Being a fish was never easy, but a new paper reports that it’s been getting harder over the last 15 years or so. According to the findings, oxygen levels are dropping in the depths of the oceans, forcing fish to move ever closer to the surface.

New research from the University of California – Santa Barbara and the University of South Carolina is warning us that fish are slowly drowning. Changes in ecology, as well as the effects of climate change on seasonal patterns, water temperature, and its gradient over different depths, have been causing deeper layers of the ocean to lose their dissolved oxygen content. This, in turn, is forcing fish to either move closer to the surface, or asphyxiate.

It may seem like a trivial matter, but this shift is causing wide-scale changes in marine ecosystems and could have a very real impact on the health of the ocean as a whole. It also raises important questions for fishery management and conservation efforts, with the authors underscoring the importance of accounting for this shift with policy to avoid further damaging marine ecosystems.

Swimming out of breath

“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, assistant professor at the University of South Carolina, “and so fish seem to be moving up to shallower regions to get to an area where the oxygen is relatively higher.”

The findings are based on 15 years’ worth of recordings, surveys, and measurements. These included measurements of dissolved oxygen in samples of water taken at varying depths, of temperature, salinity, and surveys of the average depth at which certain fish species tend to congregate. A total of 60 different species of fish were encountered often enough during these 15 years to be statistically relevant and included in the study.

Data was collected on a yearly basis, every fall, from 1995 through to 2009. The team focused on three reef features between the Anacapa and Santa Cruz islands in Southern California. These were the Anacapa Passage area, with an average depth of 50m, a seamount known as the “Footprint”, at around 150m, and the “Piggy Bank”, with an average depth of around 300m. During the surveys, the team identified all fish species that came within two meters of the submarine or were visible and within two meters of the seafloor. They also estimated the length of each individual fish.

During this time, they saw depth changes in 23 species. Four of these shifted towards deeper waters, while the other 19 moved towards the surface in response to low oxygen conditions (as shown by analysis of water samples).

The team explains that surface waters tend to be better oxygenated (have higher levels of dissolved oxygen) due to surface motions such as waves continuously mixing gases into the top layer of bodies of water. Over time, as waters mix, this oxygen also finds its way lower along the column of water. However, the team explains that warming climates make for warmer surface waters, which increases the buoyancy of these layers compared to those deeper down, reducing their ability to mix. This process is known as ocean stratification.

In addition to this, warmer water has a lower ability to dissolve and hold oxygen compared to colder water, so there’s less of this gas being mixed into the ocean to begin with.

In the end, this means less oxygen makes it to the bottom layers of water. Although salinity and temperature gradients along the column of water also influence the extent of vertical mixing, the team reports that both remained relatively constant over the study period. In other words, the trend towards lower oxygen levels seen at the study site is primarily driven by climate-associated changes in surface water temperatures. That being said, the other factors can’t be discounted completely either.

“A third of [the 60 fish species’] distributions moved shallower over time,” Meyer-Gutbrod said. “I personally think that’s a remarkable result over such a short time period.”

The team acknowledges that their study only included a relatively small area, but it did include a wide range of depths, which was the ultimate objective of the research. This narrower area actually helps reduce confounding factors, they explain, since it allowed for most conditions (apart from depth) to be constant across all the survey areas.

“Other scientists have used lab experiments to show that fish don’t like low oxygen water,” Meyer-Gutbrod said, “but what nobody’s ever done is just return to the same location year after year to see if there’s actually a change in the distribution of fish stemming from a change in oxygen over time.”

In closing, the authors explain that this trend can have quite severe negative impacts on marine ecosystems, and indirectly, on all life on Earth. Fish are simply forced to move away from their optimal depths, which will eventually result in them being pushed out entirely out of several ecosystems. According to co-author Milton Love, a researcher at UC Santa Barbara’s Marine Science Institute, we could even see a point in which species are forced into depth ranges that they simply cannot survive in.

They also cite previous research showing that many fish species also cannot tolerate high water temperatures, and are migrating towards lower depths. In the end, these factors can leave many species in an impossible situation — where they cannot breathe if too low, and can’t bear the heat if too close to the surface.

In the end, even if we start working to redress climate change right now, meaningful progress will take quite a lot of time. Until then, policymakers need to recognize and react to the pressures faced by fish species and issue regulation that protects them as best as possible, or risk wide-ranging ecological collapse in the world’s oceans.

“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 paper “Moving on up: Vertical distribution shifts in rocky reef fish species during climate‐driven decline in dissolved oxygen from 1995 to 2009” has been published in the journal Global Change Biology.