When an ecosystem becomes overfished, some species may be able to step in and fill the food chain gaps until others can recover, according to a new study published in Science. The bearded goby, a fish that lives off the coast of southwest Africa, has become the predominant prey species in the area because the rest have been overfished. The goby should be threatened under the weight of so many predators, but it isn't—in fact, researchers find it's doing better than ever, thanks to its ability to adapt while supporting the rest of the region's food chain.

The coast of southwest Africa used to be home to one to some of the most successful fisheries in the world. Fishing boats collected huge amounts of sardines and anchovies, the base of any self-respecting predator's food pyramid, until the species became heavily overfished. Now, the waters are dominated by jellyfish, unsuitable for eating by most predators in the area.

With the anchovies and sardines gone, all the predators have turned to one of the only small fish species left: the four-inch long bearded goby. The pressure of predation, combined with an increase in poisonous chemicals like hydrogen sulfides and methane in the water (which are only expected to increase with climate change) should have put the goby in danger of extinction. But the opposite has turned out to be true: somehow, the bearded goby population is just fine.

The idea of an ecosystem not suffering under the consequences of overfishing was baffling to researchers. In 2008, a large team of scientists set out to collect several types of data to figure out how the goby was surviving, even thriving.

They used acoustic imaging of the ocean at all hours of the day and night to see the goby's behavior patterns, trawled some fish themselves to open them up and see what they were eating, and chemically analyzed the water columns and sediment in the area. The researchers also set up controlled experiments with live goby specimens to test their reactions to various environmental factors.

What they were able to determine from their data was that the goby was in some ways suited to survive the situation it was in. But in others, the goby had adapted, and has actually bridged together some of the ocean's resources to help the area recover.

One of the goby's characteristics that has proved invaluable to its survival was its ability to tolerate chemically altered waters. The area has high levels of hydrogen sulfides and sinking levels of oxygen, especially down on the ocean floor. Not only could the goby live in this chemistry, it could also use it to its advantage.

Typically, when fish spend too much time in low-oxygen waters, they sustain brain and heart damage. One of the goby's predators, the hake, suffers permanent heart damage in this kind of environment.

The goby fared much better in these conditions—it still has quick escape responses, even after several hours of low oxygen supply, and its heart performance could recover with more oxygen in a couple of hours. This tolerance for the low-oxygen ocean floor gives the goby somewhere to hide from predators.

While the goby don't mind a lack of oxygen for a little while, they do need to replenish themselves regularly. Researchers observed that the species has learned to bury itself in the ocean mud during the day, and then swim up to more oxygenated waters at night to restore themselves, and possibly help them digest what may have been its biggest change: its diet.

When researchers opened up a couple of the fish to see the contents of their stomachs, they found that the goby had been eating the jellyfish that had become plentiful. They speculated the goby could be eating dead or sick jellyfish that had drifted to the bottom, or taking nibbles off live ones when they go up to the oxygenated waters at night.

Aside from the jellyfish, the goby also appear to be eating the diatomaceous mud that rests on the ocean floor, which includes decomposed plants, animals, and bacteria. The authors estimate the goby's diet is about 34 percent mud and as much as 60 percent jellyfish. It's effectively using food chain "dead ends" to fuel its numbers and sustain predator populations.

While the goby is doing all right now, researchers are unsure what its long term fate will be. They note that climate change will ramp up the inflow of low-oxygen waters, which may eliminate any gains in population that predators derive from the goby abundance.

Still, predators that survive the hostile water chemistry will need something to eat, and the bearded goby is likely to continue churning ocean waste into useable biomass. Species like the goby may be the difference between an ocean ecosystem that pulls through climate change and other human impacts, and an ecosystem that falls apart.

Science, 2010. DOI: 10.1126/science.1190708 (About DOIs).

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