Delaying efforts to keep global warming below 2 degrees Celsius is expected to have disastrous consequences for global marine fisheries.
Key points:
- Thirteen percent of important fish habitat could be lost if climate targets are exceeded
- Significant mitigation measures from 2040, in the worst case scenario, would not be enough to reverse the impacts on the oceans.
- Warmer oceans could lead to fewer fish and smaller fish
New modeling out to 2299 suggests that exceeding climate targets could result in a 13% loss of important fish habitat in the top 200 meters of the ocean.
This area, known as the epipelagic zone or “sunny zone”, is home to most of the world’s marine fisheries.
Scientists warn that fish species in this area could become less resilient and more susceptible to illnesses such as disease if 2°C rises.
The habitat decline was predicted by scientists at France’s National Center for Meteorological Research., with the help of the CSIRO and other international institutions, in a new study to Nature Communications Earth and Environment.
The researchers used the worst-case climate scenario, SSP5-3.4-OS, in which global CO2 emissions continue unabated through 2040, before aggressive mitigation efforts are made to reach net zero emissions in the years 2080.
Surface air temperatures could reach between 2.4°C and 4.1°C above pre-industrial levels in this scenario, and be brought below 2°C by the end of the century.
The ocean takes much longer to absorb and release heat, meaning there will be a significant delay in its cooling, even once CO2 emissions are reduced.
And that could have long-term consequences for fisheries managers trying to balance food demand with changing stocks.
CSIRO’s Andrew Lenton, co-author of the study, said modeling showed that even 200 years after an exceedance, the rate of recovery of a habitable ocean in the epipelagic zone was low.
“We talk a lot about getting to net zero and we absolutely need to comply with the Paris Agreement,” Dr Lenton said.
“The real risk here will be that we overshoot our emissions… and as a result we end up getting stuck in these changes which will be irreversible within our lifetime.
“We can’t really put our shows on a credit card and pay them off later. We’re going to pay a pretty high interest rate for that.”
Reheating a “double whammy” for fish
The United Nations announced in July that the planet was already on track for average surface air warming of more than 1.5°C.
So far, average ocean surface temperatures have increased by about 0.8 degrees above pre-industrial levels.
The new study suggests there could be a surface temperature peak of 19.5°C in 2072, based on an average of several models, about a decade after atmospheric CO2 peaks.
Surface temperature would drop to 18.8°C in 2182 before exceeding 19°C over the next 100 years.
A similar trend is observed for epipelagic waters, while further down (200 to 1,000 meters) in the mesopelagic zone or “twilight zone”, the average ocean temperature remains on an increasing trend throughout the projection.
As part of the study, researchers looked at how this warming could affect 72 marine species, including shrimp, scallops, mussels, lobsters, crabs, Atlantic cod and salmon.
As water temperatures increase, the metabolic needs of fish and other marine creatures also increase, meaning they expend more energy at rest and need more oxygen to survive.
Dr Lenton said this was made worse by the fact that seawater retains less oxygen when it is warmer.
“You have an increase in demand (for oxygen) and a decrease in supply, so you create a double whammy for the fish,” he said.
This would not suddenly mean underwater wastelands, but it could have serious consequences, Dr Lenton added.
“We know that life wants to adapt. There will be winners and losers.
“The fish could be smaller to allow for less metabolic demand, (so) you could get a decrease in the size of the fishery and some things (species) would not be viable.
“You will end up with a new ecosystem or an adapted ecosystem that responds to the changes.”
Climate change is already affecting fishing
More than 3.3 billion people get 20 percent of their daily protein intake from fish, with demand expected to increase. double by 2050.
Marine fish accounted for about 39% of the world’s total aquatic animal production in 2020, according to the Food and Agriculture Organization of the United Nations.
But FAO analysis indicates that the fraction of fish stocks at biologically sustainable levels has declined from 90 percent in 1974 to 64.6 percent in 2019.
Research evaluating 124 marine species distributed in 235 populations in 38 ecoregions found that their maximum sustainable yield fell by 4.1 percent between 1930 and 2010. due to warming waters.
Nineteen fish stocks, mainly located near northern Europe and Japan, were found to be 8 percent less productive.
The same study also found instances of cascading effects on zooplankton, forage fish like sardines and demersal fish on the Celtic-Gascan Shelf off Ireland and the North Sea.
Wild-caught Atlantic cod in the North Sea has achieved sustainability certification from the Marine Stewardship Council (MSC). suspended in 2019 due to declining inventories.
Research last year claimed 64 per cent of population fluctuations in the North Sea cod fishery. could be attributed to climate change while the rest came from fishing itself.
Closer to home in Australia, MSC Asia Pacific’s Matt Watson believes most commercial stocks are already facing some sort of stress from climate change.
Captains have faced geographic changes in where they find yellowfin tuna on Australia’s east coast, he said.
“In New Zealand there are some swings in the hoki fishing and some say this is also due to climate change.
“Fisheries management can help increase the level of resilience.
“You can kind of protect your resources if you have a good understanding of what kind of impacts are happening in real time.”
Fish prey also faces decline
Climate change not only affects the fish we eat, but also the creatures they eat.
Zooplankton – small creatures such as jellyfish and krill – make up about 40 percent of the world’s marine biomass and are eaten by small fish.
And zooplankton feed primarily on phytoplankton, which declines in warmer oceans.
University of the Sunshine Coast mathematician Ryan Heneghan said this change in phytoplankton size could lead to an overall decrease in zooplankton.
There could be a shift of zooplankton towards lower carbon species, he added.
“For fish that feed on zooplankton, this has implications for the quality of their diet.
“And then the big fish eat the little fish and you keep going up.”
Dr. Heneghan’s research showed that in a high emissions scenario of around 4 to 5°C of warming, global fish biomass could fall by 10 percent.
He said the impact of changes in zooplankton species had already been observed during marine heatwaves.
“There was a big heatwave off the coast of California…called The Blob in 2016,” Dr. Heneghan said.
“Temperatures were high for a few years…and then that led to not only a reduction in fish biomass, but also reproductive failure in fish communities and those kinds of things.”
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