RICHARDS ET AL.: BEHAVIORAL INTERACTION'S OF AMERICAN LOBSTER AND CANCER CRABS 



ments equally. The nonrandom arrangement of 

 treatment levels within strings could have biased 

 catch rates through gear competition. However, 

 we feel the assumption that equal numbers of 

 animals were attracted to all traps is valid for the 

 following reason. If gear competition caused the 

 reduced crab catches in lobster-stocked strings, 

 a similar pattern of catch rates would have been 

 seen in crab-stocked strings. This was not the 

 case. 



Cancer irroratus is a prey item for lobsters 

 (Squires 1970; Weiss 1970; Scarratt and Lowe 

 1972; Ennis 1973), suggesting that the decreased 

 catch of this species in traps containing lob- 

 sters may be the result of predator-avoidance be- 

 havior. Cancer borealis and H. americanus are 

 thought to compete for shelter space in rocky 

 subtidal habitats (Stewart 1972; Fogarty 1976; 

 Cooper and Uzmann 1977; Wang 1982). In labo- 

 ratory studies (Fogarty 1976), H. americanus 

 dominated C. borealis for possession of shelter. 

 This dominance appeared to be the result of 

 avoidance by C. borealis rather than overt ag- 

 gressive interactions. Such behavior may cause 

 reduced catches of C. borealis in traps containing 

 lobsters. 



The reduction in lobster CPUE when lobsters 

 were stocked is not surprising since lobsters are 

 known to be highly aggressive and generally in- 

 habit shelter alone under natural conditions 

 (Cobb 1971; Cooper and Uzmann 1980). Trap sat- 

 uration apparently becomes important for lob- 

 sters at relatively low catch levels since traps 

 stocked with 8 and 3 lobsters were equally effec- 

 tive in reducing the lobster catch. In a laboratory 

 experiment reported by Smolowitz (1978), a re- 

 duction in trap entry was seen with only 1 or 2 

 lobsters in the trap. Reduced entry was probably 

 important in the present study since escapement 

 of stocked lobsters was low (10.1%). 



Stock rates used for crabs were low compared 

 with crab catches in control traps. At higher den- 

 sities, crabs might have had a more significant 

 effect on the catch of lobsters. An increased lob- 

 ster catch might be expected in traps containing 

 C irroratus, a lobster prey item (Squires 1970; 

 Weiss 1970; Ennis 1973; McLeese 1974). How- 

 ever, the presence of live prey may not signifi- 

 cantly increase the attractiveness of an already 

 baited trap. No evidence was seen of lobster 

 predation on crabs in traps. Similarly a decrease 

 in lobster catch might be expected in traps con- 

 taining a competitor (C. borealis). However, C. 

 borealis is less aggressive than H. americanus 



(Fogarty 1976; Wang 1982) and occupies mutu- 

 ally desirable shelters through passive means 

 rather than active displacement, as shown in 

 Stewart's (1972) study. 



Trap saturation apparently was not an impor- 

 tant factor for crabs at the stock levels used, since 

 crab catches in crab-stocked traps were not re- 

 duced below the level of control traps. In labora- 

 tory observations, Miller (1978, 1979a, 1980) 

 noted that intraspecific agonistic interactions 

 among C irroratus, Hyas araneus, and C. pro- 

 ductus aggregating downstream from baited 

 traps often resulted in departure from the trap 

 area. He suggested that trap saturation in these 

 three species was due in part to "intimidation" of 

 crabs outside the trap by those inside. However, 

 at relatively low catch densities, the effects of 

 aggression may be minimal. 



The increased C. borealis catch in traps stocked 

 with 3 crabs of either species is difficult to ex- 

 plain. Release of attractants from the bait by 

 feeding activity could enhance trap entry. As 

 crab density inside the trap increases, such 

 enhancement may be countered by increased 

 aggression, reducing trap entry rates and in- 

 creasing escapement. These speculations do not 

 explain why the C. irroratus catch was not simi- 

 larly increased by a low stock density of either 

 crab species. 



Behavior 



Location Within Trap 



Behavioral interactions apparently affected 

 the spatial distribution of animals in traps. A 

 greater proportion of the crab catch was found in 

 the kitchen when 8 lobsters were stocked in the 

 parlor. This may have been the result of the avoid- 

 ance responses discussed above and may enhance 

 escapement of crabs from traps containing lob- 

 sters. Cancer borealis shifted to the kitchen in 

 both density levels of C. irroratus-stocked traps, 

 but the distribution of C. irroratus changed sig- 

 nificantly only in traps stocked with 3 C. bore- 

 alis. Perhaps the generally greater activity of 

 C. irroratus (Jeffries 1966; pers. obs.) serves as a 

 deterrent to parlor entry by C. borealis. Both spe- 

 cies may be influenced by prior residence effects 

 in which an advantage is conferred upon the in- 

 dividual^) initially utilizing a resource (e.g., 

 Sinclair 1977; Davies 1978; O'Neill and Cobb 

 1979). Such an effect may have been caused by 

 the stocking procedure. 



57 



