Home | Business News | Browse by Publication | M | Marine Fisheries Review

Testing a device to exclude ovigerous blue crabs, Callinectes sapidus, from commercial pots.

Article Excerpt
Introduction

The blue crab, Callinectes sapidus, provides the most economically important fishery in North Carolina (NCDMF, 2004) but population trends have raised concerns among fishery managers. Eggleston et al. (2004) documented a significant spawning stock-recruitment relationship, and trends in biomass decline, increasing fishing mortality, and decreasing mean size of mature females during 1987-2001. Because female crabs have a terminal molt and small mature females may continually escape through cull rings, fishing mortality may decrease their average size and subsequent recruitment in North Carolina (Wolcott and Wolcott, 2004). For these reasons, fishery managers have set a goal to maintain the stock at a level that maximizes reproductive potential (NCDMF, 2004).

Brooding female (sponge) crabs can be legally harvested in North Carolina. Since 1965, North Carolina has used five spawning sanctuaries to protect mature female crabs in the vicinity of inlets between the Atlantic Ocean and Pamlico and Core Sounds (Fig. 1). Owing to seasonal and annual fluctuations in salinity, as well as their small area, sanctuaries appear to offer minimal protection to the North Carolina blue crab spawning stock (Medici, 2004). Fishery managers in North Carolina have recently expressed interest in investigating other methods to protect the sponge crab population short of an outright prohibition against their harvest (NCDMF, 2004).

Effective protection of sponge crabs may increase the reproductive potential for blue crabs in North Carolina; this species currently has a stock status of "concern" in the state (NCDMF (1)). There are several reasons to investigate efficient, inexpensive methods to reduce capture rates of sponge crabs in the North Carolina commercial pot fishery instead of simply prohibiting their harvest. Depending on location and time of year, sponge crabs have relatively little or no market value. As such, they are often discarded at sea or landed at low-value (cull) market grades (Paul J. Rudershausen, personal observ.). The capture and subsequent release of sponge crabs can affect their brood sizes and migrations. Sponge crabs mutilate their broods while held in pots (Rittschof, 2004), but the impact of sponge crab confinement on reproductive potential has not been quantified. Prohibiting the harvest of sponge crabs would affect crab fishermen along the Outer Banks, where these crabs can constitute 25% or more of the harvest (Ballance and Ballance, 2003).

The effectiveness of a device to exclude sponge crabs but permit entry of nonsponged crabs rests on the fact that these two groups have different body proportions. A similar premise has been used to effectively exclude diamondback terrapins, Malaclemys terrapin, from crab pots (Guillory and Prejean, 1998). A partially or fully developed egg mass will result in a functionally greater body length (inter-orbital teeth to the back of the apron) and body depth of a sponge crab relative to a male or nonsponged mature female of roughly equal carapace width (Fig. 2). Blue crabs enter pots in a direction parallel to their carapace width such that this dimension is perpendicular to the face of the opening that the crab enters (Guillory and Merrell, 1993). Thus, the body length and depth of a blue crab will determine whether it fits through the opening of a crab pot. The development of the egg mass can prevent entry of a sponge crab into a pot with a restrictive opening, depending on the size of the opening, size of the crab, fullness of the sponge, and motivation of the crab to enter the pot.

Our objective was to identify one or more excluder sizes that would simultaneously reduce sizes and numbers...

View this article FREE - Now for a Limited Time, try Goliath Business News
Free for 3 Days!