FISHERY BULLETIN: VOL. 81, NO. 1 



Competition Inside Traps 



During scuba diving observations of lobster 

 traps, Pecci etal. (1978) noted an apparent domi- 

 nance of crabs over lobsters in occupation of 

 mutually desirable "niches" in traps. They re- 

 ported that when both crabs and lobsters were 

 present in traps, crabs always occupied positions 

 that were evidently preferred by both species. 

 The observations of this study contradict those 

 of Pecci et al. Both crab species were displaced 

 by lobsters. It is possible that our results re- 

 flect a prior residence advantage conferred on 

 lobsters by the stocking procedure. However, 

 our findings agree with what is known of the 

 relative aggressiveness of H. americanus, C. 

 borealis, and C. irroratus (Fogarty 1976; Wang 

 1982). 



Escapement could be a significant factor in re- 

 ducing the efficiency of traps. Skud 5 considered 

 this the most likely explanation for declining 

 catch rates for lobster over time. High escape 

 rates for two species of Cancer have been ob- 

 served by Miller (1979b) and High (1976). In this 

 study, escape of both crab species from the 

 kitchen increased when lobsters were present in 

 the parlor, probably due to the behavioral inter- 

 actions described above. Escape of crabs from 

 the parlor did not increase when lobsters were 

 stocked. This may reflect both the design of the 

 parlor head, which makes escape more difficult, 

 and the small sample size resulting from a low 

 rate of entry to the parlor. 



In summary, the behavioral mechanisms in- 

 volved in reducing crab catches in traps contain- 

 ing lobsters were 



Trap Entry and Escapement 



In the laboratory, the presence of H. america- 

 nus in a trap did not affect the number of C. irro- 

 ratus entering the kitchen, but did decrease the 

 number of C. boreal is entering. Just the opposite 

 might have been expected in light of the preda- 

 tor-prey relationship between C. irroratus and 

 H. americanus. We observed no interactions be- 

 tween animals inside the trap and those outside; 

 thus the sensory basis for avoidance by C. bore- 

 alis of traps containing lobsters is unknown. 



The proportion of C. irroratus moving from 

 the kitchen to the parlor was reduced in lobster- 

 stocked traps. The decrease in parlor entry rate 

 for C. borealis was not statistically significant; 

 however, the number of C borealis that had 

 entered the kitchen was relatively low. Reduced 

 parlor entry appeared to be the direct result of 

 interactions between animals in the two trap 

 compartments. These typically consisted of a lob- 

 ster displaying (meral spread) or lunging at a 

 crab climbing up the parlor head, resulting in 

 retreat to the kitchen by the crab. In several in- 

 stances, crabs hanging from the parlor head con- 

 tacted a lobster, which responded by displaying 

 or attacking the crab. The crab then pulled back 

 up into the parlor head and returned to the 

 kitchen. General lobster activity (fighting, ex- 

 ploring, etc.) had a similar effect on crabs in the 

 parlor head. Only 24% of C. irroratus and 10% of 

 C. borealis entering the parlor head actually 

 entered the parlor when lobsters were stocked. 

 Parlor entrants increased to 60% and 67%, re- 

 spectively, in control traps. 



1 ) For C. borealis, entry to the trap is reduced, 

 and escapement of those that enter the 

 kitchen is increased. 



2) For C. irroratus, trap entry is not reduced, 

 but entry to the parlor decreases and rate 

 of escape from the kitchen increases. 



SUMMARY 



This study demonstrated that behavioral inter- 

 actions between animals attracted to traps can 

 have significant effects on the probability of 

 their capture. The CPUE of American lobsters 

 and of two species of commercially harvested 

 Cancer crabs was significantly reduced in traps 

 containing lobsters. Such effects may be density- 

 dependent, since significantly fewer crabs were 

 caught in traps containing 8 lobsters than in 

 traps containing 3 lobsters. The proportion of 

 captured crabs occupying each trap section 

 changed significantly when lobsters were 

 stocked, and behavioral observations indicated 

 that lobsters occupy the mutually preferred posi- 

 tions in traps. The behavioral mechanisms re- 

 sponsible for decreased crab catches included 

 both reduced entry (C. borealis) and increased 

 escapement (C. irroratus and C. borealis). These 

 results reflect the behavioral and ecological rela- 

 tions of the three species. 



5 Skud, B. E. 1976. Soak-time and the catch per pot in an 

 offshore fishery for lobsters (Homarus ameriranus). ICES 

 Special Meeting on Population Assessments of Shellfish Stocks, 

 No. 8, 25 p. 



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