MBARI Collaborator Katie Thomas

How do we catch black fish?

Three members of the team (Sonke Johnsen, Katie Thomas, and Karen Osborn) are working on figuring out how deep-sea fish make themselves super black to disappear into their dark surroundings. While many deep-sea animals can be collected in beautifully pristine condition with ROVs, fish are fast swimmers and can generally avoid the ROV altogether. If we even see these fish during a dive, they can easily escape when we try to capture them.

Instead, we turn to a method that has been used since the first deep-sea expeditions: trawling. This involves dragging a net deep in the water behind the ship and pulling up everything that gets caught in it. The net we are using is small compared to some, but still is too heavy to lift without a winch and takes a team of people to deploy and recover.

Black fish are frequent components of our hauls from 1,000 to 600 meters depth, along with crustaceans, juvenile cephalopods, ostracods, pteropods, jellies, siphonophores, and many other spectacular creatures. We can target certain species by trawling at the depths they normally occupy, but we never know what we will catch in the net until it comes up, which makes every trawl an adventure!