Bottom-Water Temperature Trends in the Middle 

 Atlantic Bight During Spring and Autumn, 1964-76 



CLARENCE W. DAVIS 1 



ABSTRACT 



Annual variations of bottom-water temperatures on the continental shelf between Cape Cod and 

 Cape Hatteras were examined for the spring and autumn from 1964 to 1976. Temperatures generally 

 were highest since 1972 during both seasons. For waters between Cape Cod and Hudson Canyon, 

 maximum temperatures occurred in the spring of 1976 (8.3°C) and autumn of 1972 (12.8°); between 

 Cape Hatteras and Hudson Canyon, temperatures peaked in the spring of 1974 (9.0°C) and the autumn 

 of 1972 (15.7°C). Minimum temperatures were in the spring of 1970 (5.1°C) and autumn of 1967 (9.7°C) 

 and in the spring (4.6°C) and autumn (12.1°C) of 1970 in the respective areas. 



Some factors that affect bottom-water temperatures, the relationship of the observed tempera- 

 ture variations to temperature fluctuations in other areas of the northwest Atlantic, and the effects of 

 temperature changes on the distribution of several fish species are briefly discussed. 



INTRODUCTION 



The purpose of this study was to examine bottom- 

 water temperature data collected since 1964 in the area 

 between Cape Cod, Mass., and Cape Hatteras, N.C., to 

 reveal annual variations and any possible warming or 

 cooling trends that may have occurred. Since bottom- 

 water temperatures are less affected by localized and 

 short-term atmospheric phenomena than sea-surface 

 temperatures, they can be linked with the sources of the 

 subsurface water (and thus the direct cause of temper- 

 ature variations) during warming and cooling periods 

 (Colton 1968). Recent papers by Bowman and Wunder- 

 lich (1977) and Wright (1976) and the classic works of 

 Bigelow (1933) and Bigelow and Sears (1935) provide 

 background information to separate water masses and 

 to establish the boundary between shelf water and slope 

 water in the area discussed in this paper. The shelf- 

 slope water boundary is usually in close proximity to the 

 100 m isobath in all seasons and can be represented by 

 the 10°C isotherm (Wright 1976); these criteria will be 

 used in part to define the causes of observed tempera- 

 ture changes since 1964. A paper by Colton and Stod- 

 dard (1973) based on 1940-66 mean temperatures can be 

 used for comparing annual and long-term mean tem- 

 perature conditions. 



Changes in physical and chemical properties of the 

 oceans, including temperature, can directly or indi- 

 rectly affect fish and shellfish in numerous ways, but 

 generally, there is a noticeable lack of ecological under- 

 standing of temperature effects (Brett 1969). Beltz et al. 

 (1974) observed that over 7,000 papers have presented 

 evidence of biothermal relationships, but compara- 

 tively few of these involved bottom-water temperatures 

 in the marine environment. Probably the best single ref- 



'Northeast Fisheries Center Woods Hole Laboratory, National Marine 

 Fisheries Service, NOAA, Woods Hole, MA 02543. 



erence on the effects of the thermal environment on 

 marine fisheries are the contributions to a symposium 

 held in Rome in 1964 by the International Commission 

 for the Northwest Atlantic Fisheries (1965). Collectively 

 these papers emphasize the need for basic ecological 

 knowledge essential for an adequate understanding of 

 events in the life history of fishes. The use of long-term 

 temperature data may be helpful, either by itself or as a 

 contributing factor in quantitative models, in gaining 

 this understanding. 



Because potential users of the following data may re- 

 quire actual temperature values instead of graphical 

 data, certain of the figures and tables involve some re- 

 dundancy of my analyses. The additional information is 

 considered justified if it will encourage others to ex- 

 amine and use the presented data. 



DATA AND METHODS 



The data were collected during the period 1964-76 on 

 surveys designed and conducted by the National 

 Marine Fisheries Service, Woods Hole, Mass., (Gross- 

 lein 1969) to determine the distribution and relative 

 abundance of groundfish at randomly selected stations, 

 from Cape Hatteras to Cape Cod (Fig. 1). Approxi- 

 mately 125 bottom-water temperatures were obtained 

 during each cruise, mostly at depths between 30 and 200 

 m, with either mechanical or expendable bathythermo- 

 graphs, the latter being used after 1969. 



For analytical purposes, this area, known as the 

 "Middle Atlantic Bight" (Bigelow 1933), was divided 

 into subareas: Cape Cod to Hudson Canyon, termed the 

 Northern Bight; and Hudson Canyon to Cape Hatteras, 

 termed the Southern Bight. There was no sampling 

 south of Hudson Canyon prior to the autumn of 1967 

 and spring cruises did not commence until 1968. 



Methods and rationale for analysis are fully de- 

 scribed by Davis (1978). Briefly, the indices derived for 



