Context: The rise in oceanic temperature due to global warming is affecting the aerobic capacity of the fish and could impair their physiological performance in the future.
Issue

• The latest Intergovernmental Panel on Climate Change (IPCC) report has warned that ocean warming will continue over the 21st century and is likely to continue until at least the year 2300 even if we minimise carbon emissions.
• According to the report the amount of ocean warming observed since 1971 will likely at least double by 2100 under a low warming scenario and will increase by 4-8 times under a high warming scenario.
• Anthropogenic causes are held to be the biggest reason behind ocean warming.

Impact on fish

Ocean warming oceans can cause stress, decrease the range, increase diseases and can wipe out many commonly eaten fish.

Increase in methylmercury: The amount of methylmercury in fish higher in the food chain can change due to two reasons — ocean warming and dietary shifts due to overfishing by humans.

• Besides dietary changes, ocean warming too causes changes in the methylmercury accumulation in fish.

Creation of hypoxic and anoxic zones: Warming can aid in creation of both anoxic (waters that have no dissolved oxygen) and hypoxic (low oxygen concentration) zones

• Forced adaptation: Several species are migrating towards pole or deep waters to stay in their ideal temperature range.
• Reduction in fish size: Certain species of fish are becoming smaller and unable to move to better environments.

Limit the aerobic capacity of fish: As temperature increases, the demand for oxygen of many fish species will exceed their capacity to extract oxygen from the environment through their gills.

• As a result, the aerobic capacity of fish decreases in warming waters, and this reduction may be more important in larger fishes. This means that global warming could limit the aerobic capacity of fish, impairing their physiological performance in the future.

Impact on humans

• The impact on fish will have serious implications for our food security, as many of the species we eat could become increasingly scarce or even non-existent in decades to come.