ABOARD THE R/V THOMAS G. THOMPSON—While waiting for the weather to die down and for solutions to problems with our winch and the vehicle, the rest of our non-Nereus science program has continued. We have put both of the landers and the fish trap in the water almost every day so far at varying depths in order to get a better sense what’s swimming around where.
A lander is simply a metal tripod designed to carry a payload, and to be deployed on and recovered from the seafloor. A set of weights takes it to the bottom and anchors it there until we send a coded signal into the water and an acoustic release drops the weights letting the lander rises to the surface pulled by a string of glass spheres encased in orange plastic “hard hats.”
These landers are special because they and their payloads are designed to withstand the pressure of the deep ocean. We have two on board. The abyssal lander is currently equipped with strobes and a still camera designed for about 7,000 meters depth. It is programmed to take one photo every minute and will allow the team from Aberdeen University to identify and measure the fish that visit a baited stage beneath it.
The hadal lander is rated for full-ocean depth and is equipped with lights and a video camera, which turn on for one minute out of every five. The Aberdeen crew uses this lander to observe the behavior of species and individuals that visit the bait or that simply swim past while the cameras are on. Both also hold an amphipod trap, which captures the small crustaceans that are found at all depths in the ocean, from beach to trench.
Alan Jamieson from the Oceanlab at Aberdeen holds the record for the number of successful trench deployments—currently at 120 and growing. He also most likely holds the record for the number of deep-sea landers lost during deployments, a distinction he wears with pride, as it means he's out doing hadal research, work that carries with it an inherent and substantial risk to equipment. “Landers, particularly ones that go to 10 kilometers, don't usually live to a ripe, old age,” he noted recently.
The amount of data that comes back with the landers depends on how long they are on the bottom. Yesterday both came back after a two-day deployment, which means that Jamieson and his graduate student, Thomas Linley, have 1,800 photos and 13 hours of video to pour over in between deployments.
So far, they have seen almost exactly what they've expected. On the test deployment to 1,500 meters, eels ravaged the bait on both landers. Snubnose eel latch on with their small, powerful jaws and then twist their bodies to tear off chunks of fish. Basketwork eels have large but very weak jaws and have to wait for a shark to visit the bait and loosen chunks of soft tissue for them to feed, essentially using the sharks as a can opener.
Below 2,000 meters, the eels disappear. After 3,000 meters the sharks no longer visit the lander and the mix of species in the feeding frenzy changes. From 3,000 to 5,000 meters rattails dominate, followed by cusk eels from 5,000 to 6,500 meters—though cusk eels spend most of their time simply staring at the bait.
After 7,000 meters the number of fish visiting the lander dwindles until they disappear entirely well before 8,000 meters. The current record depth for a photograph of a fish is 7,703 meters, but theoretically they should be able to survive to 8,200 meters. After that, their physiology would have to change entirely in order to maintain the salt balance in their cells. More on that later.
Linley, Jamieson's student, also spends time in the biology lab after the fish trap comes to the surface working on his own research, which involves identifying and preserving specimens electronically. You'll learn more about his work in a video we'll be posting soon.
The third piece of equipment we are putting overboard, the fish trap, is a large mesh-enclosed cage with one-way entrances and baited containers inside to attract both large and small animals. When it comes on board, a feeding frenzy of our own occurs, as members of the science crew descend on it to retrieve any fish and amphipods that it captured. These samples head back to the cold room and bio lab inside the ship, where they begin a gradual process of reduction. First they are sorted, photographed, and identified before being dissected, sampled, sub-sampled, and preserved.
Blood samples and stomach contents as well as pieces of muscle, heart, liver, gonads, and other tissue are portioned out and frozen or pre-treated for later analysis here or on shore. We even took a sample of feces from one fish so that its gut microbiology could be analyzed. People will examine many of these for signs of cellular, genetic, and molecular adaptations to life in the deep ocean, as well as features that might identify an individual as a new species. Already we have one candidate of fish and several amphipods, but confirmation of these will require a level of taxonomic analysis that we can't do.
Yesterday's specimens also included a large cusk eel and rattail that came back after Drazen and his crew modified the trap with baited hooks outside to catch some of the fish they suspected were visiting, but not venturing inside. In addition, Jamieson's hadal lander brought back the first video evidence of cusk eels feeding on amphipods, something no one had known until now. Both the large fish and the videos of feeding behavior will help answer some very fundamental biological questions about deep-ocean ecology, including the age and growth characteristics of one of the larger hadal predators and the structure of the food web in this part of the trench.
Rattails and many other hadal species are scavengers that rely on dead animals floating down from above for a steady supply of food and there is evidence that links changes in populations of surface fish to communities of animals in the deep. Changes like this can occur naturally as a result of migration, but over-fishing could also potentially reach well below the level of nets and lines to impact entire ecosystems thousands of meters below. "It's not out-of-sight, out-of-mind," said Jamieson. "It's all one big body of water."