JUVENILE BLUE CRAB, CALLINECTES SAPIDUS, SURVIVAL: 

 AN EVALUATION OF EELGRASS, ZOSTERA MARINA, AS REFUGE 



K. A. Wilson,! K. L. Heck, Jr.,^ and K. W. Able^ 



ABSTRACT 



Field experiments were conducted to examine rates of predation on juvenile blue crabs in different den- 

 sities of eelgrass near Manahawkin, New Jersey. Tethering experiments from July to October 1985 showed 

 that crabs in eelgrass were preyed on at lower rates than those in adjacent bare sand patches. In addi- 

 tion, intermediate densities of eelgrass provided the best refuge for blue crabs while crabs in low- and 

 high-density eelgrass suffered higher rates of predation. We suggest that the root mats of high-density 

 eelgrass may reduce the ability of blue crabs to hide and bury in the substratum. There was no effect 

 of prey size (11-100 mm carapace width) on risk to predation. Predation on sand substrate declined dur- 

 ing the observation period and rates dropped to zero in vegetation in October. 



The blue crab, Callinectes sapidus, is one of the most 

 important commercial species in mid-Atlantic 

 coastal waters of the United States (Van Engel 

 1958; Williams 1984). Blue crabs are caught in abun- 

 dance from Florida into New Jersey waters, and are 

 taken in lesser numbers as far north as Nova Scotia 

 (Williams 1984). Although the Chesapeake Bay sys- 

 tem produces the greatest catches of blue crabs, 

 commercial and recreational fishing is significant in 

 many other Atlantic bays and estuaries. 



Despite the economic importance of blue crabs and 

 the large amount of prior research done on this 

 species, there are many unanswered questions about 

 the factors that influence blue crab abundance and 

 distribution (Williams 1984), and our ability to pre- 

 dict annual harvests is extremely limited. The stages 

 of the life cycle that are least understood are the 

 larval and juvenile stages, and it is these which suf- 

 fer most nonfishing mortality. 



Studies of blue crab larval transport have shown 

 that wind-driven circulation patterns influence the 

 abundance of larvae that enter mid-Atlantic coast 

 estuaries (Sulkin et al. 1980; Epifanio and Dittel 

 1982; McConnaugha et al. 1983; Provenzano et al. 

 1983; Epifanio et al. 1984; Johnson et al. 1984; 

 Sulkin 1984). In addition, we know that juvenile blue 

 crabs in most estuaries are found in much greater 



Natural 



; present 



Box 41, 



'Division of Environmental Research, Academy of 

 Sciences, 19th and the Parkway, Philadelphia, PA 19103 

 address: Marine Field Station, Rutgers University, P.O. 

 Tuckerton, NJ 08087. 



^Division of Environmental Research, Academy of 

 Sciences, 19th and the Parkway, Philadelphia, PA 19103 

 address: Dauphin Island Sea Laboratory, P.O. Box 169, 

 Island, AL 36428. 



'Marine Field Station, Rutgers University, P.O. Box 41, Tucker 

 ton, NJ 08087. 



Natural 

 ; present 

 Dauphin 



Manuscript accepted September 1986. 

 FISHERY BULLETIN: VOL. 85, NO. 1, 1987. 



abundance in stands of submerged vegetation than 

 on unvegetated substrate (Tagatz 1968; Diaz and 

 Fredette 1982; Kennish et al. 1982; Penry 1982; 

 Zimmerman and Minello 1984), and it is believed 

 that submerged vegetation provides protection from 

 predators for small blue crabs and for crabs under- 

 going ecdysis (Lippson 1973; Heck and Orth 1980; 

 Heck and Thoman 1984; Orth et al. 1984). To date 

 no studies have demonstrated that submerged vege- 

 tation actually provides protection for juvenile blue 

 crabs under field conditions nor do we have data on 

 the influence of vegetation density on survival of 

 blue crabs. Below we describe the results of a series 

 of field experiments designed to evaluate the pro- 

 tective properties of varying densities of eelgrass, 

 Zostera marina, for different size classes of blue 

 crabs. We also report on the identity of potential 

 predators and estimate the role of submerged vege- 

 tation as it influences blue crab populations in New 

 Jersey bays. 



METHODS 



Tethering experiments were conducted from July 

 to October 1985 in shallow- water seagrass meadows 

 near Manahawkin, NJ (lat. 39°N; long. 74°W). In 

 this area, sand patches are interspersed among ex- 

 tensive seagrass beds dominated by Zostera marina 

 (Macomber and Allen 1979). Large numbers of blue 

 crabs inhabit these grass beds (cf. Kennish et al. 

 1984), just as they do in eelgrass beds of Chesapeake 

 Bay (Heck and Orth 1980; Heck and Thoman 1984). 



Blue crabs were collected by seine or dip net from 

 Zostera marina and adjacent sand patches and pre- 

 pared for tethering in the laboratory. No soft crabs 



53 



