Figure 3. — Standard trap used for sampling green crab populations. The deep, sheet^metal funnel prevents escape. 

 The outside dimensions are 81 cm. long by 61 cm. wide by 34 cm. high. 



as a measure of abundance, was calculated for 

 each month and each location. 



Occasional shore surveys in the intertidal zone 

 supplemented trap catches. The surveys were at 

 low tide in widely scattered areas where trai)s 

 were not being fished and where there was good to 

 e.xcellent cover for crabs. Dining a timed search, 

 requiring at least 1 man-hoiu- of effort, all crabs 

 that could be found in the open, under seaweed 

 or stones, or in burrows, were collected, se.xed, 

 counted, and measured. Abundance was e.xpressed 

 as the number of crabs found ])er man-hour of 

 effort expended. 



CHANGES IN ABUNDANCE OF 

 GREEN CRABS 



In 1953-67 green crab i)o])ulations were samjiled 

 to detect major changes in abundance. Because the 



month of maximum catch varied from year to 

 j^ear and place to place, I have used only the 

 maximum monthly mean catch per trap per day 

 for each year when comparing years and areas 

 (fig. 4). Individual trap catches at any location 

 during one monthly trapping period (3 trajjs for 

 2 days, or 6 catches) showed an average variation 

 of ± 15 percent around the mean, with an extreme 

 of ±40 percent. This degree of variabihty in catch 

 limits the quantitative value of the trapi)ing sur- 

 veys, but the trends were consistent with general 

 observations and the results of the shore surveys. 

 The jiossibility of reducing crab populations by 

 trapping was tested by fishing 18 traps daily for 

 36 consecutive days in a 5.7-hectare (14-acre) cove. 

 A total of 33,760 crabs was removed at an average 

 catch rate of 52 crabs per traj) per day, but there 

 was no significant reduction in numbers by the 



340 



U. S. FISH AND WILDLIFE SERVICE 



