Black water diving is an unusual type of night diving where instead of focusing on a reef or patch of sand, you dive into very deep water with no ocean topography in sight.   In a typical night dive, your goal is to see what reef species are up to at night - watch the nocturnal hunters like octopus, sharks and lion fish go after prey, see crabs and lobsters roaming about more freely. 

But in a black water dive, you aren't on a reef. Your goal instead is to observe the 

 — a daily show that is the largest migration of biomass anywhere on Earth.

Diagram courtesy of the Woods Hole Oceanographic Institute 

https://www.whoi.edu/ocean-learning-hub/multimedia/ocean-twilight-zone-migration/

While there are nocturnal predators in the ocean, many marine predators need visual light to hunt so much potential prey has evolved to stay in deeper, darker waters to avoid predation. But if you are a small creature, there's not much to eat down there. If you want to eat something like algae that needs light to photosynthesize or tiny critters that eats that algae, you need to come up to the shallows. So every night, billions of organisms — from tiny zooplankton to juvenile fish and squid — ascend from deep. 

Many of the animals involved are larval or planktonic (tiny and free-drifting) forms of bigger animals: fish, cephalopods, crustaceans, jellies, etc. Black water diving lets you watch a slice of this migration — the bright lights act as a temporary reference point in the water column, and organisms drifting upward often pause or circle through the beams long enough for you to see them. The ocean is so vast and under-explored that there are untold number of species involved in this migration that have never been studied or even seen by humans. Divers are almost guaranteed to see species they’ve never seen before on black water dives, and it’s common to see a species that no one has previously documented. 

We’ve wanted to try black water diving ever since we first heard about this diversity while in Hawaii, but sea conditions were never right. Black water diving was actually pioneered in Hawaii in the 1980s - the deep channel between the Big Island and Maui is prefect for it - but it requires very calm seas to do safely.  Other scuba hotspots have since gotten in on the game (Anilao in the Philippines most famously) and since the Siladen area has calm and deep water it’s a great place to give it a try.

Every operation does black water dives a little bit differently, but at  Siladen the crew drops a vertical line of bright lights, and divers hover around it as life from deeper layers of the ocean drifts up to investigate.  Divers are paired one-to-one with a guide and are free to move around. In other places divers are in bigger groups, tethered to lines running underneath the boat. We greatly enjoyed the personal attention at Siladen, it meant Vera could focus extra hard on maintaining buoyancy with no visual indicators and Andrew could focus on photography, while the guides did most of the spotting for us with their flashlights.

The first things to appear were small worms and specks of plankton. Then we started seeing jelly-like creatures - salps, comb jellies, and jellyfish. Big numbers of small and medium-size squid arrived, sometimes bumping into each other and inking the already dark water. Mixed into the busy waters we’d also find the transparent shapes of larval shrimps. Then more complex forms drifted by — including a larval Moorish idol and a larval flying fish both looking like smaller versions of their familiar adult relatives.

We weren’t able to capture everything on camera, but here’s a small taste of what we saw:

Larval Moorish Idol, with its dorsal fin much more elongated than in the adult form.

Black Water Diving - Siladen