More Evidence that Ocean Acidification Turns Fish Into Bait

The current predator/prey balance in Earth's oceans occurs at around a water pH of 8.14. That's normal, and it's in these conditions that fish have developed the ability to detect predators by smell and thereby survive just a little bit better. In the next hundred years or so, however, that pH is forecast to change as the surface waters of our oceans absorb more and more carbon dioxide from the atmosphere and acidify. By 2100, it's expected that the average surface water pH will have plunged to around 7.8. That might feel like a small difference, but it's enough.

In the past, it's been demonstrated in a lab setting that what happens as acidification progresses is changing fish behavior. Fish become more bold (read: careless), and they become less able to sense predators. Research out today from a team based at the Georgia Institute of Technology verifies those lab results for the very first time with real-world observations, using a naturally acidic and CO2 rich "bubble reef" in Papua New Guinea. "There's no difference between the fish treated with CO2 in the lab in tests for chemical senses versus the fish we caught and tested from the CO2 reef," says GIT's Danielle Dixson. Previous reef studies have suggested as much as a five-fold increase in predatory vulnerability in CO2-dosed fish.

This real-world sort of confirmation is crucial because it examines fish that have grown accustomed to the acidic environment, living in it day to day, in contrast to fish taken from a more normal pH and then placed in acidic conditions in a lab. It's not unlike being able to see into the 2100 fish future. We might hope that fish over time adapt to their new surrounding pH, but such an adaptation doesn't seem to be the case. That's bad news.

Interestingly, it's not the fish's sense of smell that's affected. Instead, the change in pH induces neurological changes, disrupting a nervous system receptor called GABAA. Fish with this disruption lose the ability to distinguish properly between water spiked with the scent of a predator and untainted water. Fish without the disruption are more likely to choose the untainted, "safe" water.

Thickening the plot somewhat is the fact that not much else seems to be different about the affected fish. Between the acidic reef and the normal reef, species abundance, richness, and composition seemed to be unchanged. "The fish are metabolically the same between the control reef and the CO2 reef," Dixson says. "At this point, we have only seen effects on their behavior." What's more, acidic reefs tend to be safer places for fish, with fewer concentrations of predators in the first place. So, there's a possibility the projected acidification could be a wash, at least in terms of fish mortality.

Another question is whether the change is permanent in fish—whether they can adapt given more time—and whether it's permanent in fish species as a whole. That is, can the disability be spread generationally in fish? Even if the fish can re-adapt, or if fish mortality proves an even wash, acidification is dangerous. Potential effects include widespread devestation for calcifying organisms—e.g. everything with a shell—depressed immune responses in mussels, depressed metabolism in jumbo squids, and potentially widespread changes in the oceans' acoustic properties, harming creatures that use echolocation, effectively limiting their ability to "see" underwater.