106 



Abstract-Thf natural diet of 506 

 American lobsters iHomarus america- 

 niis) ranging from instar V (4 mm 

 cephalothorax length. CLi to the adult 

 stage (112 mm CD was determined 

 by stomach content analysis for a site 

 in the Magdalen Islands, Gulf of St. 

 Lawrence, eastern Canada. Cluster and 

 factor analyses determined four size 

 groupings of lobsters based on their 

 diet: <7.5 mm, 7.5 to <22.5 mm, 22.5 to 

 <62.5 mm, and >62.5 mm CL. The onto- 

 genetic shift in diet with increasing 

 size of lobsters was especially appar- 

 ent for the three dominant food items: 

 the contribution of bivalves and animal 

 tissue (flesh) to volume of stomach con- 

 tents decreased from the smallest lob- 

 sters (2871 and 399r, respectively) to the 

 largest lobsters (2'* and 11'^, respec- 

 tively), whereas the reverse trend was 

 seen for rock crab Cancer irroratus il'.i 

 in smallest lobsters to 539* in largest 

 lobsters). Large lobsters also ate larger 

 rock crabs than did small lobsters. This 

 study is the first to examine the natural 

 diet of shelter-restricted juveniles (SP{Js, 

 <14.5 mm CLi, which were thought to be 

 principally suspension feeders and to a 

 lesser degree browsers or ambush pred- 

 ators in or near their shelter However, 

 at our study site no planktonic organ- 

 isms were identified from the stom- 

 achs of SRJs, whereas formaniferans, 

 crustacean meiofauna, and macroalgal 

 debris that could be derived by brows- 

 ing, together represented only 10-14'(^ 

 by volume of stomach contents. We infer 

 that SRJs obtained bivalves by pre- 

 dation and flesh by exploiting larger 

 lobsters' meal scraps or food resei-ves. 

 Some implications of these findings for 

 lobster arti,ficial reef programs and for 

 the conservation of lobster stocks are 

 discussed. 



Ontogenetic shifts in natural diet during benthic 

 stages of American lobster iHomarus amen'canus), 

 off the Magdalen Islands 



Bernard Sainte-Marie 

 Denis Chabot 



Division des invertebres et de la biologie expenmentale 



Institut Maurice-Lamontagne 



Peches et Oceans Canada 



850 route de la Mer 



Mont-Joh (Qc), G5H 3Z4 Canada 



E-mail address (for B Sainte Mane) Sainte Maneadfo mpo gc ca 



Manuscript accepted 3 October 2001. 

 Fish. Bull. 100(l):106-116i2002). 



American lob.ster, Homaiiis amcrica- 

 niis, is a long-lived, dominant predator 

 in temperate coastal waters of eastern 

 North America (Elner and Campbell, 

 1991; Ojeda and Dearborn, 1991). After 

 the lar\'al phase, lobsters settle and 

 spend much of their time in burrows or 

 natural shelters (Cobb, 1971; Lawton, 

 1987; Barshaw and Bryant-Rich, 1988). 

 However, laboratory and in situ obser- 

 vations indicate that benthic lobsters 

 pass through successive life-history 

 phases as they grow in size, changing 

 from a shelter-restricted habit to a more 

 overt lifestyle involving daily forays and 

 seasonal migrations away from shelter 

 (Cooper and Uzmann, 1977; Cobb and 

 Wahle, 1994). A variety of classifica- 

 tions have been proposed for these suc- 

 cessive ontogenetic phases. The latest 

 scheme, by Lawton and Lavalli ( 1995), 

 recognizes five life-history phases: shel- 

 ter-restricted juvenile (SRJ, -4-14 mm 

 cephalothorax length, CL), emergent 

 juvenile (-15-25 mm CL), vagile juve- 

 nile (-25 mm CL to size of physiological 

 maturity), adolescent, and adult. 



In several decapod crustaceans, diet 

 changes as individuals grow and be- 

 come more mobile and their chela size 

 and strength increases (e.g. Lee and 

 Seed, 1992; Freire et al., 1996). Such 

 dietary shifts should occur in the lob- 

 ster as well, especially considering this 

 species' changing dependency on shel- 

 ter which, in turn, has implications 

 for foraging range and accessibility of 

 prey types (Elner and Campbell, 1987; 

 Lawton, 1987). Some studies of the nat- 

 ural diet of lobsters 12-125 mm CL 

 have found little or no differences in 



the identity or in the frequency of food 

 items that were ingested by different 

 size groups (Weiss, 1970; Ennis, 1973; 

 Hudon and Lamarche, 1987). However, 

 other studies have pointed to changes 

 in the identity and especially in the fre- 

 quency of food items ingested by dif- 

 ferent lobster size groups. Carter and 

 Steele ( 1982b). using their own results 

 and data from nonconcomitant studies 

 conducted at different sites in New- 

 foundland (Squires, 1970; Ennis, 1973), 

 have suggested that lobsters of 12-73 

 mm CL consume sea urchins, ophi- 

 uroids, and mussels more frequently 

 than larger (adult) lobsters. Scarratt 

 ( 1980) reported that lobsters consumed 

 more crabs, mussels, and fish, but fewer 

 echinoderms. as they grew in size and 

 approached maturity. This trend was 

 attributed to differential accessibility 

 of prey. Elner and Campbell (1987) in- 

 dicated that the stronger chelae of larg- 

 er lobsters would enable them to crush 

 prey that are protected by heavy shells, 

 such as gastropods and bivalves, more 

 so than the chelae of smaller lobsters. 



The natural diet of SRJ lobsters has 

 not been examined to date (Lawton 

 and Lavalli, 1995). e.xcepting rare spec- 

 imens of 12-14 mm CL. The feeding 

 appendages of SRJs are capable of 

 capturing and processing both plank- 

 tonic and benthic organisms ( Lavalli 

 and Factor, 1995). From laboratory ob- 

 servations, several authors have pro- 

 posed that SRJs may live primarily as 

 suspension-feeders, and to a lesser de- 

 gree as browsers, within the shelter or 

 as ambush predators at the shelter's 

 entrance (Barshaw and Brvant-Rich, 



