Marine life struggling to breathe free

Studies off the west coast of North America add to a growing body of evidence that the ability to breathe will shift the future distribution of marine species, potentially compromising their ability to weather further climate change, researchers say.

They suggest that oxygen loss in the warming waters of the California Current System (CCS) will lower the region’s Metabolic Index – the temperature-dependent ratio of oxygen supply to species’ metabolic demand – to below critical levels, limiting where they can live.

“Climate change isn’t just warming the oceans; it is causing oxygen to decrease, which could force fish and other ocean animals to move away from their normal range to find higher-oxygen waters,” says Evan Howard from the University of Washington (UW), lead author of a paper in the journal Science Advances.

The cool, nutrient-rich water between Mexico and Canada supports everything from phytoplankton to orcas, but among the most important to the food chain is the northern anchovy, which has been closely monitored since the 1950s.

Howard and colleagues combined historical data with recent findings about water breathability to explain population cycles.

The study found that species respond to how breathable the water is: a combination of the oxygen levels in the water and the species’ oxygen needs, which are affected by water temperature. 

Anchovy populations are known to cycle through time, but the reasons aren’t clear, Howard says. Explanations have included food supply, predator-prey interactions, competition with other species, and temperature preferences, but none has offered a full explanation.

The researchers combined observations with ocean models to fill gaps in the data and showed that the breathability index changes over time and corresponds with when anchovy populations rise and fall, and when they move deeper or closer to shore.

They then looked at the extent of anchovy habitat in the future under climate change. Projected changes in the water conditions will likely make the southern part of the anchovies’ range, off the coasts of Mexico and southern California, uninhabitable by 2100.

“This study is the first one that demonstrates on a timescale of decades that a species is responding in really close alignment with this Metabolic Index – how breathable the ocean in its habitat has become,” says senior author Curtis Deutsch.

“It adds a new, independent line of verification that species in the ocean are arranged in accordance with how breathable their habitats are.”

The inferred sensitivity of anchovy to aerobic habitat may also underlie the striking oscillations between anchovy and sardine, the researchers suggest in their paper.

“If anchovy are more hypoxia tolerant than sardine, as hypothesised in the Humboldt Current System, this could help explain why anchovy sometimes outcompete sardine in the highly productive coastal zone while sardine are restricted to the lower primary productivity offshore.,” they write. 

“In this way, temperature-dependent hypoxia traits may contribute to observed offshore niche partitioning and the anti-correlated oscillations of anchovy and sardine that have long been observed.”

Diagram full caption

In this animation, land is plotted in grey, with Baja California, Mexico in the bottom right and Vancouver Island, Canada at top left. The vertical axis shows water depth in metres, the bottom left axis shows latitude, and the bottom right axis shows longitude. 

The upper panel is the fraction of the upper ocean that is potential habitat for one reasonable combination of animal sensitivities to temperature and oxygen: When the fraction (habitat index) is 1 (blue), the animal could potentially inhabit the entire upper ocean (0 to 200 meters depth). 

When it is 0 (red), the animal does not have enough energy for its normal activities in that part of the ocean.