About 71% of Earth’s surface is covered by water, and oceans play the biggest part when it comes to regulating atmosphere. In fact, between 50-85% of the world’s oxygen comes from the oceans because of phytoplanktons. Oceans are also the largest heat sinks, absorbing millions of tons of carbon dioxide, and effectively cooling down the atmosphere. However, increases in greenhouse gas emissions have been wreaking havoc. The overall climate is becoming warmer, and one of its side effects is that fish are shrinking in size as sea surface temperatures increase.

Observations over the past several years have found that several species of fish, including sole, herring, and haddock, have shown a considerable decrease in their size. Further research found that climate change and increasing temperatures are to blame. 

According to research published in 2013 by William Cheung of the University of British Columbia and director of science at Nippon Foundation’s Nereus Program, along with other authors, the size of around 600 species of ocean fish could shrink by 14-24% by 2050 because of climate change. There is already a loss in size of several fish species such as haddock, whiting, herring, and sole in the areas of North Sea with less oxygen. Cheung and Daniel Pauly, a professor at the university’s Institute for the Ocean and Fisheries, recently published a new study in the journal Global Change Biology explaining something called the “Gill-Oxygen Limitation Theory.” According to the study, their 2013 study estimates were incorrect and the fish are in a far worse situation than what they previously thought.

An increase in temperature requires the fish to breathe in more oxygen as their metabolic rate increases. However, their gills pose a limitation on how much oxygen they can absorb, which means their oxygen intake will not be enough to support their metabolism. 

Unlike land animals, marine life has to get oxygen from the water passing through their gills. But water is a 100 times more viscous and 777 times more dense than air. While a cubic meter of air contains 250 grams of oxygen, its concentration is much less in water, not to mention its extremely small diffusion rate in water which is 10,000 times less than it is in the air. So, the surface area of a fish’s gills is very crucial for gas exchange as water contains only a small fraction of dissolved oxygen. The more the surface area, the more oxygen they could take in. But, if the fish do evolve to grow larger gills, it would decrease the gill-surface-to-body-weight ratio and weigh them down.

Scientists at the University of British Columbia found that the body size of fish decreases by 20-30% for every one degree Celsius increase in water temperature. This, in turn, affects other, larger fish which feed on them. 

Most species of fish are ectothermic or cold-blooded, and their body temperatures vary depending on the surrounding ambient temperatures. So, if there is an increase in water temperature, their body temperature also increases. An increase in body temperature also increases the rate of their biochemical reactions and metabolism. Increased metabolic rate means the fish would need more oxygen. But, as gills grow at a slower pace than their body, it puts a limitation on how much oxygen they can actually obtain from the water to support metabolism. Not getting enough oxygen would hamper their growth, though smaller species are at an advantage in situations like these.

Another factor that affects the oxygen levels in water is the productivity of phytoplanktons. As the sea surface temperatures increase, there is less mixing of waters with nutrient-rich waters because of density differences in waters at different temperatures. 

Phytoplanktons are diverse, small organisms that live on water surfaces and survive through photosynthesis. These organisms draw more than 100 million tons of carbon dioxide every day, which helps reduce temperatures and produce elemental oxygen for both land and aquatic life. According to NASA, there is already a distinct difference, or stratification, in the density of warmer – fresher water at the surface and colder, saltier water deeper down. So, the warmer the surface waters become, the less mixing there is between those waters and the deeper, more nutrient-rich waters below, which means nutrients can’t move to the surface. This decreases the productivity of phytoplanktons which results in much less oxygen.

[sources: NationaGeographic, LiveScience]