Crab Density (Age 1+): Winter dredge survey density of blue crabs aged one year and older (age 1+) 1989-2006. These are crabs measuring greater than 60 mm across the carapace and are considered the “exploitable stock.” 95% confidence intervals (1.96* std error) shown around individual points. The average range for the survey is defined as the standard deviation of the annual crab density values divided by the square root of 3.
Crab Density (Recruits): Winter dredge survey density of age 0 blue crabs (recruits) 1989-2006. These are crabs measuring less than 60 mm (2.4 inches) across the carapace. 95% confidence intervals (1.96* std error) shown around individual points. The average range for the survey is defined as the standard deviation of the annual crab density values divided by the square root of 3.
Crab Density (Female Spawning Potential): Winter dredge survey density of female spawning potential 1989-2006. These are immature and mature female crabs measuring greater than 60 mm (2.4 inches) across the carapace. 95% confidence intervals (1.96*std error) shown around individual points. The average range for the survey is defined as the standard deviation of the annual crab density values divided by the square root of 3.
Control Rule: The control rule used to manage the Chesapeake Bay blue crab fishery. An abundance of 86 million age 1+ crabs represents the overfished threshold. In 2006, abundance was above the overfished threshold and the exploitation rate was below the overfishing threshold, but slightly above the target level. The red point is the projected 2007 exploitation fraction and has a possible range of 0.44 (44%) to 0.8 (80%), based on 95% prediction limits.
Chesapeake Bay Crab Harvest: Chesapeake Bay Blue Crab harvest 1945-2006, adjusted for changes in reporting methods.
MD & VA Crab Harvest: Maryland and Virginia Chesapeake Bay Blue Crab harvest 1945-2006, adjusted for changes in reporting methods.
Abundance vs. Harvest: The relationship between the total abundance of crabs measured in the Bay-wide winter dredge survey (WDS), and the subsequent year’s harvest in pounds. Based on this relationship, the 2007 harvest is predicted to be 48.7 million pounds with a possible range of 32.3 to 65.1 million pounds. The lowest total abundance of crabs was observed in 2001. The highest abundance and the largest harvest during this time period were recorded in 1993.
VA Trawl Survey (YOY): Virginia Trawl Survey catch per tow of age 0 crabs, 1968-2006, from sites in the upper and lower rivers. Age 0 is assigned to crabs that are less than or equal to 50 mm across the carapace in September, and less than or equal to 60 mm across the carapace in October and November.
VA Trawl Survey (Age 1+): Virginia Trawl Survey catch per tow of age 1+ crabs, 1968-2006, from sites sampled in the upper and lower rivers. Age 1+ crabs are defined as those that are greater than or equal to 36 mm across the carapace in August, greater than or equal to 51 mm in September, and greater than or equal to 61 mm across the carapace in October.
VA Trawl Survey (Adult Females): Virginia Trawl Survey catch per tow of adult female crabs, 1968 through 2006, from sites in the upper and lower rivers, and the mainstem of Chesapeake Bay. All females caught from August through November are considered to be adult, in that they will likely spawn within 1 year.
Long-Term Trends
- spawning stock declined by 81%
- female size declined by 8%
- spawning stock biomass declined by 84%
- mean size at maturity diminished by 9%
In addition, larval abundance and post-larval recruitment were lower by approximately 1 order of magnitude compared with earlier years.
The initial decline resulted from poor recruitment in 1991, despite high spawning stock and larval abundance. This recruitment failure, in concert with high fishing pressure and natural mortality, led to a diminished spawning stock in 1992 and thereafter.
The Bay’s blue crab population has experienced numerous “boom and bust” cycles in the past, due to natural environmental variability and year-to-year differences in recruitment success. What is particularly troubling about the current decline is that it has now persisted for more than a decade. The decline also occurred rapidly, within 1 to 2 years, indicating a “phase shift” rather than a progressive decrease.
Prognosis
The spawning stock and larval abundance of blue crabs in Chesapeake Bay are unlikely to rebound to former high levels without significant reductions in fishing and natural mortality, along with enhanced environmental conditions conducive to successful recruitment.
The key consequences of a smaller spawning stock and poor recruitment are an increased likelihood of recruitment failure and reduced resilience to environmental setbacks such as cold snaps or tropical storms.
The demonstration of a concurrent decrease and significant association between spawning stock abundance and recruitment, larval abundance and female size is unique for the blue crab and for marine invertebrates in general, and indicates an urgent need to conserve the spawning stock for long-term sustainable exploitation and population persistence.
