Fly Fishing Offshore Weed Lines

How fly-rodders find and fish sargassum lines and patches for bluewater pelagics

A sargassum weed line in blue water
Sargassum patches and lines offer fly-rodders great promise for action. These floating, fish-aggregating blankets help concentrate offshore species popular among fly-fishermen.Tim Rahn

When it comes to fishing, I love it all. Be it giant bluefin tuna on conventional stand-up tackle or brook trout on dry flies in beaver ponds, I’m all in. But I can’t get much happier than being in a boat loaded with fly rods, hunting blue water for a patch of gold. It’s pretty hard to beat the heart-pounding, knee-knocking excitement you feel when you find what you’re looking for in a sargassum weed line and all hell breaks loose. I’ve experienced many “Discovery Channel moments” over the years while fly-fishing this way.

Much of the action is visual — prowling patches or a weed line, looking for just the right target. The angler is able to pick the species and adjust the rod weight and fly accordingly. Much like casting to trout that are feeding on top, in these visual presentations, fly choice can be critical.

Sargassum weed photographed under water
A fishes-eye view of a garden of sargassum reveals surprising beauty.Adrian E. Gray

Sargassum: The Ocean's Amazing Algae

Sargassum is a brown macro algae loaded with little air bladders, which keep it afloat. Depending on species and condition, sargassum can range from a bright yellow gold to a dark amber brown. This weed is the basis for much of the life found offshore in the deep blue. Countless species owe their entire existence to this floating plant. Over millennia, numerous species of fish, crabs, worms and shrimp have evolved specifically to live within this floating ecosystem. Such a rich abundance of life attracts hungry predators, and many species of fish come to sargassum to feed as well as to shelter within or around it.

Sargassum is at the mercy of the ocean currents, wind and waves. These forces can build up massive mats, acres across, or lines that stretch 100 miles or more. The same forces can break up weed lines in a matter of hours. At such times, searching for the vital weed can become the proverbial needle in the haystack. Pre-trip preparation will mean little if you can’t locate the golden oasis.

Massive line of sargassum weed
Massive mats of sargassum may extend to the horizon, yet before the day is over, conditions may scatter the weed.Tim Rahn

Even with so much technology at our fingertips these days, finding a weed patch or line is no sure thing. I’ve had my share of switching to Plan B after not being able to locate solid patches or lines of sargassum during a day’s fishing. There are, however, methods of discovery that will stack the odds in your favor.

Large sport-fishing yacht at dock in Cabo
Any intel on the whereabouts of weed lines offshore that you can gather at the dock will give you a leg up.Doug Olander / Sport Fishing

Good old-fashioned courtesy at the dock can go a long way in gathering some quality intel. When fishing a new area, ask the dockhands which captains they rate as best. Then, offering one of these skippers an ice-cold beverage can help get you headed in the right direction. I think folks would be surprised just how helpful good captains can be when you are polite and treat them with respect. After all, you’re not asking for their top-secret gator-trout spot.

If you are able to get some good info on where there have been patches or a solid line of weed, that’s only part of the equation. Critical information includes finding out the direction and speed at which any noted sargassum lines are being driven.

Find Weed Lines with Technology

If a captain reveals which way to head to find a weed line, but not which way it seemed to be moving, don’t sweat it: You can calculate that yourself. This is where a service like ROFF’s Ocean Fishing Forecasting comes in.

A Roffs ocean fishing forecast map
For a small fee, ROFF’s allows you to download local charts to see the direction and relative speed of the currents.Courtesy Roffer's Ocean Fishing Forecasting

ROFF’s also allows you to locate spots that are likely to have a developed grass (sargassum) lines or patches by identifying hard current breaks and eddies.

When reviewing a ROFF’s chart, look for the areas with the sharpest temperature breaks, since that’s where currents are pushing up against each other. Where the sharpest color contrasts up against each other, it’s likely there will be a rip with grass stacked along it. The best grass lines will be where these conditions have been consistent for at least three days.

You can use the chlorophyll images as well, but usually the sharp temperature breaks offer a higher percentage.

AIS (the U.S. Coast Guard’s Automatic Identification System) is another fantastic option when looking for patches and lines.

AIS: Game Changer for Weed Hunters

The U.S. Coast Guard’s National Automatic Identification System
The U.S. Coast Guard’s Automatic Identification System (AIS) is a technology that more and more recreational anglers are utilizing offshore.Courtesy U.S.C.G.

AIS has been around in the commercial sector for some time, but in the past few years, electronics manufacturers like Simrad have added AIS to some of their recreational units.

In the past, when running offshore looking for a line or large patches of weed, we might have hailed a ship we spotted on the horizon or picked up on radar to ask them if they had passed any weeds, and some were willing to offer up some info.

The problem before AIS was that you’d have to get on the radio and hail a boat simply as “the vessel at 28°46’14N and 88°50’55W,” and then wait for a response. More often than not, the response never came. Now, every commercial ship on the water has an AIS signal. Using a 16-inch Simrad NSS unit with integrated AIS, we could now hail the ship by name, which greatly increased the likelihood of a response. The ability to do this proved a game changer since it now facilitates locating promising water or avoiding searching dead water.

You can spend countless hours and hundreds of gallons of fuel running all over or you can let others help you pinpoint the sargassum. It’s true that a unit with AIS might cost more, but the fuel it can save in just a few days’ fishing can more than cover that expense. An added benefit is that you avoid wasted time and frustration.

A skipper looks over vast electronics array on a center console fishing boat
Good skippers will watch for ships in a target area and, with AIS information, try to call them to see if they've seen (or are in) thick sargassum.Chris Woodward / Sport Fishing

Whenever you’re running offshore, your head should be on a swivel. You should be actively scanning the horizon, looking for bird activity or anything on the surface. Often, you’ll spot birds before you can see sargassum.

I recall one trip out of Venice, Louisiana, when, as the continental shelf dropped off below, we began our search for solid weed lines or patches. When we passed the 1,000-foot contour, all eyes were glued to the horizon, except for our captain, Kevin Beach. He was busy studying his Simrad NSS unit and in particular, the AIS integrated with his radar. He identified two ships that were in the target area and just might have crossed paths with a sargassum jackpot. Beach reached out and the first ship responded. They said they had passed some “big weed mats” a few miles back. We adjusted course and within 30 minutes, there it was. We could see several sargassum mats a quarter-acre to nearly two acres in size — big, thick patches with the grass nicely pushed up.

Weed Lines Bountiful or Barren

A sea turtle hatchling stays close to sargassum weed.
Sargassum is home to a great diversity of marine life, including hatchling sea turtles which use it to hide from predators such as mahi.Masa Ushioda / Seapics

It’s important to keep in mind that not all sargassum is created equal. In order to maximize your success, you’ll want to look for certain things. Typically, the larger a sargassum mat, the better it will attract and hold quality fish. That said, a one-acre weed patch only a few inches thick will not be nearly as effective at aggregating fish as one half that size that’s much thicker/more dense. The deeper the weed line, the darker the color underneath the mat. That shade offers security for baitfish and helps hide the predators that lurk below.

If you’re lucky enough to find a well-defined weed line that stretches for miles in each direction, there are key characteristics to look for.

First, these lines form when currents push up against each other. Fish can be found anywhere along a good weed line, but there are a couple of situations you want to watch for. Fish often congregate anywhere the line has a bend. That bend forms a pocket into which baitfish are pushed and, logically, where predators key.

A tripletail hides in a thick mat of sargassum
Where sargassum mats look particularly dense is generally the best place to fish since these spots hold more feed for predators such as this tripletail.Adrian E. Gray

Second, look to see where the weed line appears thicker — extending deeper into the water — but more important, where the grass is being pushed up above the surface. The thicker weed offers shade, and weed pushed up indicates stronger currents, which will concentrate bait. Third, bait tends to also gather around areas in a weed line containing larger flotsam such as a pallet or timber. Something as simple as a single floating bucket in a weed line can produce a 50-pound bull dolphin or even a blue marlin.

Floating Buffet in Sargassum

Along with offering cover, sargassum is a floating buffet.

Tiny shrimp shaken out of sargassum weed
Shake some thick sargassum weed over a bucket; you'll usually see many colorful shrimps along with other crustaceans and fishes.Jeff Pierce

If you've never done this, try motoring up alongside a sargassum patch and use your baitwell net to take two or three scoops of the stuff, placing them into a bucket with seawater. Shake a handful at a time over the bucket, then toss the weed back overboard.
If you're on a good, healthy patch, your bucket will be filled with tiny shrimp and crabs, as well as a few species of forage fish and various juvenile or larval forms.

Tiny juvenile triggerfish shaken out of sargassum weed offshore
Sargassum is generally full of juvenile triggerfishes like this one and various species of filefish. They may not look like stereotypical baitfish, but predators like small mahi gobble them up.Jeff Pierce

You might be lucky enough to find a juvenile marlin, sailfish or tuna, as I have in the past. The sargassum can be an important nursery for young pelagic predators. It's really remarkable how much food is contained in a basketball-size piece of sargassum.

Wild Fishing in the Weed

Fly rod at the ready
Sargassum-weed fly-rodders are rigged and ready when a weedline is spotted.Adrian E. Gray

Heading out of Venice, Louisiana, other fly-fishing enthusiasts and I had Kevin Beach’s 39 Freeman stuffed with several fly rods, rigged and ready for just about anything we might encounter.

I like to always keep ready a good quality spinning outfit as well, rigged with a flashy, hookless popper. If we find fish scattered, or there are lots of small fish, the spinning rod allows us to efficiently cover water in order to find the right targets.

Big bull mahi turns for the camera
Spotting a serious bull dolphin (mahi) swimming near the sargassum presents a fabulous target for any fly angler.Jason Arnold / jasonarnoldphoto.com

The first five mats we pulled up to proved pretty uninteresting, holding only small dolphin and jacks but offering no marks on the sonar. As we pulled up to number six, however, we quickly identified three nice dolphin cruising the far edge, and had some solid marks 60 to 100 feet below. I stretched out a cast with the 11-weight, and the rainbow runner pattern hit the water with a satisfying plop. The fire drill began immediately as a 40-plus-pound dolphin engulfed the fly. Within another minute or so, we had three dolphin between 25 and 40 pounds hooked up on flies, all cartwheeling around the boat.

Man in the Blue Suit Comes Calling

Big blue marlin challenges underwater photographer

blue marlin

When the man in the blue suit shows up to check out the commotion with mahi by the boat, youl'll need something heavier than that 8-weight!© Doug Perrine

The first two hit the box in short order. As the 25-pound cow neared the boat, a 300-pound blue marlin materialized, ran the cow down and swallowed her boatside, breaking the leader on the 10-weight setup. That’s why I love fishing sargassum. You just never know what might show up.

We hit three more patches without much to show for it when we spotted a well-defined weed line just to the south. It stretched out as far as we could see in both directions. As we pulled up, we could see it was packed with plenty of baitfish and had some solid marks down 70 to 100 feet. The dolphin will typically be right on top, with larger ones often hanging deeper.

Wahoo Will Wake Up any Angler

Wahoo are the other fish that we commonly mark suspended under a good weed line or mat. Jumbo yellowfin tuna are also found along well-established weed lines, but they're typically marking deeper in the water column, down around the thermocline.

A free-swimming wahoo underwater
Wahoo present an exciting and tricky battle on a fly rod that will test any angler's skills.Daniel Goez

Seeing a couple solid marks down 70 feet or so, we sent a streamer deep on a fast-sinking line. On the fourth or fifth quick strip, the line came tight. When a fish that can exceed 50 miles per hour eats your fly, the line is going to come off the deck insanely fast, so you must be very conscious of that. The line can and will catch on anything, be it the rod butt, fingers, cooler handles, you name it. So many wahoo on the fly are lost in the first few seconds.

This time, the line cleared without issue. When the fish finally slowed, it was 330-plus yards into my backing. The give and take lasted 30 minutes or so before we saw color — a typical northern Gulf summer ’hoo of around 30 pounds. One final last-second run caused the fish to become tail-wrapped, and the leader parted.

More Surprises in Weed Lines

Over the next three days fly-fishing the sargassum lines, we managed to boat countless dolphin to nearly 50 pounds, battled a few wahoo, broke off a blue marlin, and had shots at another blue as well as a white marlin.

Rainbow runner
Rainbow runners are another sporting species that prowl weed lines, also attracting tuna and marlin.Pat Ford

On another sargassum trip, we located a huge expanse of mini weed patches, 3 to 5 feet wide. Every single one of these patches was jammed with juvenile rainbow runner and flying fish. When the dust settled, we had landed close to 20 yellowfin tuna to 60 pounds along with several skipjacks. It was epic. I’d been on the bow, waiting to cast to a busting fish. Each time I could get a fly to a boil within a few seconds, it was a guaranteed drag screamer.

TACKLE AND TACTICS

Fly-fishing sargassum requires the right gear. Predators that hunt the grass require some stout equipment. While we do target dolphin at times on lighter gear, such as a 7 or 8 weight, we typically fish 10-weight rods for them. If you want to fish lighter, that’s great, but you’d best have heavier rods rigged and at the ready: It’s common for a real bruiser bull to show up out of nowhere when you’re having fun with peanuts.

Fly-fishing a weedline
One never knows what might be lurking just under the sargassum mat.Tosh Brown / toshbrown.com

For the tunas (yellowfin, blackfin, skipjack) you’ll want an assortment of 10- to 15-weight rods. Probably 90 percent of what I fish offshore are 10- to 12-weight setups. Since you never know what might crash the party, you don’t want to miss a shot at a red-hot white marlin or a solid mako by not having a rig ready to go. When it comes to lines, it’s best to be ready to cover the upper water column, from floaters to fast-sinking lines. You can certainly cover most everything with a clear intermediate head and a fast-sinking line in the above mentioned weights.

Small mahi caught on fly
Even small mahi can get fussy; playing keepaway with the fly to get them fired up.Jeff Pierce

Dolphin (dorado/mahi) have a reputation for being pretty easy to catch. This is true most of the time, but I have certainly encountered some large dolphin with lockjaw. I’ve found that playing keep away with these stubborn fish often results in getting them to commit to the fly. When you have a dolphin that is refusing to eat a fly, make a cast to get its attention and then pull it away. Do that to give it just a quick look at the fly several times, and you’ll find that most will eventually fire up and explode on the fly. With this technique, I have boated several large dolphin that other anglers in the boat had given up trying to catch.

The Right Gear for Fly-Fishing Weed Lines: Rods, Reels, Lines and Flies

A tripletail caught on a fly
An 8-weight is light enough to make smaller tripletail (as well as mahi and jacks) a whole lot of fun.Adrian E. Gray

RODS

  • Dolphin, tripletail, jacks, smaller tuna — 8 weight
  • Dolphin, jacks, yellowfin/blackfin/skipjack tuna, cobia — 10 weight
  • Larger tuna, wahoo, white marlin, sailfish — 12 weight
  • Large yellowfin tuna, wahoo, blue marlin, mako sharks — 13 to 15 weight

REELS

  • 8-weight and 10-weight reels should be capable of holding a minimum of 350 yards of 50-pound-braid backing. Reels should have a strong drag system.
  • 11-weight to 15-weight reels should be capable of holding a minimum of 500 yards of 50- to 65-pound-braid backing with a well-built drag system that can apply significant, sustained pressure.
Islander LX4.0 Fly Reel
This Islander LX4.0 has been my reel of choice for 8- and 10-weight setups. I use the Islander LX4.5 reel on 11- to 15-weight setups. These reels are quite simply bombproof. Large pelagics can destroy a reel that’s anything less than that. In more than a decade of heavy use, these reels have performed flawlessly for me.Courtesy Islander Reels

LINES
When it comes to lines, choices are pretty straight-forward. Some 80 percent of your fishing will be within 15 or so feet of the surface. For this you can use a floating line or a clear, slow-sinking intermediate line.

Scientific Anglers Sonar Saltwater Intermediate fly llne
My go-to intermediate is the Scientific Anglers Sonar Saltwater Intermediate on the 8- and 10-weight rods, and the Sonar Titan Big Water Taper F/I (100-pound core) for the 11- to 15-weight outfits. The floater I fish is the Scientific Anglers Sharkwave Saltwater Titan.Courtesy Scientific Anglers

When you are marking fish down 50 to 100 feet, you might be surprised to learn you don’t need to get a fly to those depths there to catch fish you’re marking. The visibility is exceptional offshore, so getting a fly down 30 feet can often elicit a strike from a fish down 100 feet. To get down 30 feet, I fish the Scientific Anglers Sonar Titan Big Water Taper Max Sink. This line gets down quickly and has a 100-pound core to hold up to the bigger fish.

LEADERS
For leaders, I keep it simple, using straight single piece of 50- to 80-pound fluorocarbon. There is no need to get too complicated, but it can be a good idea to tie in a short section of 30-pound in the middle of your leader.

Basic offshore fly-fishing weedline leader rig
An example of a basic offshore weedline leader rigKevin Hand / Sport Fishing

The lighter line acts as an eject button, in case you hook a freight train that you just can’t stop. I once had a 400-plus-pound mako eat a small yellowfin boatside. That 30-pound section allowed me to break off that fish before losing my fly line and all of my backing.

FLIES: 9 WEEDLINE FAVES

Flies the author favors since they match the hatch by resembling in some respect the various prey found in/around sargassum lines.

Fishing fly that imitates an eel
EelJeff Pierce
Fishing fly that imitates a flyingfish
FlyingfishJeff Pierce
Fishing fly that imitates a small dolphin
"Peanut" (small) mahiJeff Pierce
Fishing fly that imitates a rainbow runner
Rainbow RunnerJeff Pierce
Fishing fly, salt firetiger Clouser
Clouser (firetiger)Jeff Pierce
Fishing fly that imitates a squid
SquidJeff Pierce
Fishing fly, Clouser
Clouser (silver bait)Jeff Pierce
Fishing fly, Wahoo Candy
Wahoo CandyJeff Pierce
Fishing fly that imitates a sargassum shrimp
Sargassum shrimpJeff Pierce

About the Author

Jeff  Pierce
Jeff Pierce started deckhanding on charter-fishing boats at the age of 10. Fly-fishing is his first love, but he does it all, be it jigging up snapper, live-baiting black marlin or fighting a 1,000-pound bluefin on stand-up. He was honored with the Federation of Fly Fishers Silver King award in 2004 for his long-standing promotion of saltwater fly-fishing. He now travels the globe selling fly-fishing gear for Scientific Anglers.Doug Olander / Sport Fishing