WILSON ET AL.: JUVENILE BLUE CRAB SURVIVAL IN EELGRASS 



3 4% 



BOt 



E 



3 30 



32 V. 



28% 



1 



^ 



a 



^ 



m 



17 % 



^ 



14% 



# ^ ^ 



11-20 21-30 31-40 41-50 51-60 61-70 71-80 81-9091-100 



Carapace Width (mm) 



Figure 2.— Blue crab body size (carapace width (CW)) and risk of predation. Hatched bars in- 

 dicate number of individuals tethered at that size and open bars indicate number of tethered 

 crabs eaten. 



Low Med 



VegeloUon Density 



High 



Figure 3.— The effect of eelgrass density on predation rates. 

 Histograms are mean rates of predation from July through October 

 on sand and at each eelgrass density. Vertical bars are + / - one 

 standard error. 



1982) and amphipods (Stoner 1982) that describe the 

 importance of seagrasses as protective cover for 

 prey. They clearly show that eelgrass provides 

 refuge from predation and increased survival for 

 juvenile blue crabs compared to that on adjacent 

 unvegetated sand substrates. 



Rates of predation on blue crabs within the three 

 densities of vegetation, however, did not conform 

 to patterns previously established, where predation 

 on crustacean epifauna is inversely proportional to 

 vegetation biomass (Stoner 1982; Leber 1985). In 



this study, risk of predation was lowest in interme- 

 diate densities rather than in high-density eelgrass. 



Savino and Stein (1982) found that attack rates 

 by largemouth bass on bluegills dramatically de- 

 clined with increasing density of artificial vegeta- 

 tion, and capture rates by the predators were lower 

 in vegetation than on bare substratum. Epifaunal 

 amphipods and caridean shrimp also suffer lower 

 rates of predation at high densities of vegetation 

 (Nelson 1979; Stoner 1980; Coen et al. 1981; Leber 

 1985). These studies and others (Vince et al. 1976; 

 Crowder and Cooper 1982; Minello and Zimmerman 

 1983) indicate that above-ground vegetation biomass 

 reduces a visual predator's search and capture effi- 

 ciencies and that vegetation may also provide a 

 matching background in which epifaunal prey may 

 hide (Endler 1978; Orth et al. 1984). 



The root and rhizome mat of seagrasses may also 

 lower search and capture efficiency of predators 

 (Orth 1977; Blundon and Kennedy 1982b; Peterson 

 1982), but in addition a high-density root mat may 

 reduce the ability of hard-bodied prey to bury and 

 hide in the substratum. For example, Brenchley 

 (1982) found that the burrowing ability of decapods 

 in dense eelgrass root mats was reduced or pre- 

 vented, and Bertness and Miller (1984) found that 

 fiddler crabs, TJca pugnax, preferred to construct 

 burrows in intermediate densities of salt marsh 

 roots. 



55 



