In the brink of extinction - many freshwater fish

Tiistai 12.12.2023 klo 17.23 - Mikko Nikinmaa

When I did my Ph.D. thesis 40-45 years ago, the topic: effects of temperature and hypoxia on respiration of rainbow trout, was hardly noticed by the general public. In 1976 we, biology students arranged a theme evening about the pollution of the Baltic  Sea and invited media. Nobody came, and when I asked a newspaper reporter why that was the case, he answered that the topic had no general interest. I wish the situation were the same today: fish would not suffer from pollution, increased temperature and decreased oxygen level. Unfortunately that is wishful thinking.

IUCN (International Union for the Conservation of Nature) has recently updated its Red List with very worrying information of fish, particularly freshwater fish. A quarter of the freshwater species is in immediate danger of extinction. The most important proximate cause is pollution, but toxicant effects cannot be separated from temperature increases and eutrophication, which causes oxygen lack. All of the previous problems increase parasite loads and cause fish diseases. In addition, overfishing and building of waterways, which has destroyed spawning sites or made it impossible to reach them, decreases fish populations.

Below I give a couple of examples of how all of the above cause the disappearance of specific species. First, eels are critically endangered species, which are characterized by their catadromous way of life and long spawning migrations. They grow and reach maturity in freshwater. Their migration from sea to freshwater feeding sites is critically dependent on smell sensing, which is dramatically disturbed by pesticides and metals. Consequently, those types of pollution may be an important cause of declining eel populations. Second, burbot is a coldwater fish. It spawns in the middle of winter, and it is ice-fished in January-February to get the fish and its eggs for soup. Now that the temperature is increasing, burbot is already living at the high end of its temperature tolerance, and may soon become extinct. Another group of coldwater fish is salmon and its relatives. In addition, it requires clear water with high oxygen content. Since both an increase in temperature and eutrophication decrease the oxygen level, salmon and its relatives may become extinct. Lampreys have succeeded in temperate waters for 500 000 000 years. Many of the species live in sea as adults, but spawn in rivers. Because of building of waterways, e.g. hydroelectric power plants, their long saga may be coming to the end. Finally, aquarium hobby is very popular throughout the world. Many of the ornamental fish do not reproduce in captivity, and are thus fished wild. This has made many species endangered because of overfishing.

Human actions cause climate change even when fossil fuel use is not involved

Tiistai 22.3.2022 klo 18.24 - Mikko Nikinmaa

Although there are still many people thinking that climate change is not occurring or at least man has nothing to do with it, there are many problems that man is causing which accelerate temperature increase, and which could be avoided by human action. The points below are such, and can be very important in driving many parts of the world uninhabitable because of either too high temperature or drought or both. So, even people who do not believe that fossil fuel burning causes temperature increase, should accept that the following affect climate and that humans could take actions to combat the changes.

1. Because of massive deforestation, Amazon rainforest appears to near tipping point, where the rainforest turns to savannah. Boulton et al write in recent Nature Climate Change (Nature Climate Change 12: 271-278; 2022) how the resilience of Amazon rainforest has decreased dramatically since the early 2000s. Other studies have also indicated that whereas we have always considered Amazonas to be a carbon sink, it has recently turned into a net emitter of carbon dioxide. The major reason for carbon dioxide emissions is the widespread forest burning.

If the rainforest starts turning to savannah, naturally the first thing that happens is that the plant and animal species living in rainforest die off, so the biodiversity decreases radically. But even if this doesn’t concern the people, who deforest Amazonas, the following should. The water cycle of South America depends on the rainforest. If Amazonas turns into savannah, because more agricultural land is wanted, the whole South America dries up, and many areas become unsuitable for agriculture (too dry). So, by trying to increase agricultural area, greedy people end up decreasing it.

And because of the loss of one big carbon dioxide sink, the temperature throughout the world increases even if fossil fuels had nothing to do with climate change.

2. Two events happening to the oceans are also causing increased carbon dioxide levels and consecutive temperature increase even without the input of fossil fuel burning, and both depend mainly on human action. First, almost a half of the photosynthetic carbon dioxide use is due to the photosynthesis of (mainly unicellular) algae. Because of the pollution, it is estimated that the oceanic photosynthesis has decreased by 10-15 %. This increased carbon dioxide load is one factor affecting global temperatures, and could be avoided by human action – proper water purification. Second, world’s oceans are overfished. The global carbon cycle depends a lot on fish accumulating carbon. When they die, the accumulated carbon sinks to the bottom of the oceans and stays there for thousands of years. As overfishing reduces fish populations, this removal of carbon from ocean surface is reduced, and the reduced removal is seen as an increase in global carbon dioxide level, leading to temperature increase.

Again this takes place without any change in fossil fuel use, but is entirely human-caused.

3. There are further a couple of vicious circles, which increasingly take place, if human actions fail to limit temperature increase. Temperature increase decreases the carbon dioxide solubility in water. Thus, if temperature increases, more carbon dioxide is given up from the ocean, leading to further temperature increase etc. There are huge natural gas (methane) stores below the permafrost. Methane is a very potent greenhouse gas. If temperature increases so that permafrost starts melting, the methane below is liberated, causes further temperature increase leading to further permafrost melting and methane liberation etc.

To prevent these vicious circles from happening, climate deeds are important even today with the brutal Russian attack to Ukraine. In the best case, it can actually speed up the change from fossil fuel-based to green energy production.