How giant sea creatures eat tiny sea creatures - Kelly Benoit-Bird

Transcriber: Andrea McDonough
Reviewer: Jessica Ruby

One of the reasons that I'm fascinated by the ocean is that it's really an alien world on our own planet. From our perspective, sitting on the shoreline or even out on a boat, we're given only the tiniest glimpses at the real action that's happening beneath the surface of the waves. And even if you were able to go down there, you wouldn't see very much because light doesn't travel very far in the ocean.

So, to answer questions about how the ocean works, in my research, we use sound. We use sonars that send out pulses of sound made up of a number of different frequencies, or pitches, that are shown with different colors. That sound bounces off things in the habitat and comes back to us. If it were to bounce off this dolphin, the signal we got back would look very much like the one we sent out where all the colors are represented pretty evenly.

However, if we were to bounce that same sound off of a squid, which in this case is about the same size as that dolphin, we'd instead only get the lowest frequencies back strongly, shown here in the red. And if we were to look at the prey of that squid, the tiny little krill that they're eating, we would instead only get the highest frequencies back. And so by looking at this, we can tell what kinds of animals are in the ocean, we can look at how dense they are, where they are distributed, look at their interactions and even their behavior to start to study the ecology of the ocean.

When we do that, we come up with something sort of surprising: on average, there isn't very much food in the ocean. So even in places which we think of as rich, the coasts, we're talking about two parts of every million contain food. So what does that mean? Well, that means that in the volume of this theater, there would be one tub of movie theater popcorn available to be eaten. But of course, it wouldn't be collected for you neatly in this bucket. Instead, you'd actually have to be swimming through this entire volume Willy Wonka style, picking off individual kernels of popcorn, or perhaps if you were lucky, getting a hold of a few small clumps.

But, of course, if you were in the ocean, this popcorn wouldn't be sitting here waiting for you to eat it. It would, instead, be trying to avoid becoming your dinner. So I want to know how do animals solve this challenge?

We're going to talk about animals in the Bering Sea. This is where you may have seen "Deadliest Catch" framed, in the northernmost part of the Pacific Ocean. We've been looking specifically at krill, one of the most important food items in this habitat. These half-inch long shrimp-like critters are about the caloric equivalent of a heavily buttered kernel of popcorn. And they're eaten by everything from birds and fur seals that pick them up one at a time to large whales that engulf them in huge mouthfuls.

So I'm going to focus in the area around three breeding colonies for birds and fur seals in the southeastern Bering Sea. And this is a map of that habitat that we made making maps of food the way we've always made maps of food. This is how many krill are in this area of the ocean. Red areas represent lots of krill and purple basically none. And you can see that around the northern two most islands, which are highlighted with white circles because they are so tiny, it looks like there's a lot of food to be eaten.

And yet, the fur seals and birds on these islands are crashing. Their populations are declining despite decades of protection. And while on that southern island at the very bottom of the screen it doesn't look like there's anything to eat, those populations are doing incredibly well. So this left us with a dilemma. Our observations of food don't make any sense in the context of our observations of these animals.

So we started to think about how we could do this differently. And this map shows not how many krill there are, but how many clumps of krill there are, how aggregated are they. And what you get is a very different picture of the landscape. Now that southern island looks like a pretty good place to be, and when we combine this with other information about prey, it starts to explain the population observations.

But we can also ask that question differently. We can have the animals tell us what's important. By tagging and tracking these animals and looking at how they use this habitat, we are able to say, "What matters to you?" about the prey. And what they've told us is that how many krill there are really isn't important. It is how closely spaced those krill are because that's how they are able to make a living.

We see the same pattern when we look in a very different ocean, further south in the Pacific, in the warm waters around the Hawaiian islands. So a very different habitat, and yet the same story. Under some conditions, the physics and the nutrients, the fertilizer, set up aggregations in the plants, the phytoplankton. And when that happens, these very dense aggregations of phytoplankton attract their predators, which themselves form very dense layers.

That changes the behavior and distribution of their predators as well, starting to set up how this entire ecosystem functions. Finally, the predators that eat these small fish, shrimp, and squid, we're talking about two- to three-inch long prey here, change how they use their habitat and how they forage. And so we see changes in the spinner dolphins that are related to the changes we're seeing in the plant life.

And just by measuring the plants, we can actually predict very well what's going to happen in the top predator three steps away in the food web. But what's interesting is that even the densest aggregations of their prey aren't enough for spinner dolphins to make it. It's a pretty tough life there in the ocean.

So these animals actually work together to herd their prey into even denser aggregations, starting with patches that they find in the first place. And that's what you're going to see in this visualization. We have a group of 20 dolphins, you notice they're all set up in pairs, that are working together to basically bulldoze prey to accumulate it on top of itself.

And once they do that, they form a circle around that prey to maintain that really dense patch that is a couple thousand times higher density than the background that they started with before individual pairs of dolphins start to take turns feeding inside this circle of prey that they've created.

And so, this work is showing us that animals can first give us the answers that aggregation is critical to how they make their living. And by looking more deeply at the ocean, we're starting to understand our interactions with it and finding more effective ways of conserving it. Thank you.