Secrets of Sargassum

Read the Weeds for More Mahi

FSF Staff August 17, 2018

Excluding fragile coral reefs, aging shipwrecks and massive seamounts, the ocean is a rather featureless expanse. Off the coast of Florida and across much of the tropics, fish migrations are largely dictated by prevailing currents. Caught in the same gyres, floating ecosystems of sargassum seaweed are the basis of life that provide structure and sanctuary for a host of predators and prey.

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Photography: Doughertyphotos.com

Comprised of two species, Sargassum natans and Sargassum fluitans, sargassum is a actually a type of brown macroalgae. These dense, holopelagic floating structures consist of branched, leafy vegetation and gas-filled floats allowing the marine plants to drift near the surface where photosynthesis can occur.

While providing essential refuge and safe breeding grounds for juvenile pelagics, sargassum also offers safety and security to numerous species of marine animals including fish, birds, sea turtles and marine invertebrates. As an intricate offshore marine ecosystem, sargassum is perhaps one of the most important structures in the entire ocean.

At times floating mats of seaweed can stretch for miles, with a range of organisms found within close proximity of fertile weed lines. In fact, marine biologists have determined that healthy sargassum seaweed can harbor over 100 species of fish and fungi, as well as over 145 species of marine invertebrates. Of all the fish that travel with these floating mats of vegetation, filefish are the most common individuals, closely followed by various jack and triggerfish that reside alongside these structures during at least one stage of their life cycles.

Generally, smaller organisms such as the sargassum crab, frogfish, porcupinefish, butterfish, sargassum nudibranch, sargassum shrimp, triggerfish and larval game fish like sailfish, mahi, swordfish and blue marlin hide in the foliage of the upper portion of the weeds. Slightly beneath the surface, larger fish like bar jack, tripletail, Spanish sardine, round scad, blue runner and flying fish all share this unique ecosystem. Additionally, research indicates that mahi typically spawn around sargassum in what is believed a method of increasing rate of survival.

Follow the food chain and it’s no wonder this sort of nursery ground attracts so many prized game fish. Don’t be fooled though, because not all offshore weedlines produce. Typically, you’ll find the poorest results on weedlines that are sparse and pale in color, which indicates dying or dead vegetation. Instead, invest your time and focus your efforts on bright sargassum with a fresh, golden hue. Brown weed with deflated floats is a sign that the vegetation is on its last leg and will soon be sinking to the bottom.

If possible, pull alongside the weedline you intend to fish and scoop up some grass. Shake the bundle of vegetation on the gunnel and carefully observe what falls out. If the weed is fertile and alive, an assortment of barely visible crustaceans will be noticeably visible. Tossing a few chunks or live chummers can also help bring out a particular weed patch’s predators from the shadows. It’s also a good idea to stay away from thin strands of eelgrass and bay grass floating in loose formations.

When you find birds dipping and diving near defined weedlines you should carefully observe their actions in attempts to pattern their behavior. A trophy mahi mahi over 30 pounds won’t have as many birds tracking it as would a school of juvenile green hornets. A mature dolphin simply can’t corral baitfish to the surface quite like a school of peanuts can. So, when you see a pile of birds it’s likely they are tight to a pack of juveniles.

Dense weed lines that pique the interest of offshore anglers are derived from the Sargasso Sea, an area in the Northwest Atlantic Ocean that is bordered by the Gulf Stream Current, the North Atlantic Current, the North Equatorial Current and the Canary Current. The counter-clockwise circulation of the North Atlantic Gyre holds several million tons of seaweed in the Sargasso Sea, and depending on the currents, thick groupings of sargassum will appear off our coastline in varying amounts.

Though massive stretches of open ocean featuring broken weedlines and scattered patches can make trolling difficult, well formed weedlines most often form alongside convergence zones of opposing water masses, which are often associated with temperature breaks, slicks and floating debris. All of which provide favorable conditions for the predator-prey interactions we desire.

If you can surf the web, there are plenty of resources that can help you locate promising water. Terrafin, ROFFS, Hilton’s Real-Time Navigator and FishTrack all produce data rich imagery from MODIS, AVHRR and VIIRS satellites and relay important oceanographic conditions to boaters in near real time. Among the many aspects to consider, 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. Additionally, by assessing chlorophyll data one could essentially determine the food indexes for a particular area and hypothesize on the presence of apex predators and the proposed location of stacked sargassum.

When hunting mahi along distinct patches of grass it’s important to remember that big dolphin aren’t always found directly under floating debris. On occasion the largest fish in the bunch will be a few hundred yards from the action, so it’s sometimes best to troll a few wide patterns around areas of prominent life before running off in search of the horizon.

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