There is a threshold at sea beyond which most boats never travel. It lies somewhere past the last dive buoy, past the familiar seamount names on the chart, past the point where the satellite phone signal still feels like a connection to the world you left in port. Cross that threshold on the Andaman Sea and you find yourself on an outer shelf where the seafloor plunges away into darkness, where canyon walls drop hundreds of metres in the space of a kilometre, and where the fish that live along those edges have rarely, if ever, encountered a lure.
This is the deep Andaman — not a single location but a zone of productive drop-offs and underwater topography that stretches along Thailand's outer continental shelf, beyond the Similan and Surin island groups and into the approaches to the Burma Banks. It is liveaboard-only territory. It is also, for the angler willing to make the commitment, some of the finest deep-water fishing in Asia.
The Topography
The Andaman Sea sits on a shelf that extends northwest from the Thai coast before dropping precipitously into the deep Andaman basin — water more than 4,000 metres deep in places. The fishing interest lies not in the abyss itself but along the shelf edges and canyon features where that depth transition occurs. Here, strong tidal currents are forced upward along canyon walls, driving nutrient-rich cold water toward the surface and concentrating baitfish along the thermal and current boundaries.
The Similan Islands (about 100km northwest of Khao Lak) sit on this shelf at depths of 15–60 metres. They are Thailand's most famous dive destination and — within their national park boundary — unfishable. But they are not the destination. The destination is the water beyond them, where the shelf edge begins and the canyon topography that holds the biggest fish starts.
The Burma Banks — technically within Myanmar's Mergui Archipelago but accessible on the same liveaboard itineraries — are a series of submerged seamounts rising from this deeper water. The canyons and ledges between and around those seamounts are precisely the environment that dogtooth tuna and large grouper exploit as their primary habitat.
Many of the most productive deep-water Andaman locations overlap with or adjoin the Burma Banks zone covered in our Mergui Archipelago fishing guide. Liveaboard itineraries frequently combine both destinations on the same trip.
Dogtooth Tuna: The Deep Canyon Specialist
Dogtooth tuna are the defining species of the deep Andaman shelf, and they deserve a complete account. Taxonomically, they are not true tunas but belong to their own genus (Gymnosarda unicolor). Biologically, they are one of the most formidable fish in the Indo-Pacific: powerfully built, equipped with crushing conical teeth, and — crucially — built to dive. When a large dogtooth feels the hook, its instinct is to run deep and toward structure. On a canyon edge at 60 metres, with 200 metres of void beneath, that instinct is the central problem of the entire encounter.
Fish in the 15–35kg range are the standard expectation on a productive deep Andaman trip. Fish above 40kg are caught regularly across the region; specimens above 60kg are documented in the Andaman Sea, though such fish are rare. The world-record class fish exceed 100kg, and the deep Andaman is among the handful of locations globally where such specimens are theoretically possible.
Dogtooth are primarily caught on jigs worked along canyon walls and shelf edges, typically in 40–100 metres of water. They hold near the bottom during tidal changes and move up into the mid-water column during active feeding periods, particularly around dawn and dusk. Current is important — the best dogtooth fishing happens when there is enough current to animate the jig but not so much that controlling depth becomes impossible.
When a large dogtooth feels the hook, its instinct is to run deep and toward structure. On a canyon edge at 60 metres, with 200 metres of void beneath, that instinct is the central problem of the entire encounter.
Slow-Pitch Jigging: The Technique That Changed Deep Fishing
The development of slow-pitch jigging in Japan over the past two decades has transformed deep-water fishing globally, and nowhere more so than in the Indo-Pacific dogtooth tuna fishery. Understanding why requires a brief explanation of what slow-pitch jigging actually is.
Conventional high-speed jigging uses a fast, rhythmic lift-and-wind motion to work a metal jig upward through the water column in an aggressive, darting action. It is effective but physically demanding and best suited to fish that are actively feeding in the mid-water column.
Slow-pitch jigging uses a shorter, parabolic-action rod — softer in the blank, with a distinctive action that loads and releases energy during the lift — to impart a completely different motion to the jig. The pitch is slower, with a long, horizontal flutter on the fall that exactly mimics an injured or disoriented baitfish. The rod tip's resilience does most of the work, reducing angler fatigue significantly.
For dogtooth tuna along canyon walls, the slow-pitch fall presentation is often decisive. Dogtooth holding tight to the bottom or mid-structure will hit a fluttering jig on the drop that they might ignore during a conventional fast-wind retrieve. The technique demands specific rod design — interchangeable with a conventional jigging rod — and jigs shaped to flutter freely rather than track straight. Our jigging tackle guide covers rod and jig selection in detail.
Jig weights for the deep Andaman range from 150g in lighter current to 400g or more when flow accelerates. Leader material is 80–100lb fluorocarbon; assist hooks should be sized for the jig and strong enough not to straighten under the initial pressure of a large fish surging toward rock.
Giant Trevally on the Drop-offs
Giant trevally at the deep Andaman shelf edges are a different animal from their reef-flat counterparts. They are larger — fish of 20–35kg are not unusual on outer shelf structure — and they patrol deeper water than the typical popping environment. Jigging along canyon edges and steep drop-offs at 40–70 metres produces GT as a bycatch of dogtooth tuna fishing, but dedicated popping passes along the shallower tops of shelf features (where depth drops from 10–20m to 60m+ within a short horizontal distance) can be spectacularly productive.
Bluefin trevally and brassy trevally also appear along these shelf structures, particularly where upwelling concentrates baitfish near the edge. Both species respond to light jigging and surface casting.
Pelagics: Marlin, Sailfish, and the Occasional Swordfish
The open water above the deep Andaman canyons holds pelagic fish. Marlin — primarily black and blue — are encountered in the outer Andaman through the season, with the peak months for blues running November through February. Trolling skirted lures or rigged baits during transit between jigging locations gives liveaboards their best chance at marlin contact.
Sailfish are present but less reliably targeted in the outer deep-water zone than at the better-known sites like the Koh Rok area or in the central Andaman. They are more of an incidental encounter in the deep canyon context.
Swordfish occupy a special category in the Andaman. They are present — swordfish are documented throughout the deep Indo-Pacific — but specifically targeting them requires deep-drop equipment (electric reels, very heavy line, large squid or fish baits deployed at 200–500m at night) that few liveaboards carry. Dedicated swordfish targeting is a specialist endeavour that a small number of experienced operators have attempted with success. If this is a primary goal, ask operators specifically about their swordfish capability before booking.
Deep Grouper and Canyon Bottom Fish
The canyon walls themselves hold resident populations of large grouper that rarely feature in standard fishing reports because reaching them requires genuine deep-water capability. Brown marbled grouper and coral grouper at 80–150m on canyon structure can reach sizes that are no longer seen on accessible reefs. Slow-pitch jigging along these walls — effectively drifting the jig across vertical structure on the fall — is the most consistent approach.
Large golden snapper and various deep-water snapper species also hold along canyon ledges. Bottom-fishing with heavy rigs and natural bait can be spectacularly productive on these species for vessels that carry the appropriate terminal tackle.
Liveaboard Selection and Logistics
The deep Andaman canyon fishery is exclusively liveaboard territory. Transit times from Khao Lak to the outer shelf edges run 8–12 hours, making day-trip logistics impossible. Most productive itineraries allocate five to eight days to allow proper coverage of multiple canyon systems and to account for weather variability.
Vessel capability matters more here than at any other Thailand destination. The outer shelf can develop significant swell on unfavourable weather windows, and a vessel that handles rough conditions safely makes the difference between extending a trip to fish a better location and running for port prematurely. Ask specifically about hull design, stabiliser equipment (if any), and the operator's weather criteria for holding versus returning.
Crew expertise is equally critical. Deep canyon jigging is a technical discipline that benefits enormously from experienced deckhands who understand current reading, jig selection, and the mechanics of fighting large fish at depth. Check our liveaboard operators Thailand overview for operators that specifically market deep-water jigging rather than generalist fishing liveaboards that treat jigging as a secondary activity.
See our 7-day Andaman fishing itinerary for a template that builds deep canyon fishing into a broader Andaman program, and the 10-day Thailand grand tour fishing itinerary if you want to combine deep canyon work with both Andaman and Gulf destinations.
Conservation and Depth
The deep Andaman canyon ecosystems are among the least-studied and least-disturbed marine environments accessible to anglers in Southeast Asia. The fish populations along these canyon walls have been subject to modest commercial pressure — some trawling occurs on the shelf — but the canyon faces themselves are generally too steep and rough for trawl nets, providing de facto refugia for large grouper and reef-associated species.
Catch-and-release for all but one or two table fish per vessel is strongly encouraged for deep-water trips. Large dogtooth tuna and grouper are long-lived, slow-growing species that represent decades of growth. Removing them from the canyon ecosystem reduces the population of precisely the fish that drew you there in the first place.
Review the catch-and-release rules Thailand guide for current best-practice on deep-water species release — decompression considerations apply to fish brought up from depth.
The deep Andaman shelf is one of Asia's genuine fishing frontiers. It will stay that way for anglers who fish it with care.
