Minimum Depth
8.50m
Maximum Depth
14.00m
Minimum Temperature
13.00°C
Maximum Temperature
30.00°C
Minimum Salinity
28.00
Maximum Salinity
36.00

From: Tunnell, 1969; Rezak et al., 1985
Seven and One Half Fathom Bank is a small topographic prominence located on the continental shelf of the northwestern Gulf of Mexico. It is 74 km south of the northwestern entrance to Padre Island National Seashore and 3.2 km offshore from Central Padre Island. The reef is an elongate structure with four distinct rises, which are oriented at an approximate 45° to Padre Island. The second rise from the south with a relief of 5.4m is greater in all dimensions than the other three rises. It used to be a freshwater lake during the last ice age when sea level was 350 feet lower. The bank is highly turbid to fairly clear water conditions and lies in an area of converging currents bathed by coastal waters from the more tropical southwestern Gulf in the summer and the northern gulf in the winter. Some molluscs, tropical fishes and invertebrates frequent this bank. There are near-shore oil platforms and jetties in this area. Recreational scuba diving and fishing are common activities.

Community Structure

Seven and One Half Fathom Bank is a small sandstone prominence and is largely covered by mats of tube-building polychaete worms, sponges, ascidians and hydroids. The reef bears an assemblage of molluscs and crustaceans of mixed warm-temperate and tropical affinities and a substantial number of species in common with the south Texas jetties and other near shore banks further north such as Herald Bank off Galveston. Sponges, hydroids, bryozoans and algae have encrusted the upper levels of the reef to such an extent that bare rock is scarcely visible.

Geological Characterization
Seven and One Half Fathom Bank crests at about 8.5 m depth from the surrounding bottom of 14m. The bottom surrounding the reef has a noticeable amount of scouring, evident on either side. The beach side of the reef has been deeply scoured while the scour on the Gulf side is not as evident because of the build up of organic material and mud. The rocky areas between the major rises of the reef consist of large segments of rock shifted from the main structure. Tabular to rounded in shape, these rise to 1.5 to 2 m above the bottom. Numerous small outcrops of rock of one to five square meter surface area dot the fringes of the reef.