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Ledges key to diving in gulf


© St. Petersburg Times, published February 11, 2001

The natural underwater terrain in the Gulf of Mexico offshore from Tampa Bay consists of hard bottom, rock piles, sinkholes, springs, potholes and ledges. Legdes makes up the majority of sites visited by divers.

Ledges are ancient coastlines formed as waves carved limestone into overhangs and cuts approximately 12,000 years ago. The sea level has risen substantially since then.

Four former coastlines shown on NOAA charts are reachable by recreational divers. The closest to shore, the Five Fathom Line (1 fathom equals 6 feet) has plentiful ledge structure, but visibility is less than 15 feet at that depth most of the year, so divers tend to go right past it.

Divers often explore the 1- to 12-foot high ledges found in 45 to 80 feet of water, found 8 to 20 nautical miles offshore. The central feature running through this area is the 10-Fathom Line, which includes approximately 75 percent of our dive sites. Visibility averages 30 to 40 feet, and many ledges extend for miles.

The 20-Fathom Line has mostly short and low ledges, much more widely dispersed. An exception is the northwest end, known as the Florida Middle Grounds, where there is a concentration of ledges up to 30 feet high in a 30-nautical mile range.

In the gulf these seem like mountains, and visibility is often 75 to 100 feet. Little of the 30-Fathom Line has been explored because of its depth and distance.

One location, the Elbow, has ledges that resemble the buttes, topping out at about 150 feet. The bottoms are 170 to 180 feet down, so extreme caution, experience and quality equipment are required.

Close inspection of a ledge reveals a limestone structure called live rock, where a vast variety of life thrives. Ecosystems are made up of soft and hard corals, sponges, anemones, Christmas tree worms, feather dusters and starfish. Crustaceans such as arrowhead crabs, stone crabs and shovelnose and spiny lobsters hide in nooks and crannies.

Marine life is concentrated on the outcropping, where the greatest protection is found. Most ledges are undercut 2 to 6 feet, but a few go to 12 feet and are large enough to hide giant jewfish, turtles and nurse sharks. The tall ledges also attract pelagic fish such as amberjack and cobia. Some of the most alive ledges are 1- to 3-feet high with low, tight holes and Swiss cheese tops offering the best protection for all inhabitants.

The high side of a ledge looks much like an overgrown field, covered with soft corals and sea fans. Bottom fish such as grouper and snapper often run up among the sameness of the reef top and lose slower predators, such as divers with spear guns. Camouflage is very effective here. Often this area extends hundreds of yards, sometimes even bridging the gap between multiple ledges.

The low side of the ledge is usually sand, broken shells or sand-covered hard bottom. Ledges collapse, creating cracks along the top and in tight holes underneath. They also break into rocks and scatter out in front of the ledge. Black and gag grouper leave the dark rocks, whiten their skin and disappear as they zigzag into the eye-numbing whiteness of the sandy areas.

Most ledges run north to south, paralleling the present coastline. Cross them running east or west for the best read on your bottom-finder. If you have a plotter, mark it every time you cross and soon you will have a connect-the-dots line depicting the ledge's layout. Dropping a marker jug or two also works well and avoids repeat searching.

Short, small ledges with frequent flat spots will be very difficult to hit again. If you find another boat on your coordinate, explore north or south until you find a new section far enough away to avoid intruding. Happy hunting!

- Chad Carney teaches scuba diving and spearfishing. Call him at (727) 423-7775 or email

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