for management.  This target abundance level corresponds/results in a harvest level that would produce 60-65 million pounds annually and is expected to preserve 20% of the blue crab spawning potential.  We have not however, seen an increase in recruitment associated with the increased abundance at this point.   

(10b) What does this indicator tell us?  Abundance of age 1+ blue crabs throughout the entire Chesapeake Bay in relation to a target and measured since 1991.

(11b) Why is it important to report this information?  To inform management agencies as to the current state of the resource and to assure informed management actions are utilized.

(12b) What detail and/or diagnostic indicators are related to this reporting level indicator?

3.  Factors Impacting Bay and Watershed Health indicators only

(7c) What is the long-term trend?  (since start of data collection)

(8c) What is the short-term trend? (3 to 5 year trend)

(9c) What is the current status?

(10c) What is the key story told by this indicator?

(11c) Why is it important to report this information?

(12c) What detail and/or diagnostic indicators are related to this reporting level indicator?

C.  Temporal Considerations

(13) Data Collection Date(s):  Winter dredge survey was initially begun in 1989; however, data is used from a consistent sampling platform from 1991+.

(14) Planned Update Frequency (e.g. - annual, bi-annual):

(a) Source Data:annual

(b) Indicator:annual

(15) For annual reporting, month spatial data is available for reporting:Data is collected from November through March and analyzed and presented in June of each year in the NCBO Blue Crab Advisory Report.

D.  Spatial Considerations

(16) Type of Geography of Source Data (point, line polygon, other): Some fixed locations, some stratified random samples.  Winter dredge survey consists of 1500+ stations bay-wide, main-stem and tributaries.

(17) Acceptable Level of Spatial Aggregation (e.g. - county, state, major basin, tributary basin, HUC): Baywide

(18) Are there geographic areas with missing data?  If so, where? Geographic limitations are due to vessel draft limitations.  No collections were made in shallow areas (<5 feet) – except during 1992-1993 in which shallow estimates of abundance were equivalent to overall abundance estimates.

(19) The spatial extent of this indicator best described as:

(a) Chesapeake Bay (estuary)

(b) Chesapeake Bay Watershed

(c) Other (please describe): _______________________

Please submit any appropriate examples of how this information has been mapped or otherwise portrayed geographically in the past.

(20) Can appropriate diagnostic indicators be represented geographically?

E.  Data Analysis and Interpretation

(Please provide appropriate references and location of documentation if hard to find.)

(21) Is the conceptual model used to transform these measurements into an indicator widely accepted as a scientifically sound representation of the phenomenon it indicates?  (i.e., how well do the data represent the phenomenon?)  Yes

(22) What is the process by which the raw data is summarized for development and presentation of the indicator?   The abundance of over wintering blue crabs in the Chesapeake Bay was estimated from the winter dredge survey (see Section 3.2.4 and Sharov et al. 2003). It is assumed that the estimated mean density of blue crabs in any year is representative of the entire distribution area for blue crabs in Chesapeake Bay. Absolute abundance is estimated by expanding crab density for every year to the total bay area, estimated at 9,812 km2 by GIS. It should be noted that the dredge survey does not sample waters less than 1.5 m depth, that account for approximately 10% of the total bay area. These shoal waters were sampled with a limited number of stations in 1992 and 1993 using a small, modified dredge. Density estimates derived from these shallow water sites were not significantly different than those derived for the area deeper than 1.5 m (Rothschild et al. 1992).

(23) Are any tools required to generate the indicator data (e.g. - Interpolator, watershed model) spreadsheet applications

(24) Are the computations widely accepted as a scientifically sound? Yes, internationally peer reviewed (Miller, 2005)

(25) Have appropriate statistical methods been used to generalize or portray data beyond the time or spatial locations where measurements were made (e.g., statistical survey inference, no generalization is possible)?  

(26) Are there established reference points, thresholds or ranges of values for this indicator that unambiguously reflect the desired state of the environment? (health/stressors only)   Control Rule for blue crab identifying biological reference points (thresholds and targets) has been identified.  Abundance estimates (this indicator) is a portion of the control rule.

F.  Data Quality

(Please provide appropriate references and location of documentation if hard to find.)

(27) Were the data collected according to an EPA-approved Quality Assurance Plan?  

If no, complete questions 28a – 28d:  No – monitoring program and assessment activities supported by the NOAA Chesapeake Bay Office.

(28a) Are the sampling design, monitoring plan and/or tracking system used to collect the data over time and space based on sound scientific principles? Yes

(28b) What documentation clearly and completely describes the underlying sampling and analytical procedures used?  VIMS and MDNR have reports detailing the procedures utilized in collecting data for the winter dredge survey.  The full stock assessment also discusses monitoring programs (Miller, 2005)  

(28c) Are the sampling and analytical procedures widely accepted as scientifically and technically valid? Yes – Internationally peer reviewed.  (Miller, 2005)

(28d) To what extent are the procedures for quality assurance and quality control of the data documented and accessible? Available on partner websites.

(29) Are the descriptions of the study or survey design clear, complete and sufficient to enable the study or survey to be reproduced? Yes 

(30) Were the sampling and analysis methods performed consistently throughout the data record? Vessel and sampling modifications have been documented and vessel efficiency studies have been conducted.  Analysis is consistent with peer-reviewed stock assessment for Blue Crab (Miller, 2005). 

(31) If datasets from two or more agencies are merged, are their sampling designs and methods comparable? Yes – the winter dredge survey has been designed to be a collection program that utilizes consistent collection patterns Baywide.  Catchability coefficients have been determined to adjust for vessel differences over the years and between jurisdictions.  

(32) Are uncertainty measurements or estimates available for the indicator and/or the underlying data set?  Yes

(33) (Do the uncertainty and variability impact the conclusions that can be inferred from the data and the utility of the indicator?  No

(34) Are there noteworthy limitations or gaps in the data record?  Please explain.

G.  Additional Information

(optional)

(35) Please provide any other information about this indicator you believe is necessary to aid communication and any prevent potential miss-representation.

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