Sharks are spending time in the deep ocean, but why? WHOI scientist explains new research
A new study indicates that the ocean’s twilight zone, below the surface layer, is critical to sharks and to popular food species, such as tuna. CAI’s Jennette Barnes talks with the study’s lead author, Camrin Braun, an assistant scientist at Woods Hole Oceanographic Institution.
Jennette Barnes: Scientists put tiny sensors on almost 350 fish from 12 species of predators, including tuna, white sharks, and swordfish. The sensors relay information about how the fish move and the water depth and temperature where they’re swimming. I asked Camrin Braun to talk about the highlights.
Camrin Braun: We collected about 42,000 days of this data that kind of tells us about … a day in the life of these predators, if you will. And … I think it's pretty well known, I would say, by the community how some of these predators … visit our local waters in the summer, for example. I think a lot of people don't know, and many scientists included, is kind of what happens when those animals leave the coastal areas where we're most familiar with interacting with them. And so we set out to kind of ask this question of, you know, what are these animals doing when they go way out into the open ocean? … And kind of what we discovered is that … it seemed like consistently there were at least some species that were pretty clearly driven to use the deep ocean for foraging or for accessing prey. … But we also found a whole bunch of other interesting behaviors and movements from these animals that really don't look like they can be for foraging at all. Or if they are, they're foraging on something that we as scientists currently don't understand.
Jennette Barnes: When you say that they're accessing prey, how do you know that's what they're doing when they're down there?
Camrin Braun: That is a really good question. In this study, the way that we did that is simply by lining them up in space and time. So we have a couple of really good examples where we have a research vessel. I say “we,” as like, the science community, had a research vessel in a particular location that was making measurements of potential prey in the deep ocean with a shipboard sonar. And so they can see these layers on — on the screens, basically on the ship. And we took that data and lined it up in space and time with the behavior that we measured from these predators. So a really good example is like a swordfish. Swordfish go to a particular depth almost every day for about 12 hours a day. And that almost always lines up with where we are pretty sure this layer is. And in the paper we have a couple examples of where we have a swordfish and a ship kind of in the same place at the same time. And we see that the predator and the prey really line up pretty well.
Jennette Barnes: Okay. Since we do a lot of reporting on the great whites tagged by [state shark scientist] Greg Skomal, I'm curious if any of that data was used here.
Camrin Braun: Yeah. Greg is a coauthor on the paper. And a lot of the white sharks that he tagged are in here. One thing that I think is important to note is that a lot of the tagging Greg is doing now is with tags like acoustic tags that many people see that, you know, ping on the receivers that are near the beaches and stuff. Those, and then there's, you know, some camera tags and just some other types of tags. Those are somewhat distinct from the type of tag that we're using here. These are specifically used for looking at long term movements of the animals — so months, to maybe a year, over kind of a large scale in the open ocean and especially recording the vertical information.
Jennette Barnes: OK. Was there anything surprising to you in your results?
Camrin Braun: Well, I think the most surprising thing is, you know, I kind of expected a lot of the predators to line up with the estimates that we have of prey in the deep ocean. And what's maybe most surprising and also probably the most interesting is that in many cases that isn't true, that the predators, in fact, don't line up with the prey. … We see them diving way deeper than what we currently think of as being kind of the core of where their potential prey probably are at depth. … I mean, we have whale sharks offshore from Cape Cod in the summer that many folks may have heard about or seen. And when they go in the open ocean, you know, they're diving to four or five thousand feet pretty consistently. ... That's way deeper than what we currently think of as being kind of a normal place for them to go to find prey. … And what this study really suggests is that that biomass might be a pretty critical link to the top predators that we're familiar with and that, you know, many of our communities, especially coastal communities, rely on to make a living or to put food on the table or whatever it is. … Right now, kind of anything that we do … to disrupt that link or to jeopardize it, whether it's deep sea mining, or fishing in the deep ocean, or, you know, climate impacts. There's a whole bunch of different ways that we might disrupt that link. But we need to quantify sort of the magnitude of that link and better understand it before we kind of get too far down the track of potentially disrupting it.
Jennette Barnes: All right. Thank you so much for doing this. I appreciate it.
Camrin Braun: Yeah, well, thank you for your interest … and talk to you again soon.