Seasonal Description of Winds and Surface and 



Bottom Salinities and Temperatures in the Northern 



Gulf of Mexico, October 1972 to January 1976^ 



PERRY A. THOMPSON, JR.- and THOMAS D. LEMING' 

 ABSTRACT 



Seasonal surface and bottom salinities and temperatures in the northern Gulf of Mexico are de- 

 scribed. The area surveyed, from October 1972 to January 1976, was between Mobile Bay, Ala. (long. 

 88°00 W), and Atchafalaya Bay, La. (long. 9r"30W), from 5 to 50 fathoms (9 to 91 m). 



INTRODUCTION 



Published hydrographic data from the Gulf of Mexico 

 generally deal with the central gulf rather than the con- 

 tinental shelf. Rivas (1968), Drennan (1968), and Wise- 

 man et al. (1976) studied surface temperature in the cen- 

 tral gulf, subsurface circulation over the shelf, and 

 effects of the Mississippi River discharge on temperature 

 and salinity of the inner shelf west of the Mississippi 

 River. Many studies have been completed on tem- 

 perature and salinity observations and seasonal trends 

 on the shelf of the northern Gulf of Mexico. The most re- 

 cent study for the northwestern Gulf of Mexico is that 

 of Temple et al. (1977) and for the northeastern that of 

 Christmas et al.' 



Because the northern shelf of the Gulf of Mexico is the 

 richest fish producing area in the gulf, considerable ef- 

 fort should be directed toward understanding the hy- 

 drography and its effects on the environment of marine 

 life. In 1972 the National Marine Fisheries Service 

 started a program to evaluate the industrial and food- 

 fish demersal fishery in the northern Gulf of Mexico and 

 to provide associated environmental information for the 

 area. Environmental variables recorded during ground- 

 fish cruises were temperature, salinity, and bottom type. 

 These variables can be correlated with densities of fin- 

 fish later to determine their influence on abundance and 

 distribution of the finfish stocks. 



In this paper we give a brief description of surface and 

 bottom temperatures and salinities and establish their 

 general seasonal variability in the northern gulf. Tem- 

 perature appears to be an important factor in the move- 



'Contribution No. 77-24P from the -Southeast Fisheries Center Pasca- 

 goula Laboratory, National Marine Fisheries Service, NOAA. 



■Southeast Fisheries Center Pascagoula Laborator,', National Marine 

 Fisheries Service, NOAA. Pascagoula, MS .39567. 



'Southeast Fisheries Center National Fisheries Engineering Labora- 

 tory, National Marine Fisheries Service, NOAA, NSTL, Bay St. Louis, 

 MS 39520. 



'Christmas, J. Y., A. Perry, and R. S. Waller, 1974. Investigations 

 of coastal pelagic fishes completion report. Gulf Coast Research Labora- 

 tory. Ocean Springs, Miss., 90 p. 



ment of juvenile groundfish from the estuaries (Franks et 

 al. 1972). Determination of seasonality would be 

 beneficial in understanding this aspect of the life history 

 of groundfish in the northern Gulf of Mexico. 



SURVEY AREA 



Initially the primary fishing grounds encompassed the 

 area from 5 to 50 fathoms (9 to 91 m) between Perdido 

 Bay, Fla. (long. 87°30'W), and .Ship Shoal, La. (long. 

 91°30'W) (Roithmayr 1965). This area was later reduced 

 to between Mobile Bay, Ala. (long. 88°00'W), and Ship 

 Shoal, La. (long. 91°30'W), because of the low density of 

 groundfish east of Mobile Bay (Gutherz et al. 1975). The 

 primary area was divided into three survey segments 

 (Fig. 1): 1) east of the Delta, SAl (long. 88°00' to 

 89°00'W); 2) off the Delta, SA2 (long. 89°00' to 90°00'W); 

 and 3) west of the Delta, SA3 (long. 90°00' to 91°.30'W). 



METHODS AND PROCEDURES 



For each cruise, 35 to 45 hydrographic stations were 

 selected both east and west of the mouth of the 

 Mississippi River. These stations were preselected from 

 fishing stations along transect lines at 20' longitude in- 

 tervals. If no fishing stations were located along a tran- 

 sect line, then the closest fishing stations to the transect 

 were selected. Hydrographic stations were separated by 

 10 to 15 km along the transect line. Because of the ran- 

 dom selection of fishing stations, the probability of 

 repeating the same hydrographic station for each cruise 

 was low. Stations were located by loran-A or radar. 



Equipment used to acquire data on the various cruises 

 is listed in Table 1. All equipment was calibrated at the 

 beginning of each cruise. Temperatures were measured 

 with an expendable bathythermograph (XBT) and 

 recorded to the nearest 0.1°C on cruises 40 to 52. Salinity 

 data were not taken during cruises 40 to 48. On cruise 52 

 surface and bottom salinities were recorded to the 

 nearest part per thousand (ppt) using a Goldbert T/C 

 refractometer. On cruises 55 to 64, salinity and tem- 



