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West coast fish are out of breath

Some fish are having trouble breathing off the west coast of North America from Baja California to Vancouver, Canada. A study looking at this problem focused on the northern anchovy, but the findings may apply to other species. The root of the problem is increasing hypoxia (oxygen deficiency) due to warmer waters. Fish require oxygen, and thus hypoxia leaves the west coast fish out of breath.

In the 1940s, the sardine fisheries off the California west coast collapsed, primarily from overfishing. But the consequences moved up the food chain affecting a variety of other species with starvation. These affected species included fish like tuna and mammals like pelicans and sea lions. Sardines are short-lived fish that feed on plankton, and they are part of a larger group of fish called forage fish. Forage fish are the second level in the food chain. They feed on the primary producers, grow quickly, and then reproduce, creating huge populations that provide food for larger fish and mammals. Anchovies are also a forage fish and thus crucial to the coastal ecosystem’s food chain.

Scientific observations collected over many years helped track fluctuations in the anchovy populations with time. These data were invaluable in clarifying decade long cycles of population growth and decline. The causal factors behind these fluctuations have also been the subject of scientific debate. However, recent research found a strong correlation between temperature, oxygen levels, and the anchovy population.

Warm water – less oxygen

Oxygen levels in the waters off the west coast are dependent on several factors. The first of these is the water temperature. The primary source of free oxygen in seawater is the atmosphere, and the amount of dissolved oxygen in the water is related to atmospheric pressure. Also, basic physics and chemistry dictate that warm surface water holds less oxygen than cold water; thus, ocean oxygen levels are temperature dependent. So, oxygen levels in the ocean waters decrease as temperatures rise.

The second factor is ocean upwelling, where deep marine waters rise to the surface. These deep waters bring nutrients that drive primary production in the food chain. Phytoplankton populations increase when more nutrients are available. But the deep water, upwelling along the coast, contains less oxygen than surface waters and has a higher acidity. 

The species that thrive along the west coast are finely tuned to the physical conditions of their environment. If those basic conditions change, then the west coast fish populations come under stress. Animals like forage fish are metabolically adapted to their environment. As water temperatures rise, their metabolism increases, and they then require more oxygen and more food. But at that point, the warmer waters hold less oxygen to meet the needs of these fish.


The recent study of the northern anchovy revealed a clear correlation between population size and the ‘breathability’ of the coastal ocean waters. The study defined breathability as a “Metabolic Index (Φ), a species-specific measure of the environment’s capacity to meet temperature-dependent organismal oxygen demand.” This index is affected by both water temperature and the amount of upwelling. However, the ongoing ocean warming caused by Anthropocene climate change shifts the whole ecosystem towards unfavorable conditions.

As ocean temperatures continue to rise, forage fish will come under increasing stress, which will decrease their populations. This decrease is bad news for west coast fish and mammal species higher in the food change, who ultimately depend on the forage populations. When commercial fisheries collapse, people suffer economic hardship, but pelicans, sea lions, and other species simply starve to death.


Changing currents cause North Atlantic ecosystem drift (Source: ArcheanWeb) –  Also:

Heat, oxygen deprivation, and extinction (Source: ArcheanWeb) –  Also:

Species adaptation to climate change (Source: ArcheanWeb) –  Also:



Climate-driven aerobic habitat loss in the California Current System (By Evan M. Howard, Justin L. Penn1, Hartmut Frenzel1, Brad A. Seibel, Daniele Bianchi, Lionel Renault, Fayçal Kessouri, Martha A. Sutula, James C. McWilliams, and Curtis Deutsch; Science Advances) –  Also:

The Role of Fishing in the Pacific Sardine Collapse (By Geoff Shester and Ben Enticknap; Oceana) –  Also:


Temperature-dependent hypoxia explains biogeography and severity of end-Permian marine mass extinction (By Justin L. Penn, Curtis Deutsch, Jonathan L. Payne, Erik A. Sperling; Science) –  Also:

Feature Image: California Anchovies (Modified) – By Transferred from en.wikipedia to Commons by User:Quadell using CommonsHelper., Public Domain,

William House
William is an earth scientist and writer with an interest in providing the science "backstory" for breaking environmental, earth science, and climate change news.