present in the Gulf of Maine and may either divert into 
Cape Cod Bay or turn eastward along the northern edge 
of Georges Bank. Similarly a clockwise (anticyclonic) 
eddy over Georges Bank causes a persistent westerly drift 
along the southern edge of the Bank which continues 
across Great South Channel. During autumn the 
southern portion of the Gulf eddy breaks down into a 
drift across the Bank, thus affecting temperatures there 
(Bumpus and Lauzier 1965). Since the Bank is well 
mixed vertically by tidal and wind forces throughout 
most of the year, subsurface temperatures there are in- 
fluenced largely by air-sea interactions and by advection 
of deeper waters. 
The identification of temperature anomalies on an 
areal and seasonal basis, along with other supportive 
data, is useful for interpreting changes in biological 
phenomena observed during these years. Examples of 
such changes are discussed in this paper. 
METHODS 
Bottom temperature data are based on mechanical or 
expendable bathythermograph observations obtained 
randomly during spring and autumn groundfish surveys 
(Grosslein‘). During each cruise approximately 75-100 
observations were made in the Gulf of Maine, mostly in 
water depths of 100-275 m, and 50-60 observations on 
Georges Bank (Fig. 1). Only those waters shallower than 
‘Grosslein, M.D. 1974. Bottom trawl survey methods of the North- 
east Fisheries Center, Woods Hole, Massachusetts, USA. Int. Comm. 
Northwest Atl. Fish. Res. Doc. 74/96, 27 p. 
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100 m on Georges Bank are considered in this report 
because of high temperature variability of greater depths 
caused by sporadic excursions of warm slope water along 
the Bank’s southern edge. Raw temperature obser- 
vations are assumed to be actual bottom temperatures 
because only a few stations in the inner Gulf of Maine ex- 
ceeded the maximum depth capability of 275 m of 
mechanical bathythermographs. Since temperatures 
below 250 m in most of the Gulf are generally isothermal 
with depth, 275-m temperatures are representative of 
bottom temperatures. Expendable bathythermographs, 
capable of reaching the greatest depths encountered, 
were used after 1969. 
Both the Gulf of Maine and Georges Bank were analyz- 
ed in their entirety and by subareas which are bands of 
one degree of longitude in width (Fig. 1). Subareas in the 
Gulf are identified by Roman numerals I-V and Georges 
Bank subareas are termed Western, Central, and 
Eastern Georges Bank. Analysis by one-degree segments 
of longitude was chosen as they define rather distinct 
physiographic regions of the Gulf and Bank (Table 1) 
and also this was a convenient arbitrary method of estab- 
lishing segment boundaries to show suspected tempera- 
ture differences and trends in varous parts of the study 
area. Analysis of temperatures by latitude was not at- 
tempted, but latitudinal variability has been docu- 
mented for both the shoal and deep waters of the Gulf of 
Maine (Bigelow 1927) and Georges Bank (Sigaev’*). 
Contoured isotherms at 1°C intervals were overlaid on 
5-min grid charts and the number of units counted to 
determine the percentage area represented by each 2°C 
temperature class interval (TCI) (Figs. 2, 3). An index of 
the mean seasonal bottom temperature was then cal- 
culated by multiplying the midpoint temperature of each 
Sigaev, I. K. 1974. Characteristic features of the hydrological con- 
ditions on the Nova Scotia Shelf and Georges Bank, 1972. Int. Comm. 
Northwest Atl. Fish. Res. Doc. 74/51, 7 p. 
Table 1.—Gulf of Maine and Georges Bank subarea characteristics. 
Area Subarea Characteristics 
Gulf of 
Maine: I Coastal,< 200 m, Jeffreys Ledge and Stell- 
wagen Bank 
Ul Western Basin, some banks and ledges 
I Intermediate between II & IV 
IV Eastern Basin, mostly>200 m 
V Coastal western Nova Scotia, entrance North- 
west Channel 
Georges 
Bank: Western Cultivator Shoal, adjacent Great South Chan- 
nel 
Central Georges Shoal, no adjacent channels 
Eastern Mostly>60 m, adjacent Northeast Channel 
Figure 2.—Distribution of spring bottom-water temperatures, 1968- 
75. (Dotted areas represent Georges Bank temperatures less than 
4°C; gridded areas represent Gulf of Maine temperatures greater 
than 8°C.) 
Figure 1.—Gulf of Maine-Georges Bank and subarea boundaries 
used in analysis of bottom-water temperature data. (Dots represent 
typical distribution of BT stations). 
is) 
