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Wild Animals/Query re fish and saltwater/freshwater


Hi There,

I have read a lot and found out that saltwater fish cannot handle freshwater and vice-versa. This seems to be due to salinity and osmosis. However, I have also read about certain fish species like perch which can handle both saltwater and freshwater environments. What I want to know is how a fish species can easily adapt itself to both saltwater- and freshwater-envinironments.

Dear Geoffrey

Thank you for your question. I also wish to thank the authors of the websites I used.

Remy Melina ( that euryhaline fish can live in both freshwater and saltwater. They can endure a wide range of salt levels and can migrate back and forth between saltwater and freshwater. When they migrate, they must undergo an acclimation period, giving time for their bodies to adjust to a different salinity than they are used to. They associate the salinity of various habitats with different stages of their life, enabling them to balance salt concentrations between their bodies and their surroundings. says that when salmon are in fresh water, they have a high filtration rate, excrete a lot of urine and the chloride secretory cells take in salt.  When they are in salt water, they have a low filtration rate, excrete a low volume of water and the chloride secretory cells excrete salt.

Kirk Janowiak (
says that fish living in the interface between fresh and salt water can change their osmoregulatory functioning depending upon the salt concentration of the water. These "brackish" water fish live in places where the salinity varies seasonally and sometimes more frequently. Many are found in estuaries and drainages of tropical waters, but some are found on and around the coasts of temperate waters. They live almost entirely in the zone of changing salinity, but several species move into saltwater zones if there are rich coral reefs near by. Their adaptations include adaptations in the renal (kidney) system, but especially in the gills. The fish often live in areas where sedimentation occurs regularly and the water can be turbid and contain particulates that might clog the gills of other fish. The gills of brackish fish can shed the particulates better and are adapted to changes in osmoregulation due to the salt concentration.
Sandeep Raghuvanshi ( brackish water fish live in areas where rivers meet the sea and have a wide tolerance of salinity in water. They can live for some time in freshwater or sea water, but for longer periods need brackish water with a salinity level between salt water and freshwater.

I hope this helps.

All the best


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Jonathan Wright


I can answer questions about wild mammals and other animals, as well as extinct animals and zoos. I am not an expert about every animal species. I can look up information from books and the internet, but can't verify if all the information is true. Please don't ask questions about: 1. Pets. I am not a vet. Please contact a vet if your pet is ill. You may need to spend some money if you want your pet to live. Don't get a pet if you don't know how to look after it and if you can't provide it with the space, food and possible companions that will help it live a healthy life. Don't take animals from the wild, unless they are ill and/or injured and you can protect them until a wildlife charity can help. It is cruel to take animals from their parents, especially if the parents will look for the babies, while putting their other babies at risk. You may be breaking the law by keeping wild animals or you may need a licence to look after some species. Please check with a local wildlife group. 2. Eggs: Please don't remove eggs from nests. The mother birds provide the right temperature for the eggs and won't sit on them if the temperature is warm enough for them to develop naturally. It is illegal to remove eggs of some species and, unless you have an incubator or a broody hen, the egg may not develop. If you are allowed to touch the eggs, you can candle them to see if they are fertile. If theys aren't fertile, they won't hatch. 3. Fights: Please don't ask about fights between different animals. These questions assume that individuals of two species fight each time they meet and that one species will always be victorious over another. This is untrue. There are cases where a live mouse has been fed to a venomous snake, bitten the snake leading to the snake's demise. 4: Diseases: Please ask doctors or other medical experts about diseases that you may catch from animals. I can't advise on how to deal with viruses, bacteria etc.


I have a zoology degree and have been interested in animals since I was two. I am a zoo volunteer at London Zoo. I have appeared on a BBC Radio Quiz, 'Wildbrain'.

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