crabs nor do we know what those conditions were. 

 A few days after the crabs were found, tempera- 

 ture, saUnity, and dissolved oxygen in tlie water 

 between the ice cover and bottom were within the 

 normal range for green crabs. Winter mortality 

 was sufficiently heavy to reduce trapping and shore 

 collections during the following summer to about, 

 one-tenth the catches of the pre\aous year. 



During the winter of 1958-59 BCF biologists 

 observed another ^\•inter kill in southwestern 

 Maine. Monthly mean water temperatures from 

 Jamuxry through March 1959 were the lowest for 

 all 3 months since 1948. Monthly mean air tem- 

 peratm'es for December and February were the 

 lowest for those months since 1917 and 1934, 

 respectively (U.S. Environmental Science Services 

 Administration, 1967). In the tidal marshes in 

 Wells, where naany crabs overwinter intertidally 

 in biuTows in banks of Spartina sod, we found 

 hundreds of dead crabs of both sexes and all sizes. 

 Trap catches in the marslies during the 1959 sea- 

 son were much smaller than in previous years, and 

 the proportion of small crabs in the catch was 

 much higher. This last observation seems to 

 confirm Waugh's (1964) and Crisp's (1964) ob- 

 servations that the smaller individuals of the 

 species suffer less from the effects of low tem- 

 perature than do the larger ones. 



CONCURRENT CHANGES IN CLAM 

 ABUNDANCE 



Although the primary purpose of this paper is 

 to report on changes in green crab abundance, the 

 importance of the species as a clam predator 

 justifies mention of changes that have taken place 

 in Maine soft-shell clam stocks as a result of 

 decreased predation. Glude (1955) demonstrated 

 correlations among the increase of green crabs, the 

 decrease of soft-shell clams, and the increase in 

 water and air temperatures. Because of the over- 

 whelming abundance of crabs, he predicted a bleak 

 futiu-e for the clam industry if the weather 

 continued mild. 



During clam population surveys in the period 

 of heavy predation, BCF biologists consistently 

 found evidence of adequate annual sets of clams; 

 however, each year class was regularly destroyed 

 as the individual clams became large enough for 

 the crabs to find. Nevertheless, the reproductive 

 potential existed for the recovery of clam stocks 



at any time when predation either decreased 

 naturally or was controlled by man. 



The decreasing abundance of crabs during the 

 late 1950's caused a lessening in the effects of 

 ]3redation on the clams and resulted in an increas- 

 ing supply of clams. Catches of clams have im- 

 proved markedly in recent years in the two south- 

 western counties of Maine and to a lesser extent 

 in the rest of the State (fig. 5). In the north- 

 eastern counties, where the decrease in crabs has 

 been most pronounced, catches have also in- 

 creased, but to a far less extent because of an 

 unfavorable market. Reports from clam diggers, 

 coastal wardens, and the resident State biologist, 

 have indicated that clam Hats in the northeastern 

 area are well popidated with market-size clams 

 (fig. 2). During the com-se of the crab surveys, 

 I have confirmed these reports by sampling a 

 number of clam fiats. Year classes of the past 

 3 to 5 years were represented consistently in the 

 samples, and usually sufficient market-size clams 

 were present to make clam digging worthwhile. 



SUMMARY OF EFFECTS OF TEMPERA- 

 TURE ON ABUNDANCE OF GREEN 

 CRABS 



Carcinus maenas is markedly affected by 

 temperature changes. Documentation is sufficient 

 to show that mass mortalities are often associated 

 \\'ith unusually cold winters and that ^dde- 

 spread increases and decreases in abundance are 

 associated with long-term warming and cooling 

 trends. 



The direct correlation of abundance of green 

 crabs with water temperature and the inverse 

 correlation of abundance of clams with abundance 

 of crabs is compared in figure 2. The close relation 

 of the very low February temperatures of 1934 to 

 the scarcity of crabs later in the same year as 

 observed by Goucher (see footnote 1), and the 

 ensuing increase in the number of clams is especi- 

 ally notable. 



There are many ways in which this crab may be 

 favorably affected by higher temperature, either 

 directly or indirectly. Scattergood ( 1952) pointed 

 out that numerous means have long been available 

 to aid in the spread of the green crab northward — 

 for examjjle, the transportation of larvae by ocean 

 currents, or of adults in fish and lobster shipments. 

 Survival and successfid reproduction did not 



CHANGES IN GREEN CRAB ABUNDANCE IN RELATION TO RECENT TEMPERATURE CHANGES 



343 



