Grammatorcynus bilineatus
Uncommon but logged year-round in Khao Lak and South Sinai, this streamlined mackerel favours warm reef water around 26 to 29 C.
Torpedo-shaped and silvery blue-grey, growing to around 70 cm, with two dark horizontal lines running along the upper flank that give the species its name. The tail is deeply forked like other mackerel and tuna, and the body lacks any bold blotches or bars, just that clean two-line marking against a pale belly. Divers usually spot it mid-water rather than tight to the reef, moving with a purposeful, steady swim rather than darting.
Typically seen cruising open water just off the reef edge rather than schooling tight over coral, often alone or in loose small groups. It keeps a fair distance from divers and doesn't linger for a close look, so sightings tend to be brief, mid-water passes rather than extended encounters. Most reports come from reef drop-offs and current-swept points where baitfish and open water meet the structure.
Unlike many of its scombrid relatives it's often found around reef structure rather than purely open ocean, giving it a reputation among the mackerel family as a more reef-associated species.
Logged year-round with no clear seasonal peak in the data, which fits its comfort range of roughly 25.6 to 29.1 C, temperatures that hold steady on these reefs most months. Plan around good general conditions at your chosen site rather than a specific mackerel season.
Reef neighbours a diver could mistake it for. Use the identification notes above to tell them apart.
Most reliably logged in Thailand, particularly around Khao Lak and Koh Lanta, with a second cluster of sightings in Egypt's Red Sea across South Sinai and Marsa Alam & the Deep South. Sightings are on the sparse side wherever it turns up, so treat it as a welcome surprise rather than a target species on either coast.
Mapping where Double-lined Mackerel is logged…
Thailand offers the best odds, especially Khao Lak and Koh Lanta, with additional sightings logged in Egypt's Red Sea around South Sinai and Marsa Alam & the Deep South. It's uncommon at every site where it's been recorded, so don't expect guaranteed encounters.
There's no detected seasonal pattern, and sightings are spread across the year. Its preferred water temperature of roughly 26 to 29 C is present on these reefs for most of the year, so timing your trip around general dive conditions works better than chasing a season.
Look for a silvery blue-grey, torpedo-shaped fish around 70 cm long with two distinct dark lines running horizontally along its upper side. It has a deeply forked tail and swims mid-water with a steady, unhurried motion rather than hovering near the reef.
They typically reach around 70 cm in length, giving them a lean, streamlined build typical of the mackerel and tuna family.
No, they pose no danger to divers. They keep their distance and are simply passing swimmers rather than any kind of threat.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Allison Tuna
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