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Author Topic: 2003 bad year seen in 2002  (Read 1156 times)
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« on: December 21, 2003, 08:11:54 PM »

CBSAC Chesapeake Bay Blue Crab Advisory Report 2002
Prepared by the Chesapeake Bay Stock Assessment Committee1: June 27, 2002
Status of the Stock: Analysis of long term fishery independent surveys conducted in Chesapeake Bay (Maryland and Virginia trawl surveys, Calvert Cliffs crab pot survey and Baywide winter dredge survey) indicate that blue crab abundance is approaching the record low and has been declining in recent years. The current status of the stock was compared to thresholds and targets endorsed by regional management agencies in January 2001. Stock abundance was above the overfished threshold and below the overfishing threshold. The low abundance puts the stock at increased risk for recruitment failure, but stock abundance does not appear to have fallen below abundances observed in past years. However, the low abundance combined with a high exploitation rate indicates a stock condition that warrants concern for the fifth consecutive year.
The recent trend in fishing mortality rate (F) is not clear. A length-based method of F estimation suggests that F may be declining. Length-based estimates of fishing mortality determined from the Maryland and Virginia trawl surveys, the Calvert Cliffs crab pot survey and the Baywide winter dredge survey declined from F = 0.91 in 2000 to F = 0.81 in 2001. The estimated 2001 fishing mortality rate is below the overfishing threshold (F10% = 1.0) but above the target ontrast, preliminary estimates of the relative exploitation rate based on the combined Baywide commercial and recreational harvest, and estimated abundances from the Baywide winter dredge survey indicate that the relative exploitation rate may be increasing. Considerably higher estimates could result if the preliminary estimates of the relative exploitation rate from the winter dredge survey were converted into fishing mortality rates. The CBSAC feels that once details in the methodology of the winter dredge-based approach are finalized, this index of annual exploitation rate will be more reliable than the length-based estimates of F.
The 2001 Chesapeake Bay commercial blue crab harvest of approximately 52 million pounds is well below the time series (1968 - 2001) average of about 75 million pounds. The low harvest in 2001 was principally a result of low exploitable stock abundance. However, the harvest was also constrained by management measures implemented in Maryland, Virginia and the Potomac River Fisheries Commission prior to and during the 2001 season.
It is apparent that F is above the target, recruitment is continuing to decline, female spawning stock biomass is near the historical low established in 2000 and that exploitable stock abundance is below the Blue Crab Decision Rule action threshold. There is a consensus among committee members that the level of risk to the stock and fishery associated with declining recruitment, low female spawning stock size and low exploitable stock size remains high and is increasing. It is important to note that estimation of fishing mortality rates is important for evaluating the effectiveness of management actions, but it is the spawning stock biomass that is relevant to the assessment of risk of recruitment failure.
Data: Five fishery-independent surveys are used to determine stock status: Virginia trawl survey, Maryland summer trawl survey, Calvert Cliffs crab pot survey, Baywide winter dredge survey and Baywide zooplankton monitoring. The first four sample crabs after settlement, the latter samples megalopal abundance in the water column. Data from the two trawl surveys and the Calvert Cliffs pot survey are based on calendar year collections through 2001. The winter dredge survey data represent seasonal collections through the 2001/02 season. For abundance indices the dredge survey is referred to as 2002 data, but for estimates of fishing mortality rates the dredge survey is referred to as 2001 data since the mortality took place in 2001. Data from the zooplankton monitoring program is based on calendar year collections. All indices are expressed as the geometric mean catch per unit effort. Modified and standardized width-age cutoff values were used to differentiate age classes for three of the four surveys (Maryland and Virginia trawl and Calvert Cliffs pot study) used to derive the abundance indices. Sliding monthly cutoff values were used to model the variable growth of age-0 crabs. Age-0 crabs are defined as being less than 50-90 mm depending on month, and age-1+ are all crabs larger than the monthly cutoff values.
Biological Reference Points: A review of targets and thresholds for Chesapeake Bay blue crabs was conducted by an expert panel convened by the Bi-State Blue Crab Advisory Committee in 2000. The panel identified exploitation and abundance thresholds, a precautionary zone in which exploitation is too high at low abundance, and an exploitation target. The overfishing threshold (F10% = 1.0) and target (F20% = 0.7) fishing mortality rates refer to the levels of spawning potential which are 10% and 20% respectively, of the spawning potential expected in a stock on which no fishing occurs. Age-specific partial recruitment was based on the selectivity of the harvest gears and established as 10% (age 0), 75% (age 1), 95% (age 2) and 100% (age 3+). The overfished threshold (Blow) is equal to the lowest exploitable stock observed in the fishery independent trawl,
Exploitable Stock Abundance (1999-01): The average exploitable abundance of age 1+ crabs for the last three years was considered to be below average for all four surveys (Maryland and Virginia trawl surveys, Calvert Cliffs pot survey and Baywide winter dredge survey). Data for all surveys combined indicate that the exploitable stock abundance has been declining since the early 1990s and is approaching the historical low..
Spawning Stock Abundance (1999-01): Mature female spawning stock abundance was below the long-term average for the Virginia trawl survey, but was at average levels for the Baywide winter dredge survey, the Maryland trawl survey and the Calvert Cliffs pot survey. Data for all surveys combined indicate that spawning stock abundance has declined since the early 1990s. It is also important to note that the 2000 and 2001 abundance estimates are the lowest of the time series.
Harvest: The three-year (1999-2001) average, commercial Baywide harvest (57 million pounds) is below the long term (1968 - 2001) average of about 75 million pounds. The 2001 Baywide harvest of approximately 52 million pounds is below average and is the lowest since the Maryland commercial crab reporting system changed in 1981. For the 1968-2001 period, Baywide commercial harvests exceeded 100 million pounds in 1966, 1981, 1983 and 1993. The 1993 harvest of 113 million pounds is the highest recorded harvest. Based on the historical relationship between winter dredge survey abundance and commercial harvest, we expect the Baywide commercial Chesapeake Bay harvest in 2002 to be less than 60 million pounds in the absence of changes to the regulations.
Management Advice: Based on a review of fishery-independent surveys conducted in Maryland, Virginia and the Potomac River it appears that: (1) there has been a long term decline in recruitment and in the age 1+ component of the stock since the early 1990s; (2) the abundance of age 1+ crabs is approaching a low in abundance not observed since the late-1960s; (3) adult female abundance in 2000 and again in 2001 was below the previous historical low set in 1968; and (4) exploitable stock abundance was below (to the left of) the blue crab action threshold for the fifth consecutive year.



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