Abundance and exploitation rate of the blue crab (Callinectes sapidus) in Chesapeake Bay
Abstract:We estimated absolute abundance of the blue crab stock in Chesapeake Bay during winter from stratified random surveys conducted baywide from 1990 to 1999, using the swept-area method. We estimated catching efficiency of the survey gear from multiple depletion experiments to correct for temporal and vessel/area differences in catchability. The survey was conducted during the winter, when crabs are dormant and "buried" in the bottom. Analysis of crab carapace width (CW) frequency distributions revealed two size modes: CW less or equal 60 mm and CW greater than 60 mm, corresponding to age-0 (recruits) and age-1+ (one year and older), respectively. Absolute density of blue crab recruits varied from 10 to 55 crabs per 1,000 m2 across years (95 million to 540 million baywide), with no significant trends over time. Abundance of age-1+ crabs declined significantly from 35 to 38 crabs per 1,000 m2 in 1990–1991 (342 million to 371 million crabs baywide) to 8.3 in 1999 (82 million crabs baywide). A stronger decline in the number of males indicates that males were exploited more intensively than females. A three-year moving average of spawning stock abundance (age-1+ females) declined twofold from the early to the late 1990s. The absolute abundance of the blue crab population in Chesapeake Bay varied from 241 million to 867 million. Over-wintering mortality was usually less than 2%, but substantially higher mortality occurred in 1994 (7.3%) and 1996 (11.9%). High correlation between January water temperature and the percentage of dead crabs provides strong evidence of the adverse effect of cold winter on survival of crabs. Large crabs were affected most by cold winter temperatures. We estimated exploitation rates for the commercial fishery by comparing abundance with total landings. The estimated exploitation rates varied from 40% to 52% from 1990 to 1998 and increased to a record high of 70% in 1999. Fishing mortality rates varied from 0.6 to 0.9 year−1 in most years and were above the level providing maximum yield per recruit (Fmax = 0.64 year−1) in nearly all years. The record fishing mortality in 1999 (F1999 = 1.6 year−1) exceeded the overfishing threshold (F10% = 1.0 year−1). Despite evidence of growth overfishing, the blue crab population supported large harvests throughout the 1990s. Increase of fishing mortality above the F10% in 1999, indicates that the population was overfished and is at risk of recruitment overfishing if fishing mortality remains at this level.
Document Type: Research Article
Publication date: March 1, 2003
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