Abstract. - During 1981 and 

 1982, 933 Atlantic cod Gadus morhua 

 and 119 thorny skate Raja radiata 

 were collected off the northwestern 

 coast of Cape Breton Island, Nova 

 Scotia. Danish seines, jigs, gillnets, 

 and trawls were used for sampling. 

 Stomach contents were analyzed to 

 investigate predator-prey relation- 

 ships between cod and snow crab 

 Chionoecetes opilio, two commercial- 

 ly important species. For compari- 

 son, skate and two toad crab species, 

 Hyas araneus and H. coarctatus, 

 were also considered. 



During spring (April, May), cod fed 

 mostly (by weight) on large soft- 

 shelled male snow crab [77-1 10 mm 

 carapace width (CW)]. In summer 

 (July) and fall (September), cod pre- 

 dation was directed more towards 

 prey species such as fish, although 

 the mean number of snow crab per 

 cod stomach also increased. The aver- 

 age number of snow crab in cod stom- 

 ach contents was higher (0.59 crab/ 

 stomach) on sand and mud bottoms 

 than on rocky and gravel bottoms 

 (0.05 crab/stomach). In contrast, 

 higher numbers of toad crab (1.35 

 crabs/stomach) were found in stom- 

 achs from cod on rocky and gravel 

 bottom than on mud and sand bot- 

 tom (0.50 crab/stomach). 



There were approximately five 

 times more snow crab (by number) 

 in the stomachs of skate than in cod. 

 The frequency of toad crab present 

 in the stomach contents of both pred- 

 ators was similar. On average, indi- 

 vidual skate consumed up to five 

 times more snow crab (by number) 

 than toad crab, while cod consumed 

 approximately equal numbers of toad 

 and snow crab. Estimates of crab 

 abundance provide evidence that cod 

 feed more on toad crab than snow 

 crab, whereas skate seem more op- 

 portunistic. 



Problems associated with using 

 stomach content analysis to assess 

 the potential impact of cod predation 

 on snow crab recruitment are dis- 

 cussed. 



Differential Selection of Crab 

 Chionoecetes opilio and Hyas spp. as 

 Prey by Sympatric Cod Gadus morhua 

 and thorny skate Raja radiata 



David A. Robichaud 



Department of Fisheries and Oceans, Biological Station 

 St Andrews, New Brunswick, EOG 2X0 Canada 



Robert W. Elner 



Department of Fisheries and Oceans. Biological Station 

 St Andrews. New Brunswick, EOG 2X0 Canada 

 Present address: Canadian Wildlife Service 

 Box 340. Delta, British Columbia B4K 3Y3 Canada 



Richard F.J. Bailey 



Department of Fisheries and Oceans, Biological Station 

 St Andrews. New Brunswick, EOG 2X0 Canada 



Present address: Department of Fisheries and Oceans, Institut Maurice-Lamontagne 

 PO Box 1000. Mont-Joli, Quebec. G5H 3Z4 Canada 



Manuscript accepted 2 May 1991. 

 Fishery Bulletin, U.S. 89:669-680 (1991). 



Cod Gadus morhua and snow crab 

 Chionoecetes opilio which in 1983 

 supported fisheries valued at $17.4 

 and $29.1 million, respectively, form 

 two of the most important fisheries 

 in the Gulf of St. Lawrence (Anon. 

 1982; Elner and Bailey 1986). The 

 snow crab resource is heavily ex- 

 ploited, and the fishery has become 

 almost entirely dependent on the 

 recruitment of crab reaching com- 

 mercial size each year (Bailey and 

 Elner 1989). Little is known about 

 the factors affecting snow crab re- 

 cruitment. Past studies on the east 

 coast of Canada have established a 

 predator:prey linkage between cod 

 and snow crab (Waiwood et al. 1980, 

 Waiwood 1981, Bailey 1982, Wai- 

 wood and Elner 1982, Lilly and Rice 



1983, Lilly 1984, Lilly and Botta 



1984, Waiwood and Majkowski 

 1984). Bailey (1982) suggested that 

 cod predation could be a major fac- 

 tor controlling the number of snow 

 crab entering the fishery each year. 

 However, Waiwood and Elner (1982) 

 suggested that cod do not control 

 recruitment of crab into the fishery, 



but simply react to the availability of 

 crabs as compared with other prey 

 species. 



Although cod has been identified as 

 an important predator of snow crab, 

 the ecological mechanisms involved 

 in the interaction are not well under- 

 stood. By analyzing cod stomach con- 

 tents, this work provides fresh in- 

 sights into cod predation on crab in 

 nature. We also discuss the problems 

 associated with using stomach con- 

 tents analysis as a means of assess- 

 ing the impact of cod predation on 

 snow crab recruitment, and evaluate 

 the use of stomach contents as a 

 biological tool for sampling snow 

 crab. For comparative purposes, 

 skate Raja radiata, a known preda- 

 tor on crab (Templeman 1982), and 

 two additional crab prey species com- 

 monly recognized as toad crab (Hyas 

 araneus and H. coarctatus) were con- 

 sidered in the study. 



Materials and methods 



Over the period 21 April 1981 to 29 

 September 1982, 933 cod and 119 



669 



