Blackwater Diving - Diel Vertical Migration

Marine ecologist Irene Middleton portrays blackwater diving off New Zealand's coast...

As I hover 10 metres below the surface in the inky darkness, I am trying to focus on the lights ahead of me rather than the 150m of water below me (not to mention the swordfish and makos that frequent the area). My husband hovers nearby, looking decidedly more comfortable than I feel. It’s 1am, we are ‘blackwater diving’ about 5 miles east of the Poor Knights Islands, and I am wondering how on earth I got myself in this situation.

The world’s largest migration of animal life occurs every day in every ocean around the world, and it happens largely unseen. As the sun sets, billions of animals, mostly zooplankton (e.g., smallish animals, including fish, crustaceans, and jellyfish), migrate up from the dark, chilly waters of the deep ocean to the shallow surface waters. Then, as the sun rises, the organisms sink back into the deep to start the process all over again. Known as the diel vertical migration, this mass movement is thought to represent a colossal game of hide-and-seek where prey items spend the day hiding in the deep waters to avoid predation and then migrate to surface waters at night to feed under the cover of darkness.

Diel vertical migration was first documented in the early 1800s by Georges Cuvier in a shallow freshwater lake. He noted that water fleas would disappear and reappear en masse on a daily cycle. Then, during World War II, navy sonar pings picked up what looked like a mid-water seabed in the oceans that disappeared each night. Scientist Martin Johnson hypothesised that the disappearing false seafloor could be marine animals migrating up to the surface. In 1945, he proved his theory by collecting zooplankton, jellyfish, and various crustaceans during an overnight excursion off California.

It wasn’t until nearly 40 years later that the public was exposed to the magic that is diel vertical migration. National Geographic published an article, that was quickly followed by a coffee table book (Within a Rainbowed Sea by Christopher Newbert), which included some of the first images of rarely seen deep water creatures near the surface at night over deep water off Kona, Hawaii. Kona became the birthplace of ‘blackwater diving’, which in the words of some of the most experienced guides, involves “throwing divers off a boat in the dark of night in open ocean waters that are, for all practical purposes, bottomless.”

So, I assume at this point that you are wondering why on earth someone would do that?! The answer comes down to the lure of the unknown. Even the most experienced divers are likely to encounter sea creatures that they – or no one – have ever seen before. As marine scientists, my husband and I are inherently curious about not only the tasty species in the ocean but also all the tiny, less charismatic (yet equally fascinating and important) ocean critters. Very few people have been crazy enough to blackwater dive in New Zealand, but the lure of the unknown and the chance to encounter the world’s largest migration in our backyard was too tempting to pass up.

Unlike Kona, northern New Zealand is not the ideal place to jump into the open ocean in the middle of the night – oceanic whitetip sharks notwithstanding. Kona, and other locations globally – Cozumel, Florida, and the Philippines – where blackwater diving has taken off have two things in common. Firstly, Kona has no continental shelf, and the seafloor drops away to about 300m within a mile of shore. And secondly, it has relatively predictable, calm conditions to ensure divers don’t get separated from the boat or surface buoy.

This predictability isn’t something that applies to New Zealand; we rarely have more than a few days (or nights) in a row of 5-knot variable winds. Also, our continental shelf is massive! To reach depths over 100m, we must travel at least 10 miles offshore. We’ve had a lot of unfruitful missions, turning around before getting to our target depth or spending uncomfortable nights being rocked around in anchorages at the Poor Knights Islands. We even ended up with a stern telling off by the local Coastguard one night after a local on the coast reported a plane going down south of the Poor Knights. We heard the report on the VHF and were thinking, “Hmm, that’s where we are, and we haven’t seen a plane.” Then we realised our massive underwater lights, flashes, and strong spotlight torches probably looked like the lights of a drowning plane. We called the Coastguard and promised to let them know the next time we were crazy enough to jump into the ocean in the middle of nowhere in the middle of the night.

It took us a while to gather the courage to plunge into blackwater diving. As marine ecologists, we understand that along with the little critters, sharks and billfish feed near the surface at night. You are in a three-dimensional space with few reference points, so sometimes, it’s hard to know if you are swimming up, down, or sideways. When you don’t have those visual references, and there is a bit of chop or water movement, seasickness is another factor to contend with.

However, on those rare occasions when the weather conditions and our work lives line up, blackwater diving in New Zealand is good – really, really, good. Like hundreds of paper nautilus swarming around you, good! We have seen juvenile tripod fish that, as adults, only occur at depths over 1000m, juvenile flying fish with bizarre moustache-like filaments that have yet to be identified by scientists, and a range of deepwater cephalopods that have only been previously seen captured in fishing nets.

We have also witnessed rarely seen behaviours: paper nautilus rising to the surface to vent and refill the air chambers they use for floatation; pelagic blanket octopus juveniles whipping at the camera with stinging tentacles they have stolen from jellyfish; and deepsea fish like ray’s bream and deepwater stargazers staring motionless at the lights suspended below the boat. Another common species we see in the open ocean – the pram-pusher shrimp – was the inspiration for the monsters Sigourney Weaver battled in the movie Alien. Thankfully, in real life, they do not exceed 20mm in length and parasitise salps rather than unsuspecting astronauts. The female captures and hollows out an unsuspecting salp and lays its eggs inside. Upon hatching, the salp becomes a nursery that the shrimp pushes around like a pram, hence the common name.

We also started exploring the open ocean during the day to see if we would bump into some of these same nighttime critters. Surprisingly, the combination of blue and blackwater diving has resulted in sightings of most, if not all, of the main target species at overseas blackwater diving meccas. Even more surprising is how often our sightings coincide with the same species being seen on blackwater dives thousands of kilometres away.

We still have a lot to learn about the deep sea, and with our vast coastline, the opportunities in New Zealand are seemingly endless. So far, scientists have used our photos to describe previously unknown life stages, we’ve discovered and written up a scientific paper describing a new-to-science boxfish, and our blackwater imagery has been shared globally.

Blackwater diving is genuinely one of the final frontiers of exploration diving for biodiversity in New Zealand, and we can’t wait for next summer (hopefully without La Niña). With any luck, we can bump into more amazing new critters while avoiding the big animals that go bump in the night!

June 2023 - Irene Middleton
New Zealand Fishing News Magazine.
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