Blackfin tuna are a major draw on the Keys hump, including the one off Marathon.

One of my favorite places to fish in the Florida Keys is known, simply enough, as the Hump. The Hump was originally called the West Hump, because it is the westernmost in a series of three humps, located on the edge of the continental shelf off of the middle Florida Keys.

In the 1970s, I began to call this area the Marathon West Hump in an attempt to promote it as a fishing destination off Marathon, due to the fact that a prominent hump off nearby Islamorada had been known for years as the “Islamorada Hump.” The Marathon West Hump lies approximately 30 miles offshore of the Middle Keys, on a course of 135 degrees from Marathon.

These humps are loosely called seamounts, because they are like mountains rising from the ocean floor. Worldwide, there are thousands of these structures. Some are remnants of underwater volcanoes; others may have once been emergent islands. Wherever you find them, seamounts are unique marine habitats. They alter currents, and provide structure for fish in the otherwise featureless depths.

Over the years we’ve developed some special techniques for fishing the Humps in the Keys. Perhaps you’ll try them on your next trip to this part of Florida, or apply what you learn to other deepwater structure you find.

Let’s take a closer look at that Hump off Marathon.

Tuna frenzy on the Islamorada Hump.

Our Hump, as we in Marathon call it, is located almost exactly in the middle, or the axis, of the Gulf Stream, where the current runs the hardest, indicated by the highest water temperature. If you were to graph the water temperature, while transiting the Gulf Stream, the surface water temperature would slowly rise, with the highest temperature in the centerline of this powerful ocean current. The shallowest part of the Hump is 480 feet from the surface, rising from a depth of 830 feet on the west, or upcurrent side, and dropping to nearly 1,200 feet on the east, or downcurrent side.

As the Stream pushes northeastward, sometimes reaching a velocity of 3 to 4 knots or more, it slams into an immovable pinnacle of solid rock, effectively blocking more than one half of the water column. In order for the water to get by this choke point it speeds up, creating an upwelling, a phenomenon where colder, oxygen-rich water rises to the surface. This is a perfect environment for maximum productivity, starting with baitfish, bigger baitfish and, finally, top-of-the-line predators.

The list of gamefish caught on the Hump includes dolphin, wahoo, blackfin tuna, huge amberjack, blue marlin, sailfish, white marlin and big sharks, chiefly makos and great whites. Fishing deep on the bottom, you might catch snowy grouper, queen snapper and a variety of other bottom dwellers.

Small feathers are deadly on blackfins.

Techniques for fishing this area have changed over the years, following advances in electronic fish-finding and position-fixing technology. During the 1950s and ‘60s, the Hump was found by running time, speed and distance courses, and then looking for the characteristic rip, caused by the Gulf Stream slamming into that 720-foot-high underwater obstruction. The success rate for locating a one-acre pinnacle in over a thousand feet of water ran about 50 percent, sometimes higher when the water was very calm, when you could see the rip caused by the powerful upwelling.

In the 1970s, we found the Hump using Loran-A, tricky and sometimes unreliable. Synchronize the sine waves, and don’t ever turn the machine off, once it locked in.

By the 1980s we had Loran-C, more reliable, but still crude by modern standards. My first Loran-C was an SRD-CLX, a huge machine about half the size of an old-fashioned bread box and costing $3,000; that’s a lot of full-day trips back in those days! This machine had the “unbelievable” capability of displaying two lines of position at the same time. Since the 14,000-microsecond delay line runs nearly parallel to the 135-degree course to the Hump from Marathon, all I had to do was to keep on that line, and run down the 43,000 line until it read 43358.2. Forget about course to destination, course steered, time to go, cross track error, and all that; information like that was still years away. To this day, the Loran-C coordinates of the peak of the hump are indelibly etched into my memory, 14032.6 and 43358.4. In fact, when I fish there now, I have to consult the chart to get the Lat/Long coordinates, which are 24-25.528-N and 80-45.328-W.