Given that fish live and breathe underwater, the idea of them drowning seems impossible. And yet still the question remains, and as with most things to do with wildlife, the answer isn't as simple as you might expect...
Can fish drown?
According to Collins Concise Dictionary, drowning is “to die or kill by immersion in liquid” – so no, fish cannot drown. However, they can suffocate when fresh water does not contain enough dissolved oxygen – either as a result of eutrophication (an excess of nutrients) or drought.
Many fish must swim constantly in order that water is always flowing past their gills. This is the case for most sharks, which is why they cannot survive when thrown back into the sea following the removal of their fins by fishers supplying the shark fin soup market. A shark with no fin is unable to swim and will therefore suffocate.
Some fish, however, are able to pump water through their mouths and over their gills, meaning that they are able to stop swimming. Examples of species that do this are nurse sharks, skates and rays.
Fish can also suffocate if their gills become damaged. This might be the result of coming into contact with fishing equipment, for example, or an altercation with a predator. Disease can also be a factor – bacteria might attach to the gills, blocking them from filtering oxygen.
Can any fish breathe out of the water?
Out of water, a fish’s feathery gills stick together, reducing the space available for gas exchange, eventually resulting in suffocation.
Some fish can survive out of the water longer than others. South America's arapaima, for example, is the largest freshwater fish in the world, reaching up to 3m in length. It has very small gills, and its modified swim bladder allows it to take in oxygen from the air. As such, the arapaima can survive out of the water for up to 24 hours.
There is a fish that can breathe air, however: the lungfish. Lungfish first appeared around 400 million years ago, and there are six species alive today, living in rivers and lakes in Africa, Australia and South America. Lungfish come to the surface to breathe, drawing air into their primitive lungs, which are an adaptation of the swim bladder. This anatomy also allows them to survive during periods of drought, when they burrow into mud.
Marine mammals and reptiles such as whales and turtles, of course, breathe like we do, by coming to the surface and taking air into their lungs. Unlike us, however, they are adapted to holding their breath for much longer periods: the longest on record is a Cuvier’s beaked whale that dived for 138 minutes, a 2014 study found.
Top image: Maze rabbitfish (Siganus vermiculatus). © Georgette Douwma/Getty
