Hammerhead sharks: the offshore schooling mystery

Hammerhead sharks: the offshore schooling mystery

Scalloped hammerheads (Sphyrna lewini) are among the most visually striking sharks in the ocean — that wide, flat head with eyes at opposite ends — and they exhibit one of the most mysterious behaviors in marine biology: they form large schools, almost always of females, at specific seamounts during specific times of year.

Nobody fully understands why. The prevailing hypothesis involves navigation via magnetic anomalies in the seabed, with the schools serving as a kind of rendezvous point. What matters to divers is that the behavior is predictable enough to plan a trip around.

The reliable schooling sites

• Daedalus Reef, Egypt (Red Sea) — summer (June–August), south plateau, 30m descent at dawn
• Layang Layang, Malaysia — April–August, 30m drop-off on the north wall
• Cocos Island, Costa Rica — liveaboard-only, Pacific, year-round with seasonal peaks
• Galápagos Islands — Darwin and Wolf in the northern archipelago, June–November
• Socorro, Mexico — December–May
• Bimini, Bahamas — winter (December–March), great hammerheads specifically

Dive technique

Hammerhead schools are usually deep (25–40m) and skittish. Descend fast, stay motionless, minimize bubbles. Flash photography spooks them. The most reliable sightings are at dawn, when the fish are still moving between deep-water daytime holding and shallower cleaning stations.