US EPA

7224987 

Journal Article 

Stock assessment of Chesapeake Bay blue crab (Callinectes sapidus Rathbun) 

Rugolo, LJ; Knotts, KS; Lange, AM; Crecco, VA; , 

1998 

NATL SHELLFISHERIES ASSOC 

SOUTHAMPTON 

493-517 

WOS:000076430800019 

This stock assessment stemmed from concerns about declines in Chesapeake Bay blue crab commercial yield, survey abundance indices, and economic performance in the 1990s. Blue crab vital rates and life-history characteristics were described. A risk-averse approach was adopted for parameter estimation. Extant (50 y) fisheries-independent and dependent data were compiled. Current spawning stock biomass was at moderate levels, as compared to historical maxima; total stock biomass was at long-term average levels since 1956. Juvenile recruitment has been variable since the 1950s. The stock is in long-term dynamic equilibrium, demonstrating variable abundance as expected in a strongly r-selected species. No evidence of a persistent stock decline was found. The decade of the 1980s was a period of above average abundance; the population modulated to average levels thereafter. Maryland commercial catch per unit effort (CPUE) has remained fairly stable from 1982 to 1995. Baywide yield varied without trend since 1945, cycling around the long-term mean with peaks in 1950, 1966, 1981, and 1993 and troughs in the mid-1950s and late 1970s to early 1980s. Baywide data since 19-15 demonstrated a fivefold increase in directed effort (f) and an exponential decline in CPUE: rapidly from 1947 to 1967, and without trend since 1970. The stock has supported maximum sustainable yield (MSY) = 37-38.5 x 10(6) kg since 1935 with no observed change in the ability of the stock to replace itself or to provide historical average yield. We judged the stock to be moderately to fully exploited at average levels of abundance. Stock-recruitment (S-R) analysis revealed weak relationships. Density-independent effects accounted for little of the variability in the S-R model. Exploitation rates for 1956 to 1996 ranged from 30-40% for peeler/soft crabs and 40-55% for hard crabs. Fishing mortality (F) ranged largely between 0.8-1.0 (F < 1.20) since 1956 within +/- 20% of the long-term mean; fully recruited F in 1996 was 0.87. Current F was below the threshold reference level of F-10% = 1.21, where F-REP = 1.17. Yield per recruit (YPR) modeling estimated F-0.1 = 0.36 and F-MAX = 0.64. The stock is strictly growth overfished with a 26% reduction in F-10% required to maximize (+2% gain) YPR. Longevity was established at 8 y. Tagging studies showed that blue crabs live considerably longer than curren convention (3 y). A life-table model reconciled the apparent disparity between this convention and the life-history parameters used in the assessment. At current total mortality (Z)approximate to 1.3, the mean age of the observed stock would be 1.5 y (153 mm CW [carapace width]; 97.3% of all individuals would be greater than or equal to 3 y of age. The dramatic rise in f since 1945 with the accompanying decline in CPUE was not associated with an increase in F. Catachability (q) has varied with f from 1956 to 1995 as a result of gear saturation. Moderate reductions in f would not result in a proportional reduction in F because of nonconstant q. The blue crab fishery is severely overcapitalized in terms of total effort.As a result of gear saturation, marginal decreases in fishing effort would not realize proportional gains in CPUE, %MSP, or YPR. Substantial economic displacement would be required to maximize YPR. Increases in %MSP and YPR could be realized through increased size limits and regulating the taking of mature female crabs, without the displacement costs associated with effort reductions alone. We recommend a risk-averse management strategy: maintain current F below F-10%. Management should be particularly averse to increases in effort that would exacerbate current economic inefficiencies, or to increases ih gear efficiency. The latter concern acknowledges the delicate interplay between F and f in the fishery. Management should be proactive: stabilize and enhance the economic viability of the fishery and provide protection for the stock through maintenance of greater than or equal to 10% maximum spawning potential (MSP). It should consider adopting strategies which increase YPR (e.g., size limit measures on both sexes) and spawning potential (e.g., limit directed fisheries on mature female crabs). 

Symposium on Blue Crab Fisheries of North America at the 88th Annual National-Shellfisheries-Association Meeting 

BALTIMORE, MARYLAND