338 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



manent populations. Since 1930, months with 

 temperatures below 32° have been rather infre- 

 quent, and the persistent spread of the green crab 

 northward would seem to indicate that sufficient 

 numbers have survived the more severe winters 

 to assure a more or less permanent population in 

 the northern part of its range. 



ROLE OF TEMPERATURE IN FAUNISTIC 

 CHANGES 



We doubt whether any marine biologist today 

 would dispute that temperature of the water is the 

 factor chiefly responsible for setting geographical 

 boundaries to the ranges of marine animals along 

 the seaboard of eastern North America. Conse- 

 quently, any alteration in the temperature or any 

 continuing trend, cither upward or downward, 

 would be of great importance both ecologically, 

 and from the commercial fisheries standpoint, if 

 the alteration in temperature is wide enough and 

 if it persists long enough to affect successful 

 reproduction of the species or the well-being of 

 the individual. 



Air temperature records, and the more limited 

 sea temperature records, show that the warming 

 trend since 1900 has not been steady, but ratlier 

 has been interrupted by periods of cooling (fig. 1). 

 In general, the amplitude of these fluctuations has 

 shown a marked decrease since about 1880 — the 

 result largely of rises in the minima. The pattern 

 of increase within years also shows that the 

 winter minima have risen while the summer 

 maxima have increased relatively more slowly. 

 The amplitude of fluctuations in winter tempera- 

 tures has, however, been quite wide (fig. 4). 



These characteristics of the warming trend are 

 important in considering the kind of faunistic 

 changes which might be expected to result. Among 

 the possible changes, we may consider the follow- 

 ing categories: 



1 . Establishment of resident populations north 

 of Cape Cod of species that formerly were summer 

 migrants. 



2. Northern extension of ranges of resident 

 populations already established north of Cape Cod. 



3. Northern extension of ranges of summer 

 migrants. 



4. Changes in seasonal movements and distri- 

 bution of permanent residents. 



5. Changes in dates of arrival and departure 

 of summer migrants. 



6. Geographical changes in the abundance of 

 permanent residents. 



In boreal latitudes, where the air climate is 

 rigorous in winter but warm in summer, causing 

 a wide seasonal difference in temperature of the 

 water (as happens in the Gulf of Maine), the crit- 

 ical questions are, How cold is the water during 

 the coldest month of the year? and How warm is 

 it during the warmest month? Perhaps it is not 

 amiss to remind the reader that the mean temper- 

 ature for the 3'ear as a whole plays only a minor 

 role in this regard in the Gulf, or in similar situa- 

 tions in general, except perhaps in the case of spe- 

 cies that live at depths so great that they pass 

 their entire lives within a very narrow tempera- 

 ture range. 



We recognize, then, that changes in categories 

 1 and 2 depend primarily on the ability of the 

 animal in question to withstand winter conditions, 

 since temperatures suitable for spawning, though 

 necessary, are not alone sufficient for the estab- 

 lishment of resident populations. In category 3, 

 the primary factor is undoubtedly the degree of 

 summer warming, and may depend on conditions 

 during only 1 or 2 months in the year, without 

 reference to any overall trend in temperature. 

 Categories 4, 5, and 6 depend to a much greater 

 extent on the complex of conditions in all seasons 

 of the year and on secular trends in temperature. 



It has long been common knowledge that no 

 shoal-water animal could maintain a permanent 

 population in estuarine situations around the shores 

 of the Gulf, or even close in along the open coast, 

 unless it were either hardy to temperatures nearly 

 as low as the freezing point of salt water or able 

 to escape the severity of winter by moving a few 

 fathoms deeper along the sloping bottom. As an 

 illustration, the mean for the coldest month was 

 below freezing point of fresh water in 9 winters 

 between 1922 and 1942 at Boston, Mass., and in 

 3 winters between 1934 and 1943 at Eastport, 

 Maine. But there has not been a winter since 1948 

 when the local fauna was exposed to a mean tem- 

 perature lower than 36° F. for as long as a month 

 either in Boston Harbor or at Eastport — localities 

 whicli taken together are representative of the 

 western-northern periphery of the Gulf. 



Since the temperature of the water at the coldest 



