Disappearing giants: How warming oceans are suffocating large fish

Since 1981, the mean global ocean temperature has risen at an average rate of 0.18 degrees Celsius per decade. This has had serious impacts on the health of marine species; as oceans warm across the world, fish that are unable to cope with climatic changes, such as cardinalfish, are disappearing at a staggering rate.

A new study conducted by an international team of researchers from McGill, the University of Montana, and Radboud University examines this alarming trend. Published in the Proceedings of the National Academy of Science (PNAS), the study explores how water temperature and oxygen availability affect the metabolism as well as survival fitness of fish. The research team was particularly interested in how rising ocean temperatures impact oxygen consumption.

“We investigate how water temperature and oxygen availability together affect aerobic metabolism (i.e., the rate of oxygen consumption) of fish,” Juan Vicente Gallego Rubalcaba, a postdoctoral fellow in the Department of Biology and author of the study, wrote in an email to The McGill Tribune.

In order to achieve their goals, the researchers developed a model based on the metabolic theory of ecology.

“We build on the equations of [the] metabolic theory of ecology (MTE) to include physical mechanisms of oxygen transfer,” Rubalcaba wrote. “This theory explains how metabolic rate increases with temperature and body mass, but does not account for oxygen uptake or availability.”

Metabolic rate refers to the rate at which an animal burns calories to produce energy. This updated metabolic-based model evaluates how water temperature and body size affect oxygen consumption and diffusion. The model assesses how oxygen is extracted from water and distributed by a fish’s gills throughout its body. 

“We developed a model based on physicochemical mechanisms of gas diffusion to calculate oxygen consumption rates as a function of water temperature, oxygen concentration, gill morphology and fish body size,” Rubalcaba wrote.

The model predicted that oxygen limitation would impose more constraints on the metabolism of larger and more active fish. To support their hypothesis, the scientists compared their predictions against actual measurements of metabolic rates in 286 fish species. The oxygen consumption rates of fish of different body sizes were studied at different water temperatures.

The results of this comparison were consistent with their predictions. Indeed, the researchers found that increased oxygen limitation, tied to the temperature increase caused by climate change, had a greater impact on larger fish.

“Metabolic rate (especially active metabolic rate) increases with temperature to a greater extent in small [fish] than in large fish,” Rubalcaba wrote. “This means that smaller species are able to increase their metabolic demand for oxygen in warmer waters, while larger species may not be able to do so.”

Since larger fish have more difficulty adapting to temperature increases, species already in decline are at greater risk. The Manaaki tuna of New Zealand, for example, experience low oxygen availability due to polluted waters that could be exacerbated by rising temperatures. 

“Constraints may be especially important in large fish, because they have small gills relative to their body size (although this is a much-debated hypothesis),” Rubalcaba wrote.

The decrease in oxygen levels caused by increased ocean temperature has massive impacts on the physiology and behavioural patterns of large fish. Fishes’ inability to extract enough oxygen from warmer water through their gills leads to shifts in body size and changes in reproductive behaviour.

“Water temperature is already rising worldwide as a consequence of climate change and many fish species need to cope with this rapid temperature change, either migrating toward colder regions or adopting different life strategies such as growing smaller to avoid respiratory constraints,” Rubalcaba wrote.

Fortunately, actions such as limiting greenhouse gas emissions and restoring marine and coastal ecosystems can be taken to mitigate oceanic warming. Given that approximately three billion people depend on fish as a primary source of nutrition, protecting fish from warming oceans must be a priority.

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