694 



Fishery Bulletin 102(4) 



The diet of the blue crab is generally mollusks, crabs, 

 and fish, depending on crab size (Virnstein, 1977; 

 Laughlin, 1982; Ryer, 1987; Hines et al„ 1990). The 

 diet of the lady crab is mainly bivalves such as My a 

 arenaria and Spisula solidissima, and some crustaceans 

 (McDermott, 1983; Ropes, 1989; Stehlik, 1993). The 

 rock crab consumes mollusks, small crustaceans, crabs, 

 urchins, and fish (Scarratt and Lowe, 1972; Drummond- 

 Davis et al., 1982; Hudon and Lamarche, 1989; Ojeda 

 and Dearborn, 1991; Stehlik, 1993). In some of the 

 aforementioned studies these crabs have been consid- 

 ered opportunistic and as such may be competitors for 

 the same prey taxa. However, differences in maximum 

 body size, chela structure, and the presence or absence 

 of swimming appendages among blue, lady, and rock 

 crabs indicate that they may have differences in diet 

 (Warner and Jones, 1976; Williams. 1984). 



Within the Hudson-Raritan Estuary, blue, lady, and 

 rock crabs are all abundant, providing an opportunity 

 to study partitioning of habitat and food resources by 

 these species. The objectives of our study were to de- 

 termine the temporal and spatial overlap of blue, lady, 

 and rock crabs in this estuary and to differentiate the 

 composition of their diets by the species, sex, and size 

 of predators, and by location of collection. 



This study has potential practical applications. Re- 

 source managers could use the results to consider when 

 and where crabs depend upon certain locations to com- 

 plete their life cycles, if dredging, filling, or sanctuaries 

 were proposed. Dietary analysis of these crabs could 

 indicate if they are a cause of mortality for young stages 

 of commercially important species. For instance, the 

 northern quahog (Mercenaria mercenaria) and the soft- 

 shell clam <M. arenaria i recently have supported and 

 presently support commercial and recreational harvests 

 in the Hudson-Raritan Estuary (MacKenzie. 1990; 1997) 

 and when young these clams are consumed by crabs. 

 The blue crab supports lucrative fisheries in the estuary 

 (Stehlik et al., 1998) and predation by various species of 

 crabs upon blue crab juveniles may affect recruitment. 



Materials and methods 



Study area 



The Hudson-Raritan Estuary, bordered by New Jersey on 

 the south and Staten Island and Brooklyn, New York, on 

 the north (Fig.l), has a surface area of about 280 km 2 . 

 The Hudson, Raritan, and Navesink-Shrewsbury rivers 

 flow into the estuary from the north, west, and south, 

 respectively. The study area is bounded on the west by 

 the 74°15' longitude line; on the east by a line between 

 the northeast corner of Sandy Hook, NJ, and the tip of 

 Rockaway Point, NY; and on all sides by the-3 m con- 

 tour. The area was divided into nine strata according 

 to physiographic features (Wilk et al. 1 ). Sandy-bottom 

 strata included Sandy Hook Bay (stratum 1), Raritan 

 Bay south of Raritan Channel (stratum 2), and Lower 

 Bay north of Raritan Channel (stratum 3). Eastern 



strata of more irregular depths were Romer Shoals 

 (stratum 4), East Bank between Ambrose Channel and 

 Rockaway, NY (stratum 5), and Gravesend Bay at the 

 mouth of the Hudson River (stratum 6). Three strata 

 were channels; Ambrose (stratum 7), Chapel Hill (stra- 

 tum 8), and Raritan (stratum 9). Raritan Channel is 

 maintained at a depth of 13.7 m, and the average depth 

 of adjacent nonchannel stations is 7.1 m. Gravesend Bay- 

 is more than 13 m deep in its center. 



The bottom of the Hudson-Raritan Estuary consists 

 mostly of soft sediments (Jones et al., 1979; Coch, 1986: 

 Wilber 2 ). The substrates in semi-sheltered southern 

 strata 1 and 2 are predominantly fine sand, silt, and 

 clay; those of stratum 3 are mainly medium sand, with 

 a mixture of sand, silt, and clay near channels; those 

 of ocean-exposed strata 4, 5, 6, and 7 are gravel, sand, 

 silt, broken shell, and have beds of blue mussels (Myti- 

 lus edulis). The bottom of Ambrose Channel is silt and 

 clay near its head and fine sand toward the ocean. 

 The sediments of the other two channels, and their 

 immediate borders, are sand, silt, and clay. Based on 

 physiographic form, temperature ranges, and sediments, 

 strata 1, 2, 3, 8, and 9 were considered inner or river- 

 ward strata, whereas 4, 5, 6, and 7 were considered 

 outer or oceanic-influenced strata. 



Collections and analyses 



Crabs were collected during monthly otter trawl sur- 

 veys of the Hudson-Raritan Estuary from June 1991 to 

 December 1994 (Wilk et al. 1 ). Sampling was done from 

 the 18-m research vessel Gloria Michelle by towing 

 a 9.1-m otter trawl with a chain sewn to the bottom 

 opening, and a 76-mm mesh net with a 51-mm codend 

 liner. Wooden trawl doors were deployed to spread open 

 the net. The net was towed once per station for 10 min 

 at 5.6 km/h to cover a distance of approximately 1 km. 

 All tows were made between 8 am and 2 pm. During 

 1991. fixed stations were towed (number of stations in 

 1991: 10 in June, 8 in July; 11 in August; 18 in Sep- 

 tember; 22 in October; 23 in November; 34 in Decem- 

 ber). Beginning in 1992, a stratified random sampling 

 design was used, in which the nine strata were divided 

 into 190 blocks of approximately 0.5 minutes latitude 

 by 0.5 minutes longitude. Each month, 40 blocks were 

 randomly sampled without replication, and the number 

 of blocks in each stratum was proportional to the area 

 of the stratum. Because of conflicting schedules, the 

 vessel was not available in May or September 1992 or 

 1994. Temperature and salinity of water 1 m above the 

 bottom were measured after each tow. During 1991 



1 Wilk. S. J.. E. M. MacHaffie. D. G. McMillan, A. L. Pacheco, R. 

 A. Pikanowski, and L. L. Stehlik. 1996. Fish, megainver- 

 tebrates, and associated hydrographic observations collected 

 in the Hudson-Raritan Estuary, January 1992-December 

 1993, 95 p. Northeast Fish. Sci. Cent." Ref. Doc. 96-14, 

 NMFS. Woods Hole, MA. 



- Wilber, P. 2000. Unpubl. data. Coastal Services Center. 

 National Ocean Survey. NOAA, 2234 Hobson Avenue, Charles- 

 ton, SC, 29405. 



