FISHERY BULLETIN: VOL. 85, NO. 1 



Juvenile blue crabs, unlike epifaunal caridean 

 shrimp or amphipods, utilize below-ground refuges 

 in seagrass beds. Our field and laboratory observa- 

 tions suggest that their primary mode of predator 

 avoidance is to bury in the substratum. Orth and van 

 Montfrans (1982) also noted burying behavior of 

 juvenile blue crabs in laboratory experiments that 

 examined predation by adult blue crabs in three den- 

 sities of artificial seagrass and root mat. Their data 

 also suggested mortality of juveniles is lowest in 

 intermediate densities of seagrass. 



We infer that at low seagrass densities the blue 

 crabs are able to bury in the substratum, but the 

 leaves and root mat of the grass do not reduce detec- 

 tion and capture efficiency of the predators as do 

 intermediate seagrass densities. Furthermore, we 

 suggest that the dense root mat and shoots of high- 

 density seagrass may reduce the ability of blue crabs 

 to bury themselves and that high blade density may 

 reduce the crabs' visual ability to detect predators. 



Based on our observations the dominant predators 

 on blue crabs appear to be toadfish, Opsanus tau, 

 the American eel, Anguilla rostrata, and other blue 

 crabs. Toadfish are extremely common in the Mana- 

 hawkin grass beds in the summer (June-September) 

 and are known to readily consume brachyuran crabs, 

 including blue crabs (Schwartz and Butcher 1963; 

 McDermott 1965; Wilson et al. 1982; Gibbons and 

 Castagna 1985). In this study, there were instances 

 where, upon recovery of tethers after a predation 

 trial, toadfish had swallowed both the crab and 

 tether and remained on the line, providing confir- 

 mation that toadfish are blue crab predators under 

 field experimental conditions. Gut contents of 

 American eels from the study area contained blue 

 crabs (K. Able, pers. obs.) and Wenner and Musick 

 (1975) found blue crabs to be a major part of the eel's 

 diet. 



Predation intensity appears to be distributed even- 

 ly over the size classes tested, although there is a 

 trend of lower predation rates on the largest blue 

 crabs (>71 mm CW). However, the sample size is 

 small for these size classes {N = 17) so the estimate 

 of predation on larger crabs may be inadequate. 

 Escape in size has been observed in other inverte- 

 brate prey (Blundon and Kennedy 1982a; Peterson 

 1982; Wilson 1985) and a similar pattern was ex- 

 pected in this study because large adult blue crabs 

 are found frequently on unvegetated substratum 

 where risk of predation is highest (Heck and 

 Thoman 1984). An additional large predator, the 

 smooth dogfish, Mtcstelus canis, occurs in Barnegat 

 Bay (Tatham et al. 1983) and we suspect it may feed 

 on blue crabs in seagrass meadows at night 



(Casterlin and Reynolds 1979). Mustelus can grow 

 to 1.5 m (Hildebrand and Schroeder 1928) and preys 

 on blue crabs in eelgrass beds (Bigelow and Schroe- 

 der 1953). Hence, predation by smooth dogfish may 

 account for loss of larger crabs and also suggests 

 that there may be a temporal as well as spatial pat- 

 tern of predation. 



Researchers have suggested that the value of 

 refuges for juvenile blue crabs and other inverte- 

 brate macrofauna is dependent on the interaction 

 of several factors including species of vegetation, 

 vegetation density, water quality, and type of pre- 

 dator (Heck and Thoman 1984; Orth et al. 1984). The 

 data from these tethering experiments clearly in- 

 dicate that eelgrass serves as protective cover and 

 that eelgrass density is indeed an important factor 

 in determining predation rates on juvenile blue 

 crabs. The unexpected result that crabs in interme- 

 diate densities of eelgrass suffered lower predation 

 rates than those in high densities underscores the 

 complexity of the interactions that determine sur- 

 vival of juvenile blue crabs. 



ACKNOWLEDGMENTS 



Support for this study was provided by the New 

 Jersey Department of Environmental Protection, 

 Marine Fisheries Administration, and the Academy 

 of Natural Sciences of Philadelphia. 



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