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'Harmless' Fish Species Survives 'Dead Zone', Turns Predator & Restores Ecosystem

Time was when a 9,000 square kilometer region off the coast of southwest Africa (from Namibia to south Africa) was one of the most productive, natural fisheries in the world. Due to the continuous churning up of deep, cold, sediment-rich water (known as the Benguela up-welling), nutrients were plentiful and many fish species, especially sardines, came here to feed and thrive off the planktonic blooms (and thus keep plankton growth in check).

However, due to a lack of any resource management policy, over-fishing took its toll, and the sardine population crashed sometime in the early 1970’s. Without the sardines to keep plankton growth in check, they quickly over-populated, and, as they died in great numbers and sank to the bottom, aerobic digestion by bacteria consumed most of the water’s dissolved oxygen, creating a vast “dead zone” unfit for most species of fish, save for the pernicious jellyfish. Normally, the take-over by jellyfish signifies an ecological “dead end” as few animals feed on jellies… but one tiny fish — the bearded goby — is defying the expectations of marine scientists and even “restoring” an ecosystem thought to be beyond remediation.

The ‘bearded goby’ (Sufflogobius bibarbatus), a small, common, prey species of fish, has become adapted to the “toxic” conditions near the sea floor of this pelagic zone. Analysis of the fish’s gut has shown that up to 60% of its diet consists of jellyfish–a marine creature few animals prey upon due to their venomous stings. Remarkably, the fish has become the pivotal player in a newly emergent ecosystem.

In a report published last month in the journal Science (‘Trophic Structure and Community Stability in an Overfished Ecosystem‘, 10 July, 2010), researchers Utne-Palm et al detail the unexpected restoration of a “dead end” ecosystem by the unusual adaptations of the bearded goby.

The tiny goby (averaging around 15 cm, or 5 inches, in length) is normally a prey species for such fish as the horse mackerel. But since the collapse of the ecosystem due to over-harvesting of the region’s sardines, and the subsequent take-over by stinging jellyfish (which feast on the over-abundant phytoplankton), the mackerel and other predatory fish have mostly stayed away.

Add to this the depletion of the water’s oxygen content and build-up of poisonous hydrogen sulfide gas due to the constant decomposition of dead jellies and plankton (sinking to the bottom) by the metabolic action of bacteria…and you have an ecosystem beyond hope of recovery.

Map showing native habitat of the bearded
goby, southwest Africa

Normally, even the gobies would be wary prey of jellyfish, but the researchers have discovered a few unexpected, physiological adaptations to these harsh ecological conditions by the simple, big-headed goby fish. For starters, the goby spends 10 or more hours a day hiding out in the toxic sludge, seemingly “holding its breath” (shutting down most of its gill respiration, but not completely) and foraging in the diatomaceous (i.e., phytoplankton rich) mud.

But at night, the gobies swim up to the surface and intermingle with the predaceous jellyfish (its slimy, thick skin may help protect it from jellyfish stings). Between these two protective habitats, the gobies — still being prey for larger fish–manage to thrive.

The marine researchers found that the bearded gobies were associated with the jellies six times more frequently than other fish species. Following body-tissue isotope analyses of the fish, researchers found that a large portion of the fish’s diet consists of jellyfish.

“We don’t know if they are eating dead jellyfish from the bottom, or if they are coming up to oxygen-filled layers to eat jellyfish, but they are eating jellyfish,” said Victoria A. Braithwaite, research team member and professor of fisheries and biology, Penn State.

So, the gobies eat the jellies, and in turn, get eaten by the larger hake and mackerel, which get eaten by sea lions, porpoises, and sea birds like gannets. All of this serves to return the little-eaten jellyfish to the ecosystem through this “harmless” goby’s anti-predator adaptation.

“It is a win-win situation where the gobies are using a resource that is usually a dead end in the ocean, the jellyfish,” said Braithwaite. “And they are using the toxic mud as a refuge. Together this seems to explain why their population is growing despite the fact that they are now being the main prey species in this unusual ecosystem.”

Additional research team members:

Anne G.V. Salvanes, Matthias Hundt and Karin Pittman, University of   Bergen, Norway; Bronwen Currie, National Marine Information and Research Centre, Namibia; Stein Kaartvedt, King Abdullah University of Science and Technology, Saudi Arabia; Göran E. Nilsson, Jonathan A. W. Stecyk, Guro K. Sandvik, Ida G. Lund, Rønnaug A.U. Strandabø and Thor A. Klevjer, University of Oslo, Norway; Megan van der Bank, Bradley Flynn and Mark J. Gibbons, University of Western Cape, South Africa; Andrew K. Sweetman, Norwegian Institute for Water Research, Bergen, Norway; Volker Brüchert, Stockholm University, Sweden; and Kathleen R. Peard, Ministry of Fisheries and Marine Resources, Namibia.

The South African Research Council and the Research Council of Norway supported this work.

Photo credit: bearded goby (Sufflogobius bibarbatus) by Victoria Braithwaite of Penn State

Map credit: Global Biodiversity Information Facility




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