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bienville
11-13-2005, 08:38 AM
. . . and what would the cause of death be?

kanicbird
11-13-2005, 11:28 AM
My understanding is that a salt water fish in fresh water causes it's cells to swell and burst, since the salt level inside the cell is greater then outside, so water enters to dilute it.

If the above is correct, then it sounds reasonable that the oppoiste happens with a fresh water fsh in salt water, water leaves the cells, and the body and it dehydrates.

Note some fish can transition between the 2.

wevets
11-13-2005, 02:47 PM
. . . and what would the cause of death be?

That's going to depend on the fish and on the concentration of salt in the water. Many fish regularly move between different concentrations of salt water, for example leopard sharks here in San Francisco Bay move easily between the nearly-fresh Delta and Petaluma marshes to the very saline south Bay and Golden Gate. Salmon and other anadromous fishes obviously also move from completely fresh water to ocean and back again.

Cause of death would probably be either tissue damage, especially to the gills, dehydration, or kidney failure depending on whether the fish was moving from high salt concentrations to low or vice versa.

lissener
11-13-2005, 05:46 PM
That's going to depend on the fish and on the concentration of salt in the water. Many fish regularly move between different concentrations of salt water, for example leopard sharks here in San Francisco Bay move easily between the nearly-fresh Delta and Petaluma marshes to the very saline south Bay and Golden Gate. Salmon and other anadromous fishes obviously also move from completely fresh water to ocean and back again.

Cause of death would probably be either tissue damage, especially to the gills, dehydration, or kidney failure depending on whether the fish was moving from high salt concentrations to low or vice versa.
It's going to depend upon the individual species, so for the sake of this question it doesn't make much sense to focus on euryhaline fish, which are clearly the exceptions.

And no, they wouldn't explode. Is the flesh of saltwater fish salty? Saltwater fishes' systems work to excrete salt from their bodies, while freshwater fishes' do not. A fish's osmotic balance will get way out of whack if it's put into water that's drastically different from the water that its metabolism has evolved a way to deal with.

I'm not sure what the exact cause of death would be; I imagine a saltwater fish would have the minerals leached out of its body, osmotically, in freshwater, while a freshwater fish's system would be overloaded with minerals in salt. These would probably entail two different causes of death.

David Simmons
11-13-2005, 06:22 PM
When saltwater fish are put into fresh water they suddenly are faced with an input of water without minerals and this lowers the concentration in their fluids which is bad, and I would think this also overloads their fluid excreting system.

Fresh water fish are faced with the opposite problem in salty water. They suddenly start to lose fluid and when they drink water it comes with a lot of minerals that their system can't get rid of.

Who knows which dies quicker? Where is that famous marine biologist, George Costanza, when we need him?

lissener
11-13-2005, 09:34 PM
. . . Fresh water fish are faced with the opposite problem in salty water. They suddenly start to lose fluid and when they drink water it comes with a lot of minerals that their system can't get rid of. . . .IIRC, saltwater fish drink; freshwater fish do not.

David Simmons
11-13-2005, 11:18 PM
IIRC, saltwater fish drink; freshwater fish do not.Yes, saltwater fish drink when they are in salt water and fresh water fish do not drink when they are in fresh water.

The salt water fish's internal fluids are less salty than their medium so they continually lose water to their surroundings and have to drink water to make up. This results in an intake of excess minerals in the water that they have to get rid of by excretion.

A fresh water fish's internal fluids are saltier than their medium so they continually take in water from their surroundings and must get rid of their excess water without getting rid of their minerals.

When a fresh water fish is put into salt water they start to lose water to their surrounds and I would think would need to drink. However, their excretory system wouldn't be able to handle all those excess minerals in their water intake.

David Simmons
11-13-2005, 11:45 PM
Thinking it over, maybe freshwater fish lack a "thirst" reflex since they get all the water they need, and then some, without drinking. In that case, in salt water they would just die from dehydration.

lissener
11-14-2005, 12:07 AM
Thinking it over, maybe freshwater fish lack a "thirst" reflex since they get all the water they need, and then some, without drinking. In that case, in salt water they would just die from dehydration.
That was my thought; why I mentioned it.

Mahaloth
11-14-2005, 11:05 AM
This thread is fascinating.

I'm just wondering how long it will be before someone comes up with some horribly cruel video of a freshwater fish in saltwater and vice versa.

We could put it in the Amazing Video thread from a couple weeks ago.

wevets
11-14-2005, 11:37 AM
It's going to depend upon the individual species, so for the sake of this question it doesn't make much sense to focus on euryhaline fish, which are clearly the exceptions.

I try to make sense... most of the time ;) . I thought the OP seemed very black and white, and that it would be important to point out that grey exists. Besides, while not euryhaline, most fishes can withstand at least some range of variation in salinity characteristics - even the ocean is not uniformly saline, varying from about 33 to 37 ppt depending on location and time.

A saltwater fish moving into water too fresh will suffer tissue damage particularly to the gills from water moving into its cells too rapidly, causing membrane damage and even causing many cells (not the whole fish) to burst. Hemorrhaging and respiratory failure would then cause death. Loss of minerals from diffusion also a worry, but in general it will be easier for the small, uncharged water molecules to move across cell membranes than charged or organically-bound (and therefore large) minerals (unbound minerals can be a different problem). The kidneys may also fail as they attempt to retain lots of water against the gradient.

A freshwater fish moving into saltwater will suffer dehydration as it loses water from cells exposed to saltier water. In this case, kidneys may also fail as they are overwhelmed with mineral- and salt-rich water, clogging their nephrons.

The gills of fishes are highly vulnerable because they need to bring the fluids in close contact to the external waters with a flimsy integument as a barrier, and the kidneys are also tasked with maintaining osmolarity of body fluids against the external concentration in both marine and freshwater fishes. In general, the problem is exacerbated by marine fishes having more dilute internal than external fluids, while freshwater fishes have less dilute internal than external fluids. Reversing the situation for external fluids causes osmoregulatory adaptations to work in a contrary direction to the needs of the fish to survive.

Note: I have never tested the ideas above by killing a fish in this manner. No animals were harmed in the production of this post. ;)

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