There's a corridor in the southern Andaman Sea, running roughly between the outer islands of Krabi Province and the open water south of Phi Phi, where Indo-Pacific sailfish concentrate in numbers that still surprise anglers who encounter them for the first time. The focal point of this fishery is the area around Koh Rok — two small islands, Koh Rok Nai and Koh Rok Nok, sitting about 47 kilometres south of Koh Lanta in Krabi Province. Between January and March, this patch of ocean produces some of the most consistent sailfish action available anywhere in the eastern Indian Ocean.
This is not a secret. Experienced billfishers know the Koh Rok corridor, and serious operators — primarily out of Phuket and Krabi — have been running trips here for decades. But it remains less crowded than comparable sailfish fisheries in other countries, the access is genuinely achievable from a Phuket or Krabi base, and the fish quality — big, acrobatic, and numerous during peak weeks — justifies the logistical effort of getting there.
What and Where
Koh Rok sits in administrative Krabi Province but is most commonly accessed from Phuket to the northwest, from Krabi Town and Ao Nang to the northeast, and occasionally from Trang-based operators to the south. The distance from Chalong Bay in southern Phuket is approximately 130 kilometres — roughly a two-and-a-half to three-hour run on a modern sportfisher in calm conditions. From Ao Nang in Krabi, the transit is shorter at around 80–90 kilometres.
The islands themselves are a marine national park — Mu Ko Lanta National Park — and the fishing does not happen inside the park boundary. The productive sailfish water lies in the open-water approaches to the islands, particularly on the current lines and bait concentrations that form in the passages to the west and southwest of Koh Rok. These are open-ocean positions that shift with conditions, and finding the right water on a given day is part of what separates an exceptional day from an average one.
The current lines are the key. The southern Andaman in this area is influenced by a complex interaction of tidal flow, wind-driven surface currents, and the deeper Indian Ocean circulation. When these align — typically during neap-to-spring tidal transitions in the peak January–March window — bait concentrations form along visible current edges, and sailfish follow them inshore. A green-blue colour change in the water, floating debris lines, and flocking seabirds are all indicators worth investigating.
On the park boundary
Koh Rok and Koh Ha (a popular dive site further south) are both within national park jurisdiction. Fishing within the park boundary is prohibited, and park rangers do patrol these waters. Responsible operators fish the approaches and open water — not inside the park — and experienced skippers know exactly where the boundaries lie.
The Species Mix
Indo-Pacific sailfish (Istiophorus platypterus) are the overwhelmingly dominant target and the reason this fishery has a reputation at all. During the January–March peak, multiple fish per day is a realistic expectation for a properly run charter on a good week. Two to five hookups in a day is common; exceptional days with ten or more encounters are documented and not as rare as they'd be in many comparable fisheries.
These are large animals. Indo-Pacific sailfish are generally accepted to run bigger than their Atlantic counterparts, and adult fish in the Koh Rok area average in the 35–55 kg range, with larger individuals present. A fish over 60 kg is a notable catch anywhere; Koh Rok produces them. They fight differently from marlin — faster, more acrobatic, with the signature leaping and bill-shaking that makes them spectacular on light tackle.
Wahoo are a consistent secondary catch in the same water. They inhabit similar temperature breaks and current lines as sailfish, and the same trolling spread that attracts sailfish will encounter wahoo. A 15–25 kg wahoo on a 20 lb class outfit is one of the more thrilling experiences in Andaman day fishing. Wire leaders are essentially mandatory for wahoo — their dentition will cut through monofilament and fluorocarbon with a single closing of the jaw.
Dorado (Coryphaena hippurus), also called mahi-mahi, appear seasonally in the Koh Rok area, particularly around floating debris and weed lines. When they're present — typically in the warmer surface water of February and March — they're aggressive and responsive, providing fast action on light tackle between sailfish encounters.
Yellowfin tuna can be found on the same bait concentrations as sailfish, particularly when frigate birds or terns are working visibly. Schools of small to medium yellowfin (5–20 kg) are typical, with larger fish occasionally present. The same trolling spread that covers sailfish will account for yellowfin, though dedicated tuna anglers would fish differently — faster trolling speed, smaller lures.
Giant trevally are not a primary feature of the Koh Rok offshore fishery, though they're present around the reef structure of the islands themselves. An operator focused on sailfish won't typically spend time working for GTs, but anglers with half a day to spare before or after the offshore run can find GT action around the Koh Rok reef edges.
Season and Conditions
The Koh Rok sailfish season follows the northeast monsoon pattern, but the peak is narrower than the broader Andaman season. The best fishing is concentrated in January through March, with February typically the single best month. December sees fish arriving and the season building; April sees it trailing off as the water temperature rises and bait disperses.
The southern Andaman in this area is influenced by both monsoon systems and sits in a semi-transition zone between the fully monsoon-dominant northern Andaman and the more complex Gulf-influenced southern waters. This means the window can be variable: some years the sailfish arrive in force by late December, others not until late January. March is reliably good in most years.
Weather during the peak season is generally settled. The northeast monsoon keeps the Andaman flat, winds are light and from the northeast or east, and visibility at sea is typically excellent. The three-hour run from Phuket in December–February conditions is typically comfortable on any decent sportfisher. As April approaches, southwesterly swell begins building and the run becomes progressively less pleasant.
Sea surface temperature in the peak season runs around 28–29°C in the zone. Sailfish prefer a specific temperature range and tend to concentrate on the edges of warm and slightly cooler water bodies — this is what produces the current lines and colour changes that experienced skippers target.
When the current lines come together between January and March, the Koh Rok corridor produces sailfish fishing that competes with anywhere in the eastern Indian Ocean.
The Trip — Day Boat vs Liveaboard
The Koh Rok sailfish fishery is primarily done as a long day trip from Phuket or a shorter day trip from Krabi. Neither option requires a liveaboard, though overnight trips to the area do exist and offer genuine advantages.
A Phuket day charter to Koh Rok leaves Chalong Bay at first light — typically 5:00–5:30 AM — and arrives on the grounds between 8:00 and 9:00 AM. This leaves five to six hours of productive fishing time before the return transit. On a good day, that's enough time to encounter multiple sailfish. The return to Phuket is typically around 5:00–6:00 PM, making it a long but not unreasonable day if conditions cooperate.
A Krabi-based day charter from Ao Nang saves an hour each way — the grounds are closer. Ao Nang has a developing but smaller charter fleet compared to Phuket. The quality of sailfish operators from Krabi has improved considerably in recent years, and for anglers already based in Krabi, the logistics are significantly simpler.
Overnight trips that anchor in the protected waters near Koh Rok allow a very early morning start on the grounds — often the most productive time — before heading back. These are offered by some operators and are worth the modest additional cost if the sailfish are concentrated in a specific area rather than spread across a wide zone.
Techniques
Live bait trolling is the dominant technique and the most productive approach for Koh Rok sailfish. The standard rig is a live scad or hardyhead baitfish on a circle hook, fished at the surface on an outrigger at 3–5 knots. When a sailfish tailing up behind the lure is spotted, experienced crews slow the boat, free-spool the bait, and allow the fish to take it before winding tight. The circle hook, with proper technique, produces clean corner-of-mouth hook-ups and excellent release survival.
Dead bait trolling with rigged ballyhoo or flying fish is also effective and produces results when live bait isn't available. Dead bait at slightly higher speeds — 7–9 knots — covers more water and can produce consistent sail strikes when the fish are spread across a wider zone rather than concentrated on a specific line.
Lure trolling with soft-head lures in the 4–6 inch range, run in a spread of four to six lures at 7–9 knots, is the fastest covering approach and works well for locating fish before switching to live bait. Many charters run a combination: lures and dead baits on the outriggers at searching speed, with a live bait rigged and ready to deploy when a sail is raised.
Light tackle teasing — raising sailfish to a teaser and then pitching a live or dead bait to a teased fish — is a specialist technique that produces the most intense encounters. Watching a sailfish lit up and chasing a teaser while a skilled angler pitch-casts a live bait from the cockpit is one of the peak experiences in sport fishing. Not all operators are set up for it, but the best billfish-focused charters run this system.
Tackle That Works
Koh Rok sailfish are typically fought on light to medium tackle, which is both appropriate for the fish size and consistent with a catch-and-release ethic. Standard sailfish outfits run 20–30 lb class rods — stand-up style for most day-charter situations — matched with conventional lever-drag reels holding at least 400 metres of 30 lb monofilament or equivalent braid. The fights are rarely marathon affairs, and most sailfish are brought to the boat within 20–30 minutes on 30 lb class gear.
IGFA-spec 20 lb class is the choice for anglers who want a genuine challenge. On a 20 lb class outfit, a large sailfish is a significant undertaking, and the fight is longer and more demanding — which is precisely why some anglers choose it. Any reputable operator will have both 20 lb and 30 lb outfits available.
Leaders of 60–80 lb fluorocarbon in the 1.5–2.5 metre range are standard for sailfish rigging. The leader must be long enough to manage the fish at the boat for unhooking but not so heavy that it inhibits the bait presentation or adds unnecessary line class below the fish's natural caution threshold.
For wahoo — which often share the same spread — wire leader sections of 49-strand braided wire in 60–80 lb are essential. Some operators rig wahoo-specific baits on wire while keeping the sailfish baits on fluoro. This is the correct approach.
The Operator Landscape
The Koh Rok sailfish run attracts the most focused billfishing operators on the Andaman coast. Look for charter operators who specifically advertise the Koh Rok or southern Andaman sailfish run rather than those offering it as a seasonal option among general offshore fishing. Dedicated billfish operators — the ones who have been running this specific trip for years — have accumulated knowledge of current patterns, historical bait concentrations, and seasonal timing that genuinely improves catch rates.
Key questions when booking: Does the boat run live bait or dead bait, or both? What hook sizes do they rig? Do they practice catch-and-release? What is the vessel's top speed and how long is the transit time? A faster boat means more fishing time — on a three-hour run each way, a boat that does 28 knots versus 18 knots is giving you an extra two hours on the grounds.
Liveaboard operators from Phuket who combine Similan trips with a run south to the Koh Rok grounds also exist, offering a composite trip that covers GT and jigging at the Similans alongside sailfish in the south. These are complex logistics but produce impressive species diversity.
Conservation and Ethics
Koh Rok sailfish are universally regarded as a catch-and-release fishery by the serious sportfishing community in Thailand, and this should be a firm criterion when choosing an operator. Sailfish are not commercially valuable, they're not particularly good eating compared to other species available in Thailand, and their population status in the Andaman — while not formally assessed — depends on a culture of responsible handling.
The mechanics of billfish release matter. Circle hooks reduce deep hooking and improve survival rates compared to J-hooks. Keeping the fish in the water alongside the boat, removing the hook with de-hooking tools rather than lifting the fish aboard, and ensuring the animal is revived before release — these are the standards that serious billfish operators and anglers practice. If your operator insists on landing and photographing the fish out of the water, choose a different operator.
The marine park at Koh Rok is equally worth respecting. The reef system around the islands is intact and supports the bait concentrations that in turn support the sailfish. Fishing within the park, anchoring on reef, and other destructive practices are not just illegal — they undermine the ecological foundation of the fishery itself.
Where to Go Next
The southern Andaman is a broader fishery than the Koh Rok headline suggests:
- Sailfish season in Thailand — the full seasonal breakdown for Andaman sailfish
- Andaman Sea fishing guide — the complete overview of the western coast fishery
- Racha Yai and Racha Noi — a closer day-trip option for GTs and reef fishing from Phuket
- Krabi — the eastern gateway for southern Andaman charters
- Catch-and-release rules in Thailand — the framework and ethics for billfish release
