114 



Fishery Bulletin 100(1) 



juvenile lobsters reared on mesozooplankton than in wild 

 lobsters, suggesting that the latter incorporated more nu- 

 tritious foods into their diet. 



The finding that early juvenile lobsters are primarily 

 predators or scavengers, if confirmed by studies at other 

 sites, has implications for the development and implemen- 

 tation of artificial reefs. Such structures are increasingly 

 being considered as a means to enhance lobster produc- 

 tivity on traditional grounds or to expand lobster habitat 

 onto less hospitable grounds (e.g. Gendron, 1998). The car- 

 nivorous benthic feeding mode of SRJs and of emergent ju- 

 veniles at our site implies that successful reefs will have to 

 be designed, localized, and weathered so that they are ini- 

 tially well colonized and subsequently regularly colonized 

 by benthic prey that are easily accessible and of high nu- 

 tritional value to juvenile lobsters. Additionally if SRJs 

 and emergent juveniles derive some protective and nutri- 

 tional benefits from the presence of larger conspecifics, 

 reefs designed to offer shelter to a full suite of lobster sizes 

 may prove to be more productive in the long term than 

 reefs offering shelter only to small lobsters. 



Importance of rock crab to lobster 



Several previous studies have noted the importance of 

 rock crab in the diet of lobster (Reddin, 1973; Evans 

 and Mann, 1977; Carter and Steele, 1982al. Boghen et 

 al. (1982) found that juvenile lobsters survived and grew 

 better on a diet containing crab protein alone than on a 

 diet of live brine shrimp iAiienua salina) or of protein 

 extracts from urchin iStrongylocentrotus droebachiensi.';). 

 mussel (Mytilus ediilis). or shrimp iPenaeus sp.). Gendron 

 et al. (2001) found that condition, somatic gi-owth, and 

 gonadal development of lobster increased with increasing 

 amount of rock crab in diet. In nature, even SRJs may ben- 

 efit from a diet including large amounts of rock crab pro- 

 tein because they preyed directly on very small rock crabs 

 (Figs. 4 and 5), and the tissue boluses they contained may 

 have been that of rock crab (see above). 



We were able to establish a positive size relationship 

 for lobster preying on rock crab (Fig. 6). The smallest rock 

 crab prey were 2-6 mm CW and belonged to the first ben- 

 thic instars of this species. In our study, apparently no 

 rock crabs larger than 50 mm CW were consumed by lob- 

 sters, and the maximum ratio of crab CW over lobster 

 CL was 0.90, even though rock crabs up to 120 mm CW 

 were seen (own personal diving obsei-vations). In the labo- 

 ratory Weiss (1970) observed that lobsters of 60-80 mm 

 CL attacked crabs offered in the size range of 62-78 mm 

 CW. Lawton and Lavalli (1995) reported that juvenile lob- 

 sters can subdue juvenile intermolt rock crabs up to ap- 

 proximately 0.40 times their own body size. Their obser- 

 vation was based on the comparison of predator and prey 

 wet masses; when expressed in terms of crab CW over lob- 

 ster CL, the maximum ratio was equivalent to about 1.27.' 

 This ratio of prey CW to predator CL is somewhat larger 

 than that derived from our stomach analvses. Because lob- 



Lawton, P. 2000. Personal comniun. Fisheries and Oceans 

 Canada, St. Andrews, New Brunswick, Canada. 



sters probably ingest only soft tissue when the prey-pred- 

 ator size ratio is sufficiently high (Weiss, 1970; and see 

 above ), our analysis of rock crab prey-size frequencies may 

 correctly estimate the minimum prey size but underesti- 

 mate the maximum prey size and the volumetric contribu- 

 tion and occurrence of rock crab in the diet of any given 

 lobster size class. Nevertheless, the present study clearly 

 shows that all lobster size classes rely on rock crab as food 

 and that the size spectrum of rock crab that is used by lob- 

 sters is broad and includes even those at the settlement 

 stage. Given the much greater economic value of lobster 

 in relation to rock crab, and the trophic dependency of the 

 former on the latter, caution should be exercised in devel- 

 oping rock crab fisheries (Gendron and Fradette, 1995). 



Acknowledgments 



We thank our diving partners F. Hazel. J.-G. Rondeau, J. 

 A. Gagne, K. Gravel, R. Larocque, J.-F. Lussier, N. Faille, 

 and A. Rondeau. We are particularly gi-ateful to J. Hudon 

 for her major contribution to the identification of stomach 

 contents and to three anonymous reviewers for construc- 

 tive comments. This is a contribution to the Canadian 

 Atlantic-Wide Lobster Studies (CLAWS) research initia- 

 tive of Fisheries and Oceans Canada. 



Literature cited 



Aiken, D. E. 



1980. Molting and gi-owth. In The biology and manage- 

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1996. A new approach to graphical analysis of feeding 

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1988. Response of megafaunal predators to synchronous 

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Barshaw, D. E. 



1989. Growth and survival of early juvenile American lob- 

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