CHANGES IN ABUNDANCE OF THE GREEN CRAB, CARCINUS MAENAS (L.), 

 IN RELATION TO RECENT TEMPERATURE CHANGES 



BY WALTER R. WELCH, Fishery Biologist 



BUREAU OF COMMERCL\L FISHERIES BIOLOGICAL LABORATORY 



W. BOOTHBAY HARBOR, MAINE 04575 



ABSTRACT 



Trap catches, shore surveys, and fishermen's reports 

 demonstrated that declining abundance of green crabs 

 in northern New England was associated with par- 

 ticularly cold winters and a cooling trend in the water. 

 Since the mid-1950's, the once great abundance of 

 green crabs in northeastern Maine has declined to a 

 scarcity or total absence. Abundance in southwestern 

 Maine and in Massachusetts has fluctuated widely, 

 and moderate abundance in Rhode Island decreased 

 consistently through 1959. 



Mass mortalities of green crabs in winter coincided 



The green crab, Carcinus maenas (L.j, has 

 undergone major changes in abundance and dis- 

 tribution along the northeast Atlantic Coast since 

 the late 1800's. Scattergood (1952) traced the 

 spread of the species northward to Casco Bay in 

 soutliwestern Maine by tlie early 1900's, to scat- 

 tered localities in central Maine by the late 1930's, 

 and finally into far northeastern Maine and New 

 Brunswick by 1951. He concluded that green 

 crabs, if j^resent at all in northeastern Maine, were 

 scarce in 1939-42. Glude (1955) narrowed the 

 period of greatest increase of green crabs along the 

 Maine Coast to 1948-51 and recorded their spread 

 to Wedgeport, Nova Scotia, by 1954 (fig. 1). The 

 Fisheries Research Board of Canada (1961) re- 

 jjorted that the species had extended its range as 

 far east as Lockeport, Nova Scotia, 64 km. beyond 

 Cape Sable. 



The green crab is of minor commercial impor- 

 tance as bait for sport fishermen south of Cape 

 Cod. A limited fishery in Maine, New Hampshire, 

 and Massachusetts sui)])lies this market. 



The major interest in the species in the United 

 States lies in its im])ortance as a j^redator. Dow 

 and Wallace (1952j jjointed out the significance of 

 the green crab as a predator on the soft-shell clam 

 (Mya arenaria). Glude (1955) and MacPhail, 

 Lord, and Dickie (1955) presented evidence to 

 show the seriousness of predation by green crabs on 



Published December 1968. 



FISHERY BULLETIN: VOL. 67, NO. 2 



with periods of severe cold. The general abundance 

 and commercial catch of the soft-shell clam (Mya 

 arenaria), a common prey of the green crab, increased 

 markedly during the period of decline of the crabs. 



Although the decrease in abundance of green crabs 

 is clearly associated with lower temperatures, the 

 relation is not necessarily direct; there are a number 

 of ways in which low temperature, in combination 

 with lowered salinity, low dissolved oxygen, and other 

 unidentified factors, may affect abundance. 



clams in Maine, New Brunswick, and Nova 

 Scotia. 



The purposes of this paper are to document 

 changes since the peak of abundance in the mid- 

 1950's and to relate these changes to concurrent 

 changes in temjjerature. 



SOURCES OF INFORMATION 



I have used data from three sources: (1) Rec- 

 ords of surface sea-water temperature kept at 

 Boothbay Harbor from 1905 through 1967; (2) 

 estimates of the abundance of green crabs and 

 soft-shell clams by biologists, wardens, and fisher- 

 men; and (3) quantitative sampling of green crab 

 populations. 



RECORDS 



Records of surface sea-water temperature were 

 kejjt at the BCF Biological Laboratory, Boothbay 

 Harbor, Maine, from 1905 tlirough 1967. From 

 1905 through 1949, thermometer readings were 

 made thrice daily at the station wharf. From 1950 

 on, instruments have recorded the temperature, 

 sensed at 1.68 m. below mean low water. Infre- 

 quent gaps in the recorded data were filled in from 

 temperatures read in flowing water tanks. Daily, 

 monthly, and annual means were derived from the 

 three daily thermometer readings and hourly 

 readings from the recordings. Figure 2 shows the 



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