Shelf Life

Where the Continental Shelf Meets the 100-Fathom Line Dreams Come True

Capt. Steve Dougherty January 28, 2015

Time spent on the water has certainly changed. In the early days of navigation, mariners regarded the 100-fathom curve as the end of the road. Long before powerful transducers transferred data to captains manning the helms of modern day vessels, primitive deckhands utilized a weighted sounding line to judge the amount of water beneath their keel, and 100 fathoms (600 feet) was typically the limit of what could manually be retrieved by hand before reaching exhaustion. While determining exact depth today is not nearly as arduous of a task, sharp-witted offshore anglers still pay close attention to the 100-fathom curve no matter their port of call or target species.

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100-Fathom Hot Spot - Destin

Over the years, scientists have mapped much of the ocean’s seafloor with advanced sonar technology and produced detailed charts with the results of their findings. When studying such charts you will notice that water depths worldwide display many similarities, with contour lines connecting points of equal depth. What makes the 100-fathom curve so unique is its relationship to the continental shelf.

Although the continental shelf’s sloping bottom shapes the seafloor, it is only when nutrient-rich currents set up over these bathymetric features that ideal feeding zones exist for a variety of highly desirable game fish.

While a coastline may appear as if it disappears into the water, a continent’s landmass may extend for many miles into the sea, creating what is known as the continental shelf. It is through the earth’s evolution that the continental shelf varies so greatly from region to region. Throughout the estimated four billion years of the earth’s history there have been numerous cycles of glaciation that dramatically influenced the rise and fall of sea level, while also carving out impressive topography. Depending on the continental shelf’s adjacent landmass, among other influences, the shelf’s width may vary intensely, which greatly influences an angler’s ability to reach prime stretches of offshore azure. In some areas the continental shelf may span for miles, while in other regions there may be little to no shelf habitat at all. Off the coast of North Florida the shelf is approximately 60 miles wide, while Palm Beach County in South Florida has just over a mile of shelf plateau. Furthermore, in some areas the 100-fathom curve is only a few miles from the 1,000-fathom curve, and it’s easy to pop out real quick and see what’s going on over the horizon. In areas where it takes a considerable amount of time and fuel to reach deep water it’s highly advised you plan ahead.

Because the continental shelf represents a major percentage of commercial and recreational exploits, countries adjacent to their respective shelf are in control of the waters and their EEZ (exclusive economic zone), which extends up to 200 nautical miles from the coastline. There are also cases where countries have rights to a continental shelf that may extend up to 350 nautical miles from the coast in order to safeguard and protect the exploitation of valuable natural resources like oil, gas, minerals and marine life.

While the continental shelf supports a complex biomass of marine organisms, it only represents a minuscule part of the ocean, estimated to cover just seven-percent of the ocean floor. Part of the reason the shelf plateau provides for so much life is because it represents the boundary where light penetration to the seafloor begins to dwindle. From the continental shelf the plateau drops off the continental slope and as light continues to diminish, darkness takes over as the ocean reaches its deepest basins.

Although the continental shelf’s sloping bottom shapes the seafloor, it is only when nutrient-rich currents set up over these bathymetric features that ideal feeding zones exist for a variety of highly desirable game fish. As an example, the Loop Current is immensely important to Florida fisheries as it transports warm water into the Gulf of Mexico through a narrow opening between Cuba and Mexico. The Loop Current heads north and then east and south as it follows the contour of the coastline before exiting the Gulf near the Florida Keys. There it flows into the Florida Straits and collides with the Gulf Stream current. The well-known Gulf Stream flows north while paralleling the Eastern Seaboard along the general trend of the 100-fathom curve, presenting exceptional fishing opportunities for a wide array of species along its entire path of travel.

Before heading offshore it’s important you do some planning and have a firm understanding of how the position of currents and water patterns influence game fish feeding habits. Offshore anglers in the northern Gulf wouldn’t dare head offshore without diligently studying a satellite-based fishing forecast, yet most South Florida anglers never even bother. This is a critical mistake because in addition to providing anglers a clear view of the Gulf Stream as well as eddies and associated filaments of promising water, satellite forecasting tools provide critical data including water temperature, clarity, color, dissolved oxygen and more.

Currents have their own signature temperature and satellites equipped with infrared technology can pinpoint convergence zones of opposing waters to determine the precise location of favorable fishing conditions. The presence of forage is also key to interactions with top predators. Chlorophyll is visible in phytoplankton, which are eaten by zooplankton, which are eaten by forage fish, which are eaten by game fish. By assessing chlorophyll data one could essentially determine the food indexes for a particular area and hypothesize on the presence of apex predators.

Anglers adept at targeting pelagics know conditions change fast. Unfortunately, promising stretches of water don’t often linger in the same area for long periods of time and anglers must learn how to react to changes and conditions observed on the water in real time. Ideal conditions exist when stable and persistent convergence zones remain over quality bottom features for consecutive days. Do some homework ahead of time and remember that it’s the combination of numerous factors that set the stage for an epic bite. While the latest satellite analyses point anglers in the right direction, anglers can also keep close tabs on contour changes and variation in bottom topography with high quality electronic charts displayed on sophisticated multifunction displays.

100-Fathom Hot Spot - Destin

While many view south Florida as the state’s premiere departure point for accessing pelagic game fish, the small town of Destin is a silent killer. Point the bow south from here and you’ll soon reach the Squiggles, an aptly named contour curve along the southeast edge of the 100-fathom curve approximately 30 miles offshore. The Nipple is another proven area along the continental shelf where blue marlin, tuna, wahoo and dolphin are all popular targets.

100-Fathom Hot Spot - The Florida Keys

The Florida Keys are world renowned for fantastic dolphin fishing and for the most consistent action with big fish many head straight offshore to the 100-fathom curve. Have you ever heard of the Ups and Downs? This area of broken bottom approximately 18 miles offshore is aptly named due to the fluctuating depths. Here converging waters stack sargassum seaweed and provide habitat for forage species.

100-Fathom Hot Spot -
Westend, Grand Bahama

The island archipelago of The Bahamas features one of the world’s largest reef systems that rises from the darkest depths of the ocean. Anglers trolling the bank for wahoo typically fish just inside of the 100-fathom curve, but along this stretch of coast the shelf plateau is very narrow. Along the Little Bahama Bank the distance from 30- to 600-feet could be less than 200 yards, which concentrates both forage and predator fish into one tight zone.

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