Threats to the biosphere from changes in the oceans are real. Global warming involves not just atmospheric heating but also sea surface warming. About half the increased warming is going to the oceans. This can have wide-ranging effects, with deoxygenation at or near the top of the list of risks.
Henry’s law states that the solubility of gasses in water is inversely proportional to temperature. What this means is that warmer water holds less oxygen. Anglers in Arkansas recognize three distinct kinds of conditions for fishing. Likely the most common fishery in Arkansas is a lake where the water temperature and hence the oxygen content supports fish such as largemouth Bass, sunfish, and the like.
If you are after smallmouth bass you are unlikely to find them in a lake, at least here in Arkansas. Smallmouth bass require a higher oxygen content that is available only in cooler water – usually clear streams that flow fast enough to avoid warming from the sun. It is not uncommon to see smallmouth bass at the cooler upstream ends of creeks and largemouth at the lower, warmer reaches.
Trout are the most demanding in terms of oxygen needs. Trout only thrive in cold water with the highest oxygen concentration. Here is Arkansas that means creeks that get the majority of their flow from springs and the cold tailwaters of impoundments.
The point of this freshwater digression is to point out that the variety and number of fish in a given locale is dependent on water temperature. This is also true in the oceans. There is a reason that megafauna such as whales spend their time in the cold, oxygen-rich waters of the arctic and Antarctic regions – that’s where their food is found in abundance. As the surface of the oceans warm, we should expect changes in where fish and sea mammals alike can survive. Just that sort of change is happening and it doesn’t look good.
Cod are an extremely important commercial fish found in northern regions of both the Atlantic and Pacific Oceans. The importance of this fish alone can not be overemphasized. The coastal regions of northern Europe have depended to a large degree on access to Cod. In the middle of the twentieth century the United Kingdom and Iceland were all but at war over fishing rights to the cod in the north Atlantic near Europe.
The trouble with cod now centers in the North Pacific. Just last week, the Gulf of Alaska was closed to cod fishing for the 2020 season. Stocks have been declining for several years, not from overfishing as occurred in the Grand Banks region of the Atlantic, but from ocean warming. The Arctic is warming much faster than the rest of the planet. Glaciers are receding, arctic ice is diminishing and now fish stocks are dwindling.
In the future, it is conceivable that other more tolerant species of fish can migrate into the warming Arctic waters but for other locales, this isn’t possible. Fish currently in the tropics are already the only species tolerant of the lower oxygen concentrations. Higher temperatures will likely create fish “deserts.”
Dr. Bob Allen is Emeritus Professor of Chemistry, Arkansas Tech University