Learning all you can about the dynamics of the Gulf of Mexico will greatly increase your haul from the “big water.”
EDITOR’S NOTE: This is part one of a two-part series. The author is a fisheries biologist and Louisiana State University professor.The Gulf of Mexico is a small, almost landlocked oceanic basin that is essentially an arm of the Atlantic Ocean. Its total surface area is approximately 615,000 square miles, the southern third of which lies in the tropics.
The greatest distance across the Gulf is more than 1,100 miles from west to east. The shortest distance is 500 miles, between the Mississippi River Delta and the northern tip of Mexico’s Yucatan Peninsula.
More than 24 percent of the area of the Gulf is deep ocean basin, almost 2 miles deep. Its deepest spot is the Sigsbee Deep, at 12,714 feet. Actually a 300-mile trench, the Sigsbee Deep is located about 200 miles southeast of Brownsville, Texas, off of the northern coast of Mexico.
The island of Cuba loosely plugs the open southeastern corner of the Gulf. Water enters the basin from the Caribbean Sea with Caribbean Current through the 100-mile wide Yucatan Channel off the west end of Cuba. While narrow, the channel is 5,400 to 6,200 feet deep.
The waters refresh the entire Gulf with the Loop Current and all its complex eddies and countercurrents. The Loop Current reaches speeds of 4.5 miles per hour.
As current meanders break off of the main Loop Current, they form large eddies. Those that spin clockwise (anticyclonic) cause warm water to flow toward the center of the eddy and sink. Because of their low-nutrient water, anticyclonic eddies are considered marine deserts.
These anticyclonic eddies, in turn, spin off counterclockwise or cyclonic eddies. Cyclonic eddies spin up from the depths of the Gulf, bringing to the surface nutrients that increase marine life.
The waters of the Gulf are also profoundly affected by the massive discharge of fresh water and sediment from the Mississippi and Atchafalaya rivers.
Besides affecting salinities and water clarity, the discharge from the rivers injects a plume of nutrients into the Gulf. The algae bloom triggered by these nutrients supports immense quantities of forage fish, such as anchovies, which in turn attract and support large numbers of predator fish. The microscopic animal zooplankters that graze on the algae also serve as a food supply for the larvae of predator fishes.
The discharge from the rivers makes the northern Gulf an important fisheries nursery ground. Because of the river discharge and Louisiana’s huge expanse of marsh, the area from Mississippi Sound to Sabine Lake is often referred to as the “Fertile Fisheries Crescent.”
Water leaves the Gulf through the 125-mile wide, 2,600-foot deep Straits of Florida, between the northern coast of Cuba and the southern tip of Florida. This Florida Current, as it is known, merges with the Antilles Current to form the Gulf Stream, one of the strongest ocean currents known anywhere.
Continental shelves make up 35 percent of the area of the Gulf. These gently sloping bottoms begin at the shoreline and extend out to waters approximately 600 feet deep. From there out, the bottom rapidly falls away on the continental slope until reaching the deep ocean basin.
The vast majority of fisheries productivity occurs in the waters of the continental shelves. The Gulf continental shelf is narrowest off of the Mississippi River Delta at about 6 miles wide, and widest in southern Florida at over 180 miles wide, and off of the Texas-Louisiana border where it is 124 miles wide.
The U.S. portion of the Gulf of Mexico continental shelf can be divided into three sections, the West Florida Shelf, the Mississippi-Alabama Shelf and the Louisiana-Texas Shelf.
The West Florida Shelf extends from the southern tip of Florida to the DeSoto Canyon, and is one of the widest continental shelves in the world. The Desoto Canyon is a deep and wide notch that cuts into the shelf south of Alabama, running northeasterly, ending off of Pensacola, Fla.
Sediments of the West Florida Shelf are mostly composed of carbonate sands, mixed in some areas with substantial amounts of quartz sands. Carbonate sediments are the broken remains of mollusks, sponges, corals, algae and foraminifera.
The West Florida Shelf also has hundreds of hard bottom areas, some of which allow the growth of tropical reef life. Most of the hard-bottom areas that project from the bottom provide excellent fishing sides.
South of Panama City are two major areas of high relief, the Whoopee Grounds and the Mud Banks. The Whoopee Grounds are found in 215 to 250 feet of water, and have rock ledges over 25 feet high. Much of the ledge has coral and coral-like invertebrate growths encrusting it. The Mud Banks is a ledge with a steep 16- to 22-foot drop located in 190 to 205 feet of water. The ledge is approximately 40 miles long.
Madison-Swanson is a 74,000-acre area south of Panama City in 200 to 330 feet of water. Within the area, habitats range from low-relief patches of reef 1 1/2 to 8 feet high to ridges and pinnacles over 50 feet high. Much of this is encrusted with sponges, sea fans, sea whips and coral-like growths.
The Florida Middle Ground, a famous fishing area, is a 380,000-acre area of hard bottom and high relief found 100 miles northwest of Tampa, Fla. The area contains rough and irregular limestone cliffs, and knobs that rise over 40 feet above the sandy bottoms around the Middle Ground in 85 to 160 feet of water.
Off of Tarpon Springs, Fla., are the Steamboat Lumps. This hard-bottom area of more than 66,000 acres is relatively low relief. It does, however, provide excellent fish habitat.
Farther to the southern part of the Gulf on the Florida Shelf are the Dry Tortugas, an area of banks about 70 miles west of Key West. One of the banks is high enough to create seven small islands, also known as the Dry Tortugas. The area of the Dry Tortugas is huge, covering about 750 square miles. The banks are made up of limestone and ancient reefs. Near the Dry Tortugas are the Tortugas Bank and Riley’s Hump.
The Mississippi-Alabama Shelf extends from the DeSoto Canyon westward to the Mississippi River Delta. The sediments in the eastern part of the area tend to be more carbonate, while nearer the Mississippi River Delta the sediments become mostly terrigenous, or derived from land.
Among the most interesting bottom features of the area are small peaks of cemented-together sediments called “pinnacles.” Their average height is 20 feet, with some as tall as 50 feet. The greatest numbers of these pinnacles are in depths of 330 to 370 feet. It is thought the pinnacles were formed by the cementing action of algae in periods of lower ocean levels.
West of the pinnacles area are dense fields of reef-like mounds from 3 to 3,000 feet in diameter and 3 to 60 feet in height. Most of these are in water depths of 240 to 270 feet. Also found west of the pinnacles are low ridges that run parallel to shore in 220 to 250 feet of water. The highest of these ridges is only 26 feet, with most of them being much smaller.
Hard bottoms are also located in more nearshore waters on the Mississippi-Alabama shelf. Southeast Banks are located about 17 miles offshore, south-southwest of Mobile Bay, in 69 to 87 feet of water. Southeast Banks is a rock rubble field with about 13 feet of relief.
Located 10 1/2 miles southwest of Dauphin Island is Southwest Rock. This is a rock outcrop 23 to 29 feet wide and 3 to 5 feet high in water depths of 65 to 72 feet. Twenty-three miles south of Mobile Bay is the 17 Fathom Hole, a 160-foot deep depression in the bottom, surfaced with rock rubble, shell and course sand. It is in water depths of 98 to 105 feet.
Big Rock and Trysler Grounds are located approximately 28 1/2 miles offshore of the Alabama-Florida state line. Big Rock is a large mound, 16 feet high. Trysler Grounds is a 6- to 10-foot high area made up of small rocks. Big Rock and Trysler Grounds are in 98 to 105 feet of water.
The Louisiana-Texas Shelf is largely covered with muddy or sandy sediments eroded from land sources and deposited by the Mississippi River. This sediment deposition has taken place in the last 5,000 years, since the latest sea level rise.
The Mississippi River (including its distributary, the Atchafalaya River) delivers 220 million tons of sediment to the Gulf per year. As large as this number is, it is only half of what the river carried at the time that the United States was settled by European colonists. The reduction in sediment load has mostly happened since 1950, with construction of large dams on the Missouri and Arkansas rivers.
These huge sediment deposits are up to 49,000 feet thick offshore of the Louisiana-Texas state line. The sediments cover a layer of rock salt almost 10,000 feet thick. The weight of the sediments has caused the salt beneath to flow and push up through weak places in the overlying sediments, forming banks, or “lumps” as they are sometimes called by fishermen.
Over 130 of these flat-topped mountain-shaped banks are found on the Louisiana-Texas coast. One of the most well-known is Sackett Bank near the Mississippi River Delta, known to fishermen as the Midnight Lump.
The East and West Flower Gardens located off of the Louisiana-Texas state line are the highest banks, coming near enough to the water’s surface to be encrusted with true hard corals. Carbonate rock and rubble covers most of the tops and sides of the banks.
The waters of the Gulf of Mexico are strongly affected by the Mississippi River, which discharges 420 billion gallons of fresh water per day into the Gulf. This discharge ranks the Mississippi as the seventh largest river in the world.
The open waters of the Gulf, uninfluenced by the Mississippi River, has salinities of 36.0 to 36.5 parts per thousand (ppt). One study measuring salinities in the northwestern Gulf showed that during May, the lowest-salinity month, salinities of all nearshore waters off Louisiana and Texas to Galveston Bay were less than 24 ppt. During August, the highest-salinity month, the only water less than 30 ppt was a very narrow nearshore band off central Louisiana. Bottom waters of the mid and outer shelf remained fully marine all year.
In the eastern Gulf, the lowest salinity reading in May was 31.5 ppt, just off the Mississippi barrier islands. All other eastern Gulf readings were 33 ppt or higher. Below Tampa Bay, all waters were 36 ppt or above, except for local areas off of Charlotte Harbor and the Everglades. In August, the pattern was very similar, except that the lowest-salinity water was 33 ppt off of Cedar Key.
Dissolved oxygen levels in the Gulf of Mexico average about 5 parts per million (ppm) in the summer, which is when they are lowest. Water with dissolved oxygen levels at or below 2 ppm is considered hypoxic, or severely oxygen-depleted.
Areas of hypoxic water occur annually from mid-May through mid-September in bottom waters on the continental shelf off Louisiana. This hypoxic area, often called the “Dead Zone,” is triggered by nitrogen and phosphorus in the discharge of the Mississippi River. The lighter, nutrient-rich fresh water from the river forms a layer over denser marine waters. The vast nutrient supply in the discharge water triggers large algae blooms.
The remains of these algae and the feces of the zooplankton that graze on the algae sink to the bottom, where they reduce oxygen levels when they decay.
The hypoxic bottom water does not mix with the oxygenated layer above it because of salinity and temperature differences, causing stratification. The hypoxic zone in the northern Gulf can reach 4.5 million acres in size, and usually persists until tropical storms in the late summer or cold fronts in the fall force water mixing.
Besides influencing salinity and dissolved oxygen, the Mississippi/Atchafalaya River system discharges enough muddy water to create turbidity as much as 65 miles down-current from the delta.
Another type of turbidity found in the western Gulf of Mexico is the layer of muddy water found along the sea bottom, called the nepheloid layer. At times 30 to 50 feet thick, the nepheloid layer is created by the re-suspension of muddy sea floor sediments by swift bottom currents and turbulence.
It may, in shallow water, extend from the bottom to the surface, but is usually overlaid by clear water. A nepheloid layer is commonly found on the continental shelf of Louisiana west of the Mississippi River and on the Texas coast. The West Florida Shelf, with its absence of silt and clay on the bottom, and with little river runoff, has much clearer water than the Louisiana-Texas Shelf.
The gradually-deepening slope of the continental shelf ends at about 600 feet deep. From there, the bottom very rapidly falls away to the relatively flat abyssal (deep sea) plain over 3,200 feet deep. The area of steeply sloping bottom is known as the continental slope. In total, the continental slope makes up about 20 percent of the total area of the Gulf.
The most common sediment type on the continental slope is silty clay, found in all parts of the Gulf. Some pure clay sediments are found in slope areas of the western Gulf, but not the eastern Gulf. Eastern Gulf slope sediments have more sand in them than western Gulf slope sediments.
Less plankton, the basis for marine food webs, is found in waters over the continental slope and in the open Gulf than in continental shelf waters. In the area north of the Yucatan Channel and in the DeSoto Canyon region, deep ocean waters ride up the continental shelf to the surface. These nutrient-rich waters spur plankton growth. In general, however, waters of the western Gulf are more productive than waters of the eastern Gulf.
Be sure to pick up the June issue of Louisiana Sportsman, in which the author will explore the specific types of habitat that exist in the Gulf and how they impact fish and anglers.
