Explainer: Why warming waters mean an uncertain future for fish populations

Hundreds of dead fish line the shoreline of a sandy beach. (Photo: NPS via Wikimedia Commons)

The year 2023 has already shattered climate records. From the hottest summer and the lowest Antarctic sea ice extent to global sea temperatures going off the charts, the evidence of climate change is mounting.

The effects of a rapidly warming Earth are being felt far and wide, by both the human and animal world. But marine life is disproportionately affected. Just this week, 120 river dolphin carcasses were found floating in a tributary of the Amazon river in Brazil. Experts suspect that high temperatures — water temperatures in the region reached 39 degrees Celsius, more than 10 degrees higher than the average for this time of the year — could be responsible for the deaths.

But how does global warming affect fish? And what impact could it have on their collective future? Let’s find out.

Too hot to handle

The world’s oceans have been heating for many years now. 2023, however, was a bit different. From early March to May, average ocean surface temperatures climbed two-tenths of a degree Celsius. It surprised experts who are accustomed to seeing such surges. Then, each day from July 31 to August 31 witnessed warmer global average ocean surface temperature than the previous record from March 2016.

Such spikes are obviously not good for marine life. A 2023 study by the Woods Hole Oceanographic Institution shows that warming waters threaten the ocean’s biggest fish, such as marlin and skipjack. They could lose up to 70 percent of their environment or habitat by 2100. According to the study’s researchers, longer periods of warming can be harmful because of the animals’ temperature needs. And since large fish are often highly migratory, it could result in some species moving to northern or deeper waters in search of normal temperatures.

High ocean temperatures are also expected to cause big population movements. For example, the Gulf of Mexico is warming especially quickly and it could impact species of marlin and tuna. It also threatens conventional fish distribution. In India, for example, the Arabian Sea, which is witnessing rising temperatures and frequent weather events, is now seeing an invasion of species like jellyfish, puffer fish, and leather jacket fish, among others.

The rise in water temperatures also affects fish populations and mortality in rivers, backwaters and ponds. In Kerala, the month of May has typically recorded the most fish kills — a phenomenon in which a large number of fish die in an area and float on the surface within a short period of time. In the long run, it could affect populations in the state, such as the endangered red line torpedo barb and critically endangered Tameen barb, and species that get ready to spawn in the monsoon such as Garra mullya and olive barb. Removing active breeders from the system could seriously hinder species restoration.

Out of breath

Another concern is that ocean oxygen levels have dropped more than 2 per cent since 1960 and are expected to decline up to 7 per cent more by 2100. Some areas, such as the northeast Pacific, have lost over 15 per cent of their oxygen. According to the IPCC’s 2019 special report on the global oceans, from 1970 to 2010, the volume of “oxygen minimum zones”, where big fish cannot thrive but jellyfish can, has increased by 3-8 per cent.

This decline in oxygen is driven by warming waters, weaker currents, and increased bacterial activity fuelled by fertiliser runoff and microplastic pollution. It affects freshwater bodies too. This year itself, Gorewada Lake in Nagpur and Jubilee Lake in Jharkhand saw a high number of fish deaths due to factors such as scorching heat, pollution and a high rate of evaporation that led to depleted dissolved oxygen levels.

Even otherwise, fish will try to swim away from waters with low oxygen, but if they can’t escape, they become sluggish. Low oxygen levels affect their growth, reproduction, activity levels, and survival. Larger fish are more affected because they need more oxygen. Overfishing has already reduced the number of large fish, and deoxygenation is expected to make that worse.

It can lead to reduced fish populations and fisheries, more jellyfish (disrupting marine ecosystems), and an increase in dead zones (where no marine life can survive).

Cause and effect

Lastly, the ocean acts as a sponge for greenhouse gases. Warming waters change the pH levels as time goes on and emissions stay high, making it more acidic. This results in a lower availability of carbonate ions — essential ions that help marine life such as clams, oysters and crabs build shells or skeletons. They might seem small and insignificant, but have a key role to play in the marine food web.

Higher temperatures also cause coral bleaching, effectively reducing the nutrient habitat for fish populations. There has also been a phenomenal rise in cases of Harmful Algal Blooms, a challenging water pollution problem, in oceans, and even the Arabian Sea, increasing the fish mortality rate.

With our planet only expected to get warmer in the coming years, climate-related pressures will pose the biggest problem for marine life in the coming decades. It’s one more reason to get a handle on greenhouse gas emissions, address climate change and protect our oceans.