SIZE COMPOSITION AND GROWTH OF YOUNG ROCK CRAB, 

 CANCER IRRORATUS, ON A ROCKY BEACH IN MAINE^ 



Jay S Krouse^ 



ABSTRACT 



Monthly hand collections of small rock crab, Cancer irroratus, were made from an intertidal zone in 

 East Boothbay, Maine, from June 1972 through April 1975. An analysis of size and sex frequencies 

 indicated: 1) young-of-the-year crabs (<5 mm carapace width) entered the intertidal area in late 

 summer-early fall and remained there through the second fall with a resultant width range between 15 

 and 40 mm; 2) a deceleration and/or cessation of growth in winter; 3) an emigration of crabs >40 mm 

 carapace width from the intertidal area associated with declining winter temperatures and/or 

 behavioral changes; 4) sex ratios approximated a 1:1 relationship; and 5) small male and female rock 

 crabs (<60 mm carapace width) had a common growth rate. 



While searcing beneath the rocky substrate of an 

 intertidal zone for juvenile American lobster, 

 Homarus americanus Milne Edwards, whose early 

 distribution and abundance is generally unknown, 

 I discovered numerous small rock crab, Cancer 

 irroratus Say, burrowed under the rubble. 

 Because rock crab is a valuable commercial species 

 as well as an important food source of lobsters 

 (Ennis 1973), I believe it important to describe the 

 distribution of young crabs in their natural envi- 

 ronment along with other life history information 

 (size structure, sex ratio, and growth). 



METHODS 



Rock crabs were carefully hand collected about 

 once a month during extreme low slack tides from 

 the intertidal zone of Grimes Cove, East Boothbay, 

 Maine (Figure 1). The rocky substrate of this 

 unsheltered seaward cove consists of rocks of 

 assorted sizes intermingled amongst areas of 

 bedrock, sand, and pulverized shells. By using 

 large boulders as landmarks, it was possible to 

 sample consistently the same general area near 

 the low water mark. Unfortunately, for various 

 reasons, samples could not be obtained for all 

 months of the study. 



After two biologists concurrently expended 1 h 

 gathering crabs, their catches were immediately 

 returned to the laboratory where sex and carapace 



'This study was conducted in cooperation with the National 

 Marine Fisheries Service, NOAA, Department of Commerce, 

 under Public Law 88-309, as amended, Commercial Fisheries 

 Research and Development Act, Project 3-153-R. 



'Maine Department of Marine Resources, West Boothbay 

 Harbor, ME 04575. 



Manuscript accepted April 1976. 



FISHERY BULLETIN: VOL. 74, NO. 4, 1976. 



width (distance between the two most posterior 

 notches on the anterolateral border) to the nearest 

 millimeter were recorded. The sex of crabs < 10 mm 

 carapace width (CW) was determined under a 

 dissecting microscope. 



Width-frequency histograms were compiled by 

 2-mm increments for rock crabs caught each 

 month from June 1972 through April 1975. 



RESULTS AND DISCUSSION 

 Size Composition and Seasonal Distribution 



Since there were no discernible differences in 

 size distribution between male and female crabs, 

 the data for sexes were combined in monthly 

 width-frequency histograms (Figure 2). This 

 similarity in size composition of male and female 

 crabs <60 mm CW suggested a common growth 

 rate for both sexes up to this size, unlike the 

 marked size disparity of larger male and female 

 rock crabs (>60 mm CW) caught in commercial 

 lobster traps which was primarily attributed to a 

 decrease in the growth rate of females after the 

 onset of sexual maturity (Krouse 1972). 



Modal groups, which most likely represented 

 one or, perhaps, more molt classes, were quite 

 conspicuous in each of the monthly histograms. 

 However, due to extensive overlapping of modes I 

 was unable to quantitatively follow these modal 

 groupings from month to month for purposes of 

 estimating mortality rates. 



Inspection of monthly histograms revealed that 

 young-of-the-year crabs (recently metamorphosed 

 from megalops to first crab, <5 mm CW) initially 



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