Can Fish Drink Water? Do they ever get thirsty and need a drink!

Have you ever been asked a question, opened your mouth quickly to answer, and then stopped because you realize you really don’t know what to say? I have, several times, with one of them being when asked ‘can fish drink water?’

I thought when asked, of course, they do, after all they live in water so why wouldn’t they drink it? Then I had the thought that I’ve never seen a fish pee, which would be the result of them drinking, so maybe they don’t. I then decided I was now slightly confused and decided to look into it.

Saltwater fish drink water, while freshwater fish do not even though they do need water to survive.  Once water enters a fish’s body, saltwater fish mainly urinate it out through their gills,  since they release the water through osmosis from their body.  Freshwater fish release most of their water from the urinary tract.

Why Do Saltwater Fish Drink, Whilst Freshwater Don’t?

To keep it fairly simple, freshwater fish do not drink because they are always absorbing water through their gills and skin due to a process called osmosis. Saltwater fish, on the other hand, are always losing water through their gills and skin also due to osmosis, so they need to drink almost constantly. Osmosis involves a flow of water across the membranes of a fish in order to move solutes (salt) from areas of high concentration to low in an attempt to even the distribution out.

Freshwater fish absorb water through their gills as well as their skin.

The reason why saltwater fish and freshwater fish react differently to the process of osmosis is all down to the salt. Freshwater fish have bodily fluids and blood that is much saltier than the water they swim in, whilst for saltwater fish the opposite is true. This means that when more salt is introduced through the osmosis process, it can cause the saltwater fish to dehydrate. To avoid this, they take water through their mouths and direct it into the digestive tract, hence, drinking it.

It is worth mentioning here, that like humans, fishes bodies only need a certain concentration of salt to function to the best of their ability.  Water cannot be allowed, in fish, to just diffuse freely through their gills. This is because when this happens saltwater fish would shrivel up and die, whilst freshwater fish would explode.

To avoid ‘exploding or shriveling’ fish their gills are equipped with special cells that pump salt in or out of their blood selectively. As mentioned above, in freshwater fish these cells constantly draw water in, and in saltwater fish, they constantly draw salt out. The kidneys of the saltwater fish are also used to help with this process.

can fish drink water

Do Goldfish Drink Water?

The Goldfish is the most common species enquired about when it comes to whether fish drink. Believe it or not, it is that often asked that it even has its own joke:

‘We once owned a goldfish that had a urine infection. We knew because the bowl kept overflowing.’

However, the answer, of course, as we now know, is that they don’t, they are not an exception to the freshwater/saltwater rule. There are, nonetheless, some fish that do not fall into saltwater fish drink and fresh water don’t categories. These include:

  • The Shark whose blood is naturally very salty and virtually matches the salt content of the sea. This means they are never overloaded on salt and hence have no need to drink.
  • The Salmon who lives both in freshwater and saltwater dependent on the time of year. This means that they have to adapt to drinking and not drinking varying upon where they are.
  • Not all sea living animals drink salt water. Whales and other sea mammals, for example, get their water from their food. Should they drink seawater, it is believed, that they would suffer from the same dehydration that humans do.
  • The Nikpoh Lyrad (Archerfish) drinks water into a chamber of its stomach. It does not, however, need this water to survive; rather to spit at its enemies with tremendous speed.

Also interestingly, but debatable, a freshwater lake in Australia was (allegedly) emptied of all fish during a drought leading to the water level not dropping at the same fast rate as it had been doing. This has led to fishkeepers in the area arguing that the freshwater fish in there must have been drinking the water!

So if fish can drink water, Do They Also Urinate?

It’s easy for us to see that our fish poop, the little strings hanging from their butts give it away. However, it’s not quite so clear whether our fish urinate or not. After all, they’re in the water and you’re hardly going to see it!

From what we know already, that saltwater fish drink and freshwater don’t, the natural assumption would be that the aforementioned urinate and the latter don’t. However, this would not be the truth as both freshwater fish and saltwater fish urinate.

Saltwater fish and creatures need to maintain a balance of salt in their bodies which is made more difficult with them living in a saltwater ocean!

A recent study by the Virginia-Maryland Regional College of Veterinary Medicine, in fact, claims that fish basically urinate all the time. They do this either through their gills or urinary pores with the latter being filtered via the kidneys.

Freshwater fish mainly use their urinary pore to urinate which is located near their anus. They release large amounts of undiluted urine as they have to get rid of all the water they take on board. Saltwater fish, on the other hand, urinate mainly through the gills. Their urine contains high amounts of salt and low amounts of water meaning it is extremely concentrated.

They have astonishing gills that help them drink the saltwater, process it and then eliminate all the excess salt via excretion.

A Word of Caution

As some fish drink and all fish urinate they can suffer from urinary tract and kidney diseases. Renal dropsy, for example, is common in goldfish and other carp species, which is caused by a parasite, Sphaerospora auratus. Fish affected with this condition display swelling in their abdomen from fluids that accumulate as the kidneys fail. This condition is generally fatal.

Carl Broadbent
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