Feeling the Squeeze

Low oxygen levels are pushing fish into shallower waters, with potentially devastating impacts for fisheries and ecosystems
Harrison Tasoff
Blue rockfish closeup while swims close to corals

Blue rockfish were among the species that migrated upward over the course of the study. Photo Credit: Southwest Fisheries Science Center ROV Dive Team

Fish do require oxygen to breathe; it’s just that they get what they need from the oxygen dissolved in water rather than in the air.

Unfortunately, oxygen concentrations are dropping throughout the oceans. A new study out of UC Santa Barbara and University of South Carolina is the first to document more than a dozen species moving to shallower water in response to low oxygen conditions. The research, published in Global Change Biology, spans 15 years of surveys and measurements. The authors stressed the importance of accounting for the findings in fishery management and conservation, or risk implementing strategies wildly out of step with conditions under the waves.

“This study finds that oxygen is declining at all the depths we surveyed: from 50 meters to 350 meters,” said lead author Erin Meyer-Gutbrod, “and so fish seem to be moving up to shallower regions to get to an area where the oxygen is relatively higher.” Now an assistant professor at the University of South Carolina, Meyer-Gutbrod started this analysis as a postdoctoral scholar at UC Santa Barbara.

The consequences of this trend could be severe. “This appears to be verification of a rather frightening hypothesis. Namely, that fishes are being moved out of their optimal habitats,” said co-author Milton Love, a researcher at UC Santa Barbara’s Marine Science Institute.  “And the end point of that is that, ultimately, they will be driven out of at least some of their habitats.” Certain species may eventually be pushed to areas where their physiology can’t handle the conditions, he noted.

What’s more, results from other studies suggest that increasing surface temperatures are driving many fish deeper. This means fish habitat could be compressed from the top by heat and from the bottom by oxygen availability.

An additional concern is how this habitat compression interacts with fishing pressure. This trend could concentrate the fish, making them potentially easier to catch. But increased landings would belie fish stocks that are actually in dire straits. “If you throw your net in the water and you get a ton of fish — more than you’re used to getting — you may think, ‘Oh, it’s a good year for the fish. Maybe the population is recovering,’” Meyer-Gutbrod said. “But instead, it could be that all the fish are just squished into a tighter area. So you could have fishery regulations changing to increase fish allowances because of this increase in landings.” The result would be catastrophic for the fishing industry and the ecosystems on which it depends.

According to the researchers, this is why it’s crucial that we understand what’s going on, predict how it will play out, and adapt management strategies to incorporate this reality.

“The phenomenon will play out as conditions dictate,” Love said. “But our response to it is within our power.”

MSI Principal Investigators