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Ask Dr. Pauly: How is global warming affecting fisheries?

The news about the impact of global warming on the oceans — or, more precisely, of ocean warming and deoxygenation — are gradually becoming more serious and wide-ranging, so much so that a brief review of the issues involved may be helpful. Here, contrary to my earlier columns, I will add a number of scientific or other references to the summary below of these issues, so that readers get a feel of the existing literature. My excuse for being associated with several of these references is that I do work on these topics.  

1. Elevated temperatures increase the oxygen requirements of fish while decreasing the oxygen content of the water (see also Oceana Magazine, Winter 2015). This effect is worse in oxygen-poor zones of the ocean, which are spreading. One major effect of these challenges is that the maximal size that fish and invertebrates (such as lobsters and squids) can reach declines, along with the size at which they become mature, which reduces the number of eggs they can produce; 

This graph – adapted from one that appears in the Science Advances paper linked above – shows how water temperature affects the growth and size of Atlantic cod in two different regions. In Iceland, they reach much larger sizes than in French waters.

2. Another major effect of increasing ocean temperatures is that fish populations (which usually occur over a range of latitudes) do well on the cool edge of their distributions and badly on the warmer edge. Thus, rising ocean temperatures cause fish populations to shift their distribution ranges poleward, and the fish populations in tropical waters become depleted

3. The population range shifts described above are well documented throughout the world and are known to cause many fish “stocks” that are traditionally exploited by a given country (or state) to move away from the waters of that country or state and into the waters of other countries (or states); 

4. This situation can cause fisheries management problems in the best case, e.g. between South and North Carolina on the eastern seaboard of the United States, and actual conflicts or “fish wars” in other cases, with the worst cases occurring in tropical countries, whose fish will tend to slowly disappear

5. Therefore, this will require arrangements to be made between countries and states to either share or swap exploitation rights. However, such arrangements must be made quickly because the fish populations will not be waiting for poetical decisions to move where they must;  

6. Unfortunately, we can also expect a recurrence of heat waves such as the one that affected the Pacific Northwest in the summer of 2021, killing an estimated 1 billion shore animals along the coast of British Columbia. Heatwaves make fisheries management procedures and agreements vain. 

In other words, if we want to have fish in the ocean and fisheries to exploit them in the longer term, we need to reduce our greenhouse gas emissions urgently and massively.  

This will also involve getting rid of bottom trawlers, which not only use huge quantities of fuel to drag giant nets along the seafloor, and thus emit huge amounts of carbon dioxide into the atmosphere, but also stir up the carbon that had been buried in sediments on the seafloor.

If we don’t reduce our greenhouse gas emissions, we will be in trouble. 

Dr. Daniel Pauly is the founder and principal investigator of the Sea Around Us project at the University of British Columbia’s Institute for the Oceans and Fisheries, as well as an Oceana Board Member. This column appears in the Climate Issue (Spring 2022) of Oceana Magazine. Read it online here.

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