Magdalena Bay · Mexico · Pelagic & blue water
Thetis Bank
Striped marlin hunting sardines offshore of Magdalena Bay, October to early December
Thetis Bank is one of six high spots on the Ridge, a 56 km (35-mile) seamount north-west of Magdalena Bay, and the offshore ground worked each autumn when striped marlin gather to hunt sardines. The diving is in open water: boats cover ground looking for bait driven up from below, and put swimmers in when they find it. More than 30 marlin can be in sight at once when a hunt is running.
Where it is and how deep
No published position for this site — the map shows Magdalena Bay and its other sites; operators know the mooring.
The dive
Nothing about this one starts in the water. Boats run offshore and then spend the day covering ground, watching for working birds and for bait breaking the surface, because the bank is a search area rather than a mark you tie into. When something is found, swimmers slide in a few metres from it, swim hard to stay with the action for as long as it holds, then climb back aboard and reposition for the next pass.
Depths worked here run from about 5 to 30 m, but the encounter itself is shallow. What you see sits in the top few metres of open ocean, and the deepest you go is a short breath-hold drop to hold position rather than chase the action from the surface. More than 30 marlin can be in view at once when a hunt is running, lancing through the bait ball and turning back for another pass, and the whole thing can be over in a minute or last most of a morning.
You are rarely alone with them. Orcas, mako sharks, bottlenose dolphins, sea lions, wahoo and yellowfin tuna all hunt the same bait, and humpbacks, fin whales and the rarer sei whales pass through the same water. Currents are gentle inside the bay but can run strong out here, which is why swimmers go in for short bursts and the boat stays close to pick them up.
Background
Anglers knew Thetis Bank long before divers did. Gary Graham found its autumn marlin bite by accident in 1979, and in good autumns since, such as 1999 and 2000, perhaps 200 striped marlin could be raised in a week's trolling. The fish follow the sardines, so the bank is the ground of choice when the bait is still to the north, and frigate birds flying out from roosts in the Magdalena Bay mangroves mark where the action is.
Marine life and season
- Marlin & sailfish
- Whales
- Orcas
- Tuna
- Dolphins
- Seals & sea lions
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| Off-season | Off-season | Off-season | Off-season | Off-season | Off-season | Fair | Fair | Good | Prime | Prime | Good |
Season follows Magdalena Bay: The baitball season runs Oct–Dec and peaks in November; outside it the marlin disperse and operators do not run trips.
Know before you go
- Currents are gentle inside the bay but can run strong offshore; work to the boat's pick-up plan instead of trying to swim back up-current.
- Mako sharks and orcas hunt the same bait as the marlin; enter alongside a ball rather than into it and let the animals set the distance.
- There is no bottom, no line and no fixed reference, so a swimmer who loses the boat in swell has nothing to hold position on.
Questions divers ask
Where is Thetis Bank?+
Offshore on the Pacific side of Baja California Sur: it is one of six high spots on the Ridge, a 56 km seamount north-west of Magdalena Bay. There is no fixed dive position; boats search the area for bait rather than tie into a mark.
When is the season at Thetis Bank?+
The window runs from October to early December, with October and November the reliable months for marlin hunting bait offshore.
How deep is the diving at Thetis Bank?+
The profile for diving in Magdalena Bay is 5 to 30 m, but the marlin encounters themselves happen in the top few metres of open water.
Sources
- Magdalena Bay Diving, Baja California - Mexico — Dive The World
- Offshore Fishing in Magdalena Bay — Sport Fishing Magbay
- Fishing Magdalena Bay's Offshore Thetis Bank — Mexfish
- Magdalena Bay — Nautilus Liveaboards
In The Logbook
- Where to dive in every month of the yearTrip planning · 14 min read
- Where to dive with seals and sea lionsBig animals · 17 min read
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