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 the second installment in a two-part series. Part one appeared in the May issue.Aside from unvegetated soft mud, sand or clay bottoms, which are habitat types of a sort themselves, the inshore and offshore waters of the Gulf of Mexico contain especially productive habitats.
The eight habitat types of special importance are marshes, seagrasses, mangroves, oyster reefs, live/hard bottoms, artificial structures, coral reefs, and sargassum. Each will be discussed in more detail.
Marshes
Gulf of Mexico marshes consist of emergent grasses and sedges rooted in soft sediment bottoms. They are regularly flushed by gentle tides, get at least some freshwater input and are not exposed to strong wave action. Salt marsh vegetation is dominated by Spartina alterniflora, commonly called smooth cordgrass, or oyster grass. At very slightly higher elevations and lower salinities, the dominant vegetation is marsh hay cordgrass, usually called wire grass, and needle rush or needle grass.
Coastal marshes are extremely productive of marine life, but the challenge of dealing with constantly changing salinities, ranging from nearly fresh to very saline, is very stressful.
The tremendous productivity of marshes is based not on the marsh grasses being directly grazed upon. Rather, as the dense stands of plants grow and die, they break down into detritus, which serves as fuel for the web of life in marshes. Additionally, algae and other microorganisms that grow on the grasses are also eaten.
The marsh grass itself also provides complex shelter for young fish, shrimp, crabs and other sea life.
More than half of the wetlands in the United States are found on the Gulf of Mexico, and 66 percent of that is salt marsh. The Mississippi River Delta is the largest single wetland system in the country, with nearly 725,000 acres of emergent wetlands, although large amounts of it are being lost to subsidence and erosion.
Away from the northcentral and northwestern Gulf, the influence of salt marshes is reduced. Marshes fringe the Florida coast eastward in a narrowing band to Cedar Key. South of Cedar Key, salt marshes mix with mangroves in declining amounts until mangroves dominate the southern Florida Gulf Coast.
In central and southern Texas, marshes are sparse and are mostly made up of plants other than grasses and sedges. They are completely absent from the Laguna Madre because of very high salinities, lack of freshwater inflow and high summer temperatures. There, they are replaced by unvegetated sand flats.
Seagrasses
Seagrasses, also called submerged aquatic vegetation, or SAV for short, are vascular (non-algal) plants that spend their entire life cycle (except for blossoms in some species) submerged in saline waters. Salt water is a very hostile environment for vascular plants, and only five species make up virtually all of the SAV in the Gulf region: shoalgrass, clover grass, manatee-grass, widgeon grass and turtle grass.
Most species of seagrasses spread by runners or root sprouts, and can make dense beds of one species called “seagrass meadows.” Most seagrass meadows also include many species of algae. Algae and seagrass don’t compete with each other. Algae take their nutrients directly from the surrounding water, while seagrasses rely on the sediments for their nutrients. Too many nutrients in the water, however, can cause dense growths of algae to smother seagrasses.
Seagrass meadows are very productive habitats. The dense growths of SAV provide hiding places and shelter for small animals and the young of larger predator fish. They also reduce wave action and erosion, promote water clarity, increase bottom area with their root structure and provide much surface area on which other small sea life can live.
Dead leaves and stems shed from the plants provide detritus for creatures to eat in the plant beds and in areas distant from the beds. Many species of highly desirable predator fish forage in seagrass meadows.
The single most important requirement for SAV is enough water clarity to allow sunlight to penetrate. Mud bottoms are preferred by most SAV, although shoalgrass and turtle grass will grow in sand. Clover grass will grow in nearly liquid mud, as well as in highly polluted areas.
The ideal salinity range for most species of seagrasses found in the Gulf is 20-40 parts per thousand (ppt). Clover grass has the narrowest range of salinity tolerances. Shoalgrass has the widest range, and also the highest optimal salinity at 45 ppt. Wigeon grass is included in the seagrasses, but is really a freshwater plant with high salinity tolerances. Ideal areas for seagrass meadow development are low-wave-action, shallow-water areas with restricted circulation.
About 2.5 million acres of seagrasses are found in Gulf of Mexico estuaries and nearshore areas, with approximately 95 percent being in Texas and Florida. Probably 1 million acres of clover grass alone exist on the West Florida Shelf, especially the Big Bend area. Florida Bay and the area around the Florida Keys have large areas of turtle grass, manatee grass and shoalgrass.
In between, these same species are found with clover grasses, especially in shallower waters. Turtle grass, manatee grass and shoalgrass beds have historically been dense in Tampa Bay, but some of these have been lost. In Florida, wigeon grass is found mainly in the lower-salinity areas near river mouths.
In Texas, more than 90 percent of the seagrasses are found on the lower coast. By far the largest amount, 186,000 acres, is found in Laguna Madre, including Baffin Bay. Shoalgrass is the main seagrass in the upper Laguna Madre, and manatee grass dominates the lower Laguna Madre. Seagrasses are declining in Laguna Madre.
Another 25,000 acres of mostly shoalgrass is in the Corpus Christi Bay complex, which includes Nueces and Redfish bays. Slightly less than 23,000 acres of seagrasses can be found in Matagorda, San Antonio and Aransas bays. Almost all of the seagrasses of Galveston Bay have disappeared.
Mississippi has the third largest amount of seagrasses, with over 122,000 acres documented in 1976. Almost all if it is within the Gulf Islands National Seashore. That acreage has almost certainly declined dramatically since then.
Louisiana has less than 14,000 acres of seagrasses, most of which are in Lake Pontchartrain and Chandeleur Sound. Five species of seagrass exist in Chandeleur Sound, while most of the seagrass present elsewhere in the state is wigeon grass, which prefers low salinity. Seagrasses have completely disappeared from White, Calcasieu and Sabine lakes.
In Alabama, an assessment done in the early 1980s showed less than 4,000 acres of seagrasses. That acreage has also declined since then.
Mangroves
Three species of mangroves are found fringing Gulf of Mexico waters: black mangrove, red mangrove and white mangrove. Like marsh grasses and sedges, mangroves thrive in areas of soft sediments, regular tides, some fresh water and low wave action. Again like in marshes, the tidal flux in mangrove areas creates challenging conditions of salinity changes for sea life.
In spite of the challenges, mangroves provide excellent habitat for fish and other sea creatures. Red mangroves support themselves above the water level on strong arching roots that interlock between trees. These prop roots provide a firm place for algae and small animals to grow in an otherwise soft mud environment. They also provide shelter for more than 220 species of fish. Detritus from the trees also fuels the food web.
In Florida, where all three mangrove species are found, red mangroves grow between the average low tide mark and the average high tide mark. Inwards of red mangroves, the black mangrove grows just above and just below the average high tide mark. White mangroves grow on slightly higher ground that is only rarely washed by tides.
Mangrove distribution is limited by freezing temperatures. Most hardy is the black mangrove. It is common on the southern Texas coast, occurs in scattered patches in the northern Gulf and is common again below Cedar Key, Fla.
After a significant freeze, all the woody vegetation dies, but the plant seems to spring back immediately in years without hard freezes. Red mangroves have been reported as far north on the Florida coast as Cedar Key, but they become much more common to the south, where they grow with white mangroves.
Oyster reefs
Oyster reefs in Gulf of Mexico states are built by a living mollusk, the eastern oyster, itself an important commercial species. Ideal growing areas for oysters are fairly calm estuaries, with firm bottoms, continuous water flow and low sedimentation rates. Ideal temperatures for oysters are 50-79 degrees, and best salinities are 12-25 ppt. Oysters are filter feeders, and need good water circulation to bring them food and oxygen. Oysters produce large amounts of waste that can accumulate on a reef that has poor water movement over it.
Oyster reefs generally have an upper zone of live oysters over a core of buried shell from dead oysters. As young oysters, known as spat, settle on and attach to the shells of live and dead oysters, a reef is built. Reefs range in size from small patches and mounds to ridges that are miles long. Mature oyster reefs often extend above the low tide line.
Oyster reefs have so many nooks and crannies that one square foot of reef can provide 50 square feet of surface that other organisms can grow upon. The rugged surface of a reef also provides many hiding places for eggs, larvae and young fish, as well as other marine creatures that serve as food for mid-level or higher predators.
Several offshore snappers and groupers in their early life stages use oyster reefs, as do adult members of the drum family such as spotted seatrout and red drum. More than 300 species of sea life have been identified on oyster reefs.
Oysters are found from Oyster Bay near Cape Sable, Fla., northward in estuaries around the Gulf through Texas, except in Laguna Madre.
Louisiana has the largest quantity of oyster reefs, 87 percent of which are managed on private leases. The state leases about 400,000 acres for oyster farming; however, actual reef covers less than half the area. An additional two million acres is reserved by the state as public grounds, primarily for seed oyster production to stock on private leases.
Florida holds nearly 184,000 acres of oyster reefs, of which less than 14,000 acres are open to oyster harvest. Of the open areas, nearly 65 percent are public, most of which are located in Apalachicola Bay and St. George Sound in the panhandle.
Oyster reefs are found in all Texas estuaries, especially between Galveston Bay and Corpus Christi Bay. Oyster reefs do not occur in Laguna Madre because of unfavorable conditions. Something less than 20,000 acres of oyster reefs exist in Texas, 88 percent of which are public.
Seventeen natural reefs are managed by the state in Mississippi. Those, and 6 privately-leased reefs ranging in size from 5 to 100 acres, make up over 10,000 acres of reef in the state. Almost all of the commercial oyster harvest comes from western Mississippi Sound.
Artificial structures
Man-made structures that provide hard surfaces occur throughout the Gulf of Mexico in inshore, nearshore and offshore waters. Except in Louisiana, most of the structures in shallow waters, besides bridges, are related to navigation or erosion control. Jetties, pilings, groins and breakwaters are common in all states.
Most of these are constructed of large granite, limestone or sandstone rocks, which provide valuable hard bottom in otherwise soft or sandy bottom habitats. The large rocks also have many crevices and holes that provide shelter for small creatures that larger fish prey on.
Finally, they serve to concentrate numbers of predator fish because the structures provide ambush points and create water current discontinuities that concentrate small forage fishes.
Louisiana’s waters are unique in that thousands of small to medium oil and gas platforms and wellheads dot its coastal lakes and bays. These structures effectively provide the same effects as jetties and breakwaters, but on a much more extensive scale.
Waters offshore of Louisiana, and to a lesser degree East Texas and Mississippi, hold thousands of oil and gas platforms famed for providing excellent fishing. Four thousand platforms stand in waters of the Fisheries Conservation Zone (federal waters), and many hundreds more are found in state territorial waters.
As many as 29,000 fish have been estimated to use a single platform at one time. Structure-oriented species such as red snapper, gray snapper, various groupers, amberjack, almaco jack, gray triggerfish, Atlantic spadefish and sheepshead, as well as blue runner, are common at offshore oil platforms. Platforms are also visited by pelagic species such as tunas, marlins, king mackerel and wahoo. The number of fish and the variety of species is greatest at platforms off western Louisiana and near the Mississippi River Delta.
Even more than with other artificial reefs, debate exists as to whether oil platforms actually create habitat that increases fish populations or merely serve as fish attractors. Whatever the case, increasing numbers are being removed as the oil and gas deposits they mine are being depleted. The Louisiana Rigs to Reefs Program has converted more than 150 decommissioned platforms to submerged artificial reefs.
Other offshore artificial reefs have also been created in the other four Gulf states using obsolescent military equipment and scuttled ships, as well as the occasional oil platform. Alabama is unique in having created a special area for fishermen to create their own private artificial reef structures.
Live/hard bottoms
In inshore and nearshore (within 60 feet deep) waters, live/hard bottoms are confined to Florida from Hernando Beach to the Florida Keys, and to a lesser degree the South Texas shelf from Corpus Christi to Brownsville. The shallow-water Florida live/hard bottoms are limestone outcroppings encrusted with algae, sponges, hard and soft corals, hydroids, anemones and bryozoans.
The Texas nearshore hard bottoms of note are East Bank, Steamer Bank, Little Mitch Bank, Four Leaf Clover, 9 Fathom Rock and 7½ Fathom Reef. These lie in 45 to 130 feet of water. None rise more than 16 feet over the surrounding bottoms. Few live encrusting animals exist on these banks. Because of their low relief above the surrounding mud bottom, the banks are mostly submerged in a nepheloid layer of muddy bottom water, and suffer from heavy sedimentation.
Fourteen other named banks are located on the South Texas shelf in water depths of 200 – 300 feet. They are Big Dunn Bank, Small Dunn Bank, Blackfish Ridge, Mysterious Bank, Baker Bank, Aransas Bank, Southern Bank, North Hospital Bank, Hospital Bank, South Baker Bank, Sebree Bank, Big Adam Bank, Small Adam Bank and Dream Bank.
On the shallow continental shelf south of Matagorda Bay is an area of reefs drowned by ancient rising sea levels. They range from 3 to 72 feet in height, and are surrounded by a nepheloid layer of water 50 – 65 feet thick. Presently, only small amounts of coral-like algae are found on these reefs.
The Louisiana/Texas Shelf has many other banks, all of them caused by dome-like extrusions (diapirs) of salt upwards from deep salt deposits. These can be put in two categories: midshelf banks and shelf-edge banks.
Midshelf banks typically rise from depths of 260 feet or less, and are 13 to 164 feet high. The tops and sides of midshelf banks are limestone, sandstone, claystone and siltstone. Named midshelf banks include Sonnier Bank, Fishnet Bank, Claypile Bank, 32 Fathom Bank, Coffee Lump, Stetson Bank, Phleger Bank and 29 Fathom Bank.
Shelf-edge banks rise from deep bottoms (around 600 feet) near the edge of the continental shelf. They are the big boys of offshore structure in the Gulf of Mexico, ranging from 115 to 500 feet high. Like midshelf banks, these are salt diapirs capped with carbonate rocks. Many have live bottom on them and two — the East and the West Flower Garden Banks — are encrusted by true hard corals.
Ninety-five species of reef fish have been observed on shelf-edge banks. Shelf-edge banks include East Flower Garden Bank, West Flower Garden Bank, Geyer Bank, Rankin Bank, Elvers Bank, MacNeil Bank, Appelbaum Bank, Bright Bank, McGrail Bank, Alderdice Bank, Rezak Bank, Sidner Bank, Ewing Bank, Jakkula Bank, Bouma Bank, Parker Bank, Sackett Bank (Midnight Lump), Diaphus Bank and Sweet Bank.
On the Mississippi-Alabama Shelf, the pinnacles, found in 330-370 feet of water, offer a surprising amount of hard bottom for coral-like live bottom animals to inhabit. The pinnacles seem to be reef structures at a stage in between growth and fossilization. It is believed that they were full-blown ancient coral reefs before being more deeply submerged by rising sea levels at the end of the last ice age.
The pinnacles have a diverse population of reef fish, many of which are common in the Caribbean but rare elsewhere in the northern Gulf. The pinnacles are the first elevated structures that the Loop Current from the Caribbean encounters. This current carries numbers of tropical reef fish larvae, which may settle on the pinnacles.
Coral reefs
Large complex coral reefs are not common in the U.S. part of the Gulf of Mexico, although some do exist. Far more common are isolated coral colonies and solitary corals, which are found throughout the Gulf.
Large coral reefs exist in the waters of the Dry Tortugas Islands 73 miles west of Key West, Fla. These reefs form an oval ring-like structure, 16.7 miles by 7.5 miles. The area consists of corals, exposed limestone covered with live bottom growths and grass beds.
The East and West Flower Garden Banks are thriving coral reefs located on the tops of shelf-edge banks. The East Flower Garden is located 120 miles and the West Flower Garden 107 miles southeast of Galveston, Texas. They are the northernmost thriving tropical coral reefs in North America.
The East Flower Garden Bank is pear-shaped, and covers about 16,500 acres. Its shallowest waters are about 66 feet deep. The West Flower Garden Bank has three peaks, and covers slightly less than 34,000 acres. Its shallowest point is 49 feet deep.
The West Florida Shelf contains the most coral colonies and solitary corals. The Florida Middle Ground contains some hard corals, but most of the area is classified as live bottom. True (stony) corals have difficulty surviving winter water temperatures below 64 degrees.
The shelf waters between Tampa Bay and Sanibel Island have significant coral communities as well as sponges and other bottom creatures.
Pelagic sargassum
Unlike other Gulf of Mexico habitats, sargassum habitat moves. It is comprised of three species of brown algae. Most sargassum occurs in the Atlantic Ocean, but significant quantities enter the Gulf of Mexico. Sargassums are high-salinity plants. In low salinities, the plants lose the bright golden color of healthy plants, and turn brown as they die.
Healthy mats of sargassum contain a fantastic array of crabs, shrimp and other invertebrates that duplicate the exact color of sargassum. Among the 135 species of fish closely associated with sargassum mats is the sargassum fish. It is not only colored exactly like sargassum, it has flesh tabs growing from its body that resemble sargassum plants, and its fins are on leg-like extensions that it uses to climb around inside sargassum mats.
Sargassum appears to be extremely important habitat for some life stages of many species of fish of interest to fishermen. Many very small jacks use sargassum heavily, and even larger individuals hunt near sargassum. The gray triggerfish is also heavily associated with sargassum. Cobia, as well as many adult blue water species, including billfish, tunas and dolphin hunt the fringes of sargassum mats.
