But fish are increasingly threatened by global warming. Greenhouse gases released mainly by humans burning coal, oil, and natural gas have led to a sharp rise in mean global temperatures over the last 50 years. Temperatures are expected to rise 1.4-5.8°C  more by the end of the century. As a result, the water in rivers and lakes is heating up, and even the oceans have warmed in the last 50 years.

Warmer waters, as well as changes in rainfall, currents, and sea level, are already affecting the world’s fish and fisheries. As global warming continues, the pressure on populations already strained by overfishing, pollution, and habitat loss will increase. And while slightly warmer water may not sound so bad to many of us, its effect on fish and aquatic ecosystems, and ultimately on the global food supply and economic stability, could be severe.

HOT, HUNGRY, AND GASPING FOR AIR

Fish are more sensitive to temperature than many animals because they cannot maintain a constant body temperature like we do – in most cases, their body is exactly the same temperature as the water they are swimming in. Different species can live in very cold or very hot water, but each species has a range of temperatures that it prefers, and fish can’t survive in tempera tures too far out of this range.

When fish encounter water that is too cold for them, their metabolism – the chemical engine that drives their body – slows down and they become sluggish. As the surrounding water warms up, their metabolism speeds up – they digest food more rapidly, grow more quickly, and have more energy to reproduce. But fish need more food and more oxygen to support this higher metabolism.

If there is not enough food, all of a fish’s available energy goes to fuelling its high metabolism, and less energy is available for growth and reproduction. Rainbow trout grow signifi cantly more slowly when their water temperature is raised only 2°C and food is limited, and fish such as salmon, white fish, and perch are all expected to grow more slowly if food supply does not increase as temperatures rise.

WATERY LAYERS THAT REFUSE TO BUDGE


Water may all look the same to us, but for fish, the world is made up of very distinct layers – each with its own temperature and supply of food and oxygen. In temperate lakes, as water near the surface heats up in the spring, it becomes lighter and floats on top of the cooler, denser layers below. More plants and animals live in the top layers, where nutrients are used up quickly, but oxygen diffuses in and is produced. by aquatic plants.

The cool bottom layers have less oxygen but lots of nutrients from decaying plant and animal matter, and they provide a thermal refuge for fish that find the summer surface waters too warm. As surface waters cool in the fall, they become heavy enough to drop down and mix with the bottom layers – this “turnover” moves nutrients up to the o the surface and sends oxygen below.

As global temperatures rise, the top layers will become even warmer and lighter, making mixing harder. Earlier arrival of spring also increases the period of time when lakes remain layered, or stratified.

Even the ocean contains layers that are affected by climate change. Both temperature and salinity contribute to the density of seawater – cold, salty water is heavier than warm, fresh water. In the Gulf of Alaska, surface temperatures have risen and more fresh water is flowing into the sea from melting glaciers and increased precipitation. This lighter layer of warm, fresh water has reduced vertical mixing in the Gulf, and there are now less nutrients to feed the small organisms that fish depend upon.
 
Carbon dioxide (CO2) is the main pollutant causing climate change. It rises through the atmosphere and captures heat, intensifying the effect of the greenhouse gases that keep the earth warm. This has dramatic consequences for the globe’s climate system – more extreme weather like droughts, floods, and storms; rising sea levels and changes of large ocean currents, and changes ofregional weather systems during events like
The Child.

Climate change threatens fish and fisheries as temperatures rise in oceans, lakes and rivers.