Sainte Marie and Chabot NatLiial diet of Haniaiui ameiicaniis off tlie Magdalen Islands 



111 



reflect mainly changing lobster preferences and differen- 

 tial accessibility of prey types. 



Ontogenetic shifts in diet 



There was clear evidence of a progressive dietary shift with 

 increasing lobster size at our study site. Smaller lobsters 

 relied to a greater extent than larger lobsters on soft or 

 easily acquired food items (flesh, sessile juvenile bivalves, 

 macroalgae, meiobenthic crustaceans, and foraminiferans). 

 Larger lobsters fed on bigger, more mobile and also more 

 nutritious prey, including crustaceans that were protected 

 by heavy shells, and fish. Fishes were probably taken by 

 predation (see Weiss, 1970) because there was no fishing 

 activity at or near our study site that might have provided 

 lobsters with fish bait or discards. 



The most striking ontogenetic changes in volumetric 

 contribution of prey types occurred for rock crab and bi- 

 valves, the former increasing from 0.07 to 0.53 and the lat- 

 ter decreasing from 0.28 to 0.02 from the smallest to the 



largest lobster size class, respectively (Fig. 5). Only lim- 

 ited comparisons with other studies are possible, given the 

 differences in methods and in the size range of lobsters 

 examined. However, the observed trends of increasing im- 

 portance of rock crab and of decreasing importance of bi- 

 valves with increasing lobster size were consistent with 

 the analyses of Scarratt (1980) and of Carter and Steele 

 ( 1982b), and they suggest that lobsters are not simply op- 

 portunistic or unspecialized feeders (see Elner and Camp- 

 bell, 1987). 



Multivariate analysis of lobster diet resulted in size 

 groupings that are quite consistent with Lawton and La- 

 valli's (1995) size classification of the early life-history 

 phases based on a broad set of behavioral and ecological 

 criteria. Major shifts in diet in the present study occurred 

 at about 7.5, 22.5, and 62.5 mm CL (Fig. 2). The two clas- 

 sifications differ in the smaller size for the transition from 

 the first to second group (7.5 mm in our diet-based classifi- 

 cation compared with 14.5 mm CL in Lawton and Lavalli's 

 scheme), but the size for transition from the second to the 



