Stehlik et al.: Distribution patterns of various crab species in the Hudson-Raritan Estuary 



705 



Predator/ 

 stratum 



Lady 

 Lady 

 Rock 

 Rock 

 Rock 

 Rock 

 Blue 

 Lady 

 Blue 

 Lady 

 Blue 

 Lady 

 Blue 

 Blue 

 Rock 

 Blue 

 Rock 

 Rock 

 Rock 

 Lady 

 Lady 

 Rock 

 Lady 

 Lady 



I 1 1 1 1 1 1 1 1 1 



100 90 80 70 60 50 40 30 20 10 



Percent similarity in diet 



Figure 10 



Cluster analysis dendrogram of the diets of blue, 

 lady, and rock crabs from the nine strata of the 

 Hudson-Raritan Estuary. The vertical line at 50% 

 similarity defines groups A-F. 



42.00 



40 00 - 



38.00 



36.00 - 



Trophic niche breadth was greatest in male and fe- 

 male blue crabs (5.234 and 6.563) and male lady crabs 

 (6.166) (Table 4). It was narrowest for female rock crabs, 

 but sample size was low. Overlap was highest within 

 species: blue crab males and females (0.819), and lady 

 crab males and females (0.861). Overlap was lowest 

 between lady and rock crabs, sexes combined (0.427 i. 



Discussion 



Temporal and spatial overlap within the estuary 



The scatter plots (Fig. 5) and spatial niche overlap 

 indices indicate substantial likelihood of co-occurrence 

 and encounter among blue, lady, and rock crabs in the 

 Hudson-Raritan Estuary. However, the species were not 

 all active in the study area at the same time. Seasonal 

 migration and winter torpor are two mechanisms that, 

 at times, prevent interspecies encounters. Rock crabs had 

 low temporal overlaps with blue and lady crabs because 

 when rock crabs migrate in from the coastal ocean, lady 

 crabs migrate out and blue crabs become less active and 

 sometimes bury themselves. Although otter trawling does 

 not adequately sample buried blue crabs, commercial crab 

 dredgers catch large numbers of overwintering blue crabs 

 from December through March in and near the Raritan 

 and Chapel Hill channels (Stehlik et al., 1998). 



Temporal overlap between blue crabs and lady crabs 

 was fairly high because of their co-occurrence in the 



-76.00 



-74.00 



-72.00 



-70.00 



-68.00 



-66.00 



Figure 11 



Presence of blue, lady, and rock crabs at stations from the 

 fall 1992 bottom trawl survey, (Northeast Fisheries Science 

 Center, Woods Hole, MAl. Each point represents presence at 

 a station. Occurrence inside the estuaries! boxed symbols) 

 was derived from the literature cited in this article. 



warm months. It was expected that intra-estuarine spa- 

 tial separation might minimize contact between these 

 species because they are reported to prefer different 

 substrates. The blue crab is known to occupy a variety 

 of substrate types, including sand, mud, and submerged 

 vegetation (Milliken and Williams, 1984; Wilson et al. 

 1990), whereas the lady crab is primarily collected on 

 sand (Williams and Wigley, 1977). The lady crab bur- 

 ies itself in sand more readily than in mud (Barshaw 

 and Able, 1990) and it is able to forage more efficiently 

 in sand than in sand-gravel or sand-shell substrates 

 (Sponaugle and Lawton, 1990). However, as shown in 

 Figures 5 and 6, lady crabs were not confined to sandy 

 strata but were most abundant on the fine-grained sedi- 

 ment strata 1, 2, and 9. 



The pattern of seasonal estuarine use by blue and 

 lady crabs is not unique to the Hudson-Raritan Estuary. 

 Other estuaries in which the two Portunidae are abun- 

 dant in summer months but uncommon in winter are 

 Barnegat Bay, NJ (Milstein et al., 1977; pers. observ. ), 

 Delaware Bay (Winget et al., 1974), and Chesapeake 

 Bay (Haefner and Van Engel, 1975). 



Rock crabs undergo seasonal migrations from coastal 

 waters into and out of estuaries, but the timing differs 

 by latitude. In Canada, the Gulf of Maine, and northern 

 Massachusetts, rock crabs are much more abundant in 

 immediate coastal waters, estuaries, and in the inter- 

 tidal zone in warmer months (Krouse, 1972; Scarratt 

 and Lowe, 1972). Rock crabs are more numerous in 

 Narragansett Bay, Rhode Island, in warmer months 

 (Jeffries, 1966; Clancy 4 ). Juveniles are present inside 



Clancy, M. 2002. Personal commun. Boston University, 

 College of General Studies, Division of Natural Science, 

 Boston, MA, 02115. 



