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Wild Animals/Query re fish and saltwater/freshwater



I have read a bit 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 Gerald,

Well...I'm not sure how much you already know from your readings, so I'm not sure I'll be telling you anything you don't already know here.  But...

Anadromous (moving from saltwater to freshwater to spawn) and catadromous (moving from freshwater to saltwater to spawn) fish have the special challenge of keeping their body tissues at a constant salinity/osmolality despite their changing environment.  They use the same basic mechanisms that freshwater and saltwater fish use to maintain water balance, but they are able to adjust their mechanisms (probably while they are in transition in brackish areas) from one to the other.

You can find a nice overview of how salmon (anadromous) maintain homeostasis while switching from salt to fresh water here:

And an overall review of both here:

Basically, anadromous and catadromous fish literally switch from freshwater osmoregulation mechanisms to saltwater osmoregulation mechanisms (or vice versa) as they travel between the two habitats.

The physiological details of these mechanisms are complex, but if you are interested in reading about them, I can refer you to sources to learn about the sodium-potassium pump and other players in this metabolic drama.  :)

Hope this helps.


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Dana Krempels, Ph.D.


I'm an evolutionary biologist with a passion for animals. Ask about natural history, behavior, ecology, evolution. PLEASE NOTE:

If you have found an "orphaned" or injured wild animal or bird:
Please don't waste time asking questions on the internet, as the answers may come too late. DO NOT FEED THE ANIMAL, and DO NOT HANDLE IT unless it is in imminent danger. (Many wild "orphans" are not orphans at all!) If you are absolutely sure it is orphaned, keep it warm and quiet, and find a LICENSED WILDLIFE REHABILITATOR HERE. Don't try to raise a baby yourself, or rehabilitate an injured anmal. Many a well-intentioned rescuer will do more harm than good, especially with baby birds and baby rabbits.

Without geographic location, time of day and habitat, I can't help. A clear picture is always best.

It's impossible for me to I.D. an animal call without hearing it myself.

I'm not an expert on comparative strengths of different animals (more complicated than you might think!) nor bite forces.

I refuse to answer "Which of these two animals--X or X--would win in a fight?".

These hypothetical matchups range from impossible (Grizzly Bears and Gorillas don't even occupy the same continent.) to ridiculous (Someone asked me "Who would win a fight between a Great White Shark and a tiger?").

The vast majority of animals--even the fierce and powerful--are not as warlike as Homo sapiens, and it's childish to project our aggressiveness onto them.


I have been the fortunate caregiver to a group of Black-tailed Jackrabbits rescued from the Miami International Airport, and not releasable in this area because they are not native. I also have rehabbed and released Eastern Cottontails, and am in contact with many very experienced wildlife rescuers who regularly handle injured or orphaned rabbits and hares.

House Rabbit Society

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I have a Ph.D. in Biology, with main areas of expertise in evolutionary biology, genetics, botany, and ecology.

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