Estimation of Annual and Semi-Annual Survival of Adult Female Blue Crabs and Assessment of the Effectiveness of the Virginia Blue Crab Sanctuary using Tag-Return Methodology
Master of Science (M.Sc.)
Virginia Institute of Marine Science
The blue crab fishery is the most valuable commercial fishery in Chesapeake Bay. The Chesapeake Bay stock recently experienced a period of overfishing, which has resulted in below average abundances, and the spawning stock has experienced an 84 % decline in biomass relative to levels in the late 1980s. The status of the stock is determined by comparing current estimates of mortality to biological reference points. Given the current focus on blue crab conservation, there is a need to obtain reliable, empirical estimates of survival to compare to the biological reference points. A tagging program was initiated on the terminally-molted, mature female component of the Bay-wide blue crab stock to estimate annual and semi-annual survival rates and to assess the effectiveness of the Virginia blue crab spawning sanctuary. Crabs were obtained from five fishery-independent research surveys throughout Chesapeake Bay and were measured, tagged, and released on-site. Tagging was conducted primarily during winter (late October to March) and summer (May to August) from November 2001 to March 2005. Recaptures of tagged crabs were reported by commercial and recreational fishers.
Annual survival rates and tag recovery rates were estimated independently for the winter and summer tagging data using a Brownie model. The two independent estimates of annual survival based on winter tagging (0.08 ± 0.02 SE) and summer tagging (0.08 ± 0.02 SE) data were virtually identical and very low. The estimated tag recovery rate was 24 % based on the winter tagging data and 17 % based on the summer tagging data. The estimated monthly survival rate during winter, 0.87 ± 0.02 SE, was significantly higher than the monthly survival rate during summer, 0.74 ± 0.02 SE. The low estimates of annual survival are consistent with (i) historical estimates of the percentage of age 2+ females in the winter dredge fishery, and (ii) recent estimates of survival derived from estimates of exploitation rate obtained from the ratio of catch to pre-season abundance.
To assess the effectiveness of the spawning sanctuary, mature females were tagged and released inside and outside the sanctuary in the summers of 2002, 2003 and 2004. A comparison of the probability of recapture for crabs tagged outside the sanctuary to crabs tagged inside the sanctuary using relative risk provided a means of assessing the sanctuary effectiveness quantitatively. Probability of recapture for crabs released outside of the sanctuary was 6.3, 5.2, and 2.8 times the probability of recapture for crabs tagged inside the sanctuary for 2002, 2003 and 2004, respectively. Consequently, a significant proportion of adult female blue crabs remained in the sanctuary to spawn and was not captured by the fishery. Hence, the blue crab spawning sanctuary in Chesapeake Bay is an effective means of protecting females migrating to or residing in the spawning grounds.
These findings indicate that survival rates of mature female blue crabs in Chesapeake Bay have remained extremely low during a period of low abundance, which may be preventing stock recovery. Although the blue crab sanctuary is effective in protecting the females that have entered its borders, it only offers protection for 3.5 months of the year. A low annual survival rate suggests that very few adult females live long enough to spawn in more than one year. Current management must be altered for sustainable exploitation of the blue crab in Chesapeake Bay. This study represents one of the few to derive field estimates of semi-annual survival of an invertebrate species using Brownie models. This investigation also serves as one of the few empirical tests to date of the effectiveness of a marine reserve designed to protect spawning stock.
© The Author
Lambert, Debra M., "Estimation of Annual and Semi-Annual Survival of Adult Female Blue Crabs and Assessment of the Effectiveness of the Virginia Blue Crab Sanctuary using Tag-Return Methodology" (2005). Dissertations, Theses, and Masters Projects. William & Mary. Paper 1539617829.