"This is why we all need to breathe in oxygen and we breathe out carbon dioxide. Fish do that too."
https://newsroom.unsw.edu.au/news/science-tech/how-oxygen-sucked-out-our-waterways-kill-fish
"the more carbon dioxide that people pumped into the atmosphere by burning fossil fuels, the more the oceans would absorb. The ocean would continue to soak up more and more carbon dioxide"
https://earthobservatory.nasa.gov/features/OceanCarbon
"The overload of carbon dioxide (CO2) in our ocean is literally causing a sea change, threatening fragile, finite marine life and, in turn, food security, livelihoods and local to global economies.
Like a sponge, our ocean are absorbing increasing amounts of CO2 from the atmosphere. Over the past 200 years, the world’s seas have absorbed more than 150 billion metric tons of carbon from human activities. Currently, that’s a worldwide average of 15 pounds per person a week, enough to fill a coal train long enough to encircle the equator 13 times every year. CO₂ concentrations are now higher than at any time during the past 800,000 years, and the current rate of increase is likely unprecedented in history."
https://www.noaa.gov/ocean-acidification-high-co2-world-dangerous-waters-ahead
"The reason for the declining oxygen levels in oceans is that warmer waters hold less dissolved oxygen and ocean temperatures are rising at an alarming rate. According to another new study released this week and published in the scientific journal PLOS Climate, a majority of the world’s ocean surface has consistently exceeded the normal range since 2014.
High water temperatures threaten ecosystems such as coral reefs and kelp forests, which fish feed on."
https://news.yahoo.com/climate-change-will-deplete-ocean-oxygen-kill-fish-studies-show-182130028.html
"The depletion of oxygen in our oceans threatens future fish stocks and risks altering the habitat and behaviour of marine life, scientists have warned, after a new study found oceanic oxygen levels had fallen by 2% in 50 years."
https://www.theguardian.com/environment/2017/feb/20/fish-under-threat-oxygen-depletion-oceans-study
"A study published last year shows that oxygen levels have been declining for more than 20 years—faster than anticipated. Areas with low levels of oxygen are expanding, causing fish, shrimp and other organisms to flee or die, and their feeding habits to change."
https://www.lung.org/blog/sharks-need-oxygen-too
As Ocean Oxygen Levels Dip, Fish Face an Uncertain Future
https://e360.yale.edu/features/as-ocean-oxygen-levels-dip-fish-face-an-uncertain-future