Population dynamics and stock assessment of the blue crab in North Carolina.

摘要

The blue crab (Callinectes sapidus) is an ecologically important estuarine predator and represents North Carolina's most important commercial fishery. Recent fishery-dependent and -independent data suggest the population is declining. The goal of this study was to increase our understanding of the status and population dynamics of the blue crab in NC by addressing the following objectives: (1) estimate population demographics of blue crabs in salt marsh creeks, (2) construct a discontinuous model of blue crab growth in NC using growth rates estimated from free-ranging blue crabs, and (3) provide a comprehensive stock assessment for the blue crab in NC. A series of complementary laboratory and field studies assessed the nursery role of salt marsh habitats for the blue crab (Callinectes sapidus). Population demographics and movement patterns of juvenile and adult blue crabs were quantified in two tidal salt marsh creeks near Beaufort, North Carolina, USA during June--October 2001. While there are many studies that report estimates of population density, mortality rates, or movement rates for blue crabs, this study represents one of the first attempts to estimate all quantities concurrently. Juvenile crabs were mobile within the interstices of the vegetated marsh surface during flood tide, and were equally distributed buried in intertidal marsh and adjacent mud areas during ebb tide. Juvenile crabs may experience a spatial refuge from cannibalism in the marsh surface since adult conspecifics are physically impeded by dense vegetation and rarely move far into marsh habitats. This spatial refuge in the vegetated marsh surface may be significant, since cannibalism represents a large source of mortality for this species. The relatively high use of the marsh surface by juvenile blue crabs, combined with a general lack of sampling these complex habitats, suggest that crab densities may be even higher in salt marsh systems than previously thought. Growth models commonly used in fisheries and ecological modeling assume growth is a continuous function of age. While this approach is appropriate for finfish, the validity of these models for crustacean species, which grow discontinuously, has been questioned. There is a critical need to compare the predictions of discontinuous and continuous models simultaneously to identify if potential biases are introduced by the assumption of continuous growth for the blue crab. The blue crab stock in North Carolina currently sustains heavy exploitation by the commercial fishery, and information on the recreational fishery is generally lacking. There has been a systematic increase in commercial landings from 1987--1999, followed by a period of reduced landings from 2000--2002. During this period fishery-independent indices of abundance have remained stable, or have shown a significant decline. In no case have any indices of abundance shown an increasing trend. Moreover, adult and spawning stock abundance (SSB) during 2000--2001 were at the lowest levels recorded since 1987, and the mean size of mature females has declined significantly during 1987--2003. Increases in the index of relative SSB in 2002--2003, however, may indicate a recovery. We detected a significant stock-recruit relationship for the blue crab in North Carolina using certain estimates of recruit abundance and recommend an urgent need for conservation of the spawning stock. We encourage decision makers to use the information and recommendations in this dissertation as soon as possible to manage the blue crab fishery in NC in a sustainable manner.