Seasonal Bottom-Water Temperature Trends in 
the Gulf of Maine and on Georges Bank, 1963-75 
CLARENCE W. DAVIS! 
ABSTRACT 
Spring (1968-75) and autumn (1963-75) bottom-water temperatures in the Gulf of Maine and on 
Georges Bank were analyzed to investigate a suspected warming trend in the region. During the 
spring the mean temperature in the Gulf of Maine increased rather steadily from a low of 5.4°C in 1968 
to a high of 6.4°C in 1974. Various subareas of the Gulf had more frequent and greater oscillations but 
exhibited the same overall warming trend. Mean spring temperatures on Georges Bank fluctuated 
from 3.8°C in 1968 to 6.3°C in 1974 and declined by nearly 2°C in 1975 with similar characteristics in 
eastern, central, and western subareas of the Bank. 
During the autumn in the Gulf, bottom-water temperatures reached a minimum of 5.4°C in 1966, 
increased to a maximum of 8.4°C in 1973 and 1974, but declined to 8.0°C in 1975. Subareas of the Gulf 
generally showed the same temperature trends from 1963 to 1968; especially notable are the cooling 
trend west of long. 69°W which commenced in 1971, and a decrease in all five subareas in 1975. Georges 
Bank temperatures in autumn declined from a maximum of 13.1°C in 1965 to a minimum of 10.4°C in 
1969, reached another peak of 12.6°C in both 1973 and 1974, but declined to 11.6°C in 1975. Subareas of 
Georges Bank generally followed the same pattern with the eastern third of the Bank usually 2°C or 
colder than either of the other subareas in the autumn. 
The average bottom-water temperatures during spring were 5.0°C on Georges Bank and 6.1°C in 
the Gulf of Maine; temperatures in the autumn were 11.7°C and 7.2°C, respectively, for these areas. 
INTRODUCTION 
This study summarizes variations in bottom-water 
temperatures observed during spring (1968-75) and 
autumn (1963-75) groundfish surveys conducted by the 
Gulf of Maine-Georges Bank area by the National 
Marine Fisheries Service, Woods Hole, Mass. Relatively 
high temperatures were observed during 1973 and 1974 
and raised the question of whether there had been a sig- 
nificant upward trend in average temperatures or simply 
a couple of anomalous years since the surveys began in 
1963. 
Recent papers by Colton and Stoddard (1973), Colton 
(1968a), and Schopf (1967) have summarized the dis- 
tribution of bottom-water temperatures from 1940 to 
1967 in the Gulf of Maine and contiguous waters. A con- 
temporary paper by Karaulovsky and Sigaev’? sum- 
marizes similar data for 1962-72 and provides an inter- 
mediate comparison between the earlier papers and this 
present study. These previous studies are not strictly 
comparable with the present study because of the 
variability of the data bases and different analytical 
methods; nevertheless, comparisons are useful for the 
identification of long-term trends, and the analyses are 
considered adequate for approximating annual changes 
in spring and autumn bottom temperatures in the Gulf of 
Maine and on Georges Bank. 
‘Northeast Fisheries Center, National Marine Fisheries Service, _ 
NOAA, Woods Hole, MA 02543. 
*Karaulovsky, V. P., and I. K. Sigaev. 1976. Long-term variations 
in heat content of the waters on the Northwest Atlantic Shelf. Int. 
Comm. Northwest Atl. Fish. Res. Doc. 76/2, 9 p. 
Numerous authors, including Dow (1966, 1969) and 
Sutcliffe et al. (1977) for Gulf of Maine waters, have used 
surface-water temperatures to correlate fish catch with 
environmental factors. Flowers and Saila (1972) used 
both surface and bottom temperatures to estimate yield 
of American lobsters, Homarus americanus, and found 
the latter parameter considerably more accurate. 
Although a study of bottom-water temperatures alone 
represents only a partial analysis of the temperature 
structure of the region, they are sufficient to reveal major 
changes in the thermal environment and are par- 
ticularly relevant to the distribution of benthic 
organisms. The remainder of the temperature profile, 
from surface to near bottom, is not included in this 
study. Salinity profiles are also excluded since only sur- 
face and bottom data were routinely obtained until the 
most recent years. For these reasons, specific identifica- 
tion of subsurface water masses is speculative. It is 
known, however, that the major source of subsurface 
water in the Gulf of Maine is the inflow of relatively 
warm slope water through the Northeast Channel 
(Bigelow 1927; Colton 1968b). Therefore, major changes 
in the average bottom-water temperatures in much of the 
Gulf may be closely related to changes in the volume and 
extent of this inflow. 
Georges Bank water is derived largely from the Gulf of 
Maine, but it is also sporadically influenced by intru- 
sions of slope water along its southern boundary (Bum- 
pus’). In the spring a counterclockwise (cyclonic) eddy is 
‘Bumpus, D. F. 1975. Review of the physical oceanography of 
Georges Bank. Int. Comm. Northwest Atl. Fish. Res. Doc. 75/107, 32 p. 
