How vulnerable are fish to osmotic shock? Any research?

[GroktheCube]

I've always heeded the standard advice of "within .002 or drip", but I've never really seen any research on exactly how severe a salinity swing needs to be for it to actually cause osmotic shock. Obviously exposure to completely fresh water (or any water below 1.008 for that matter) would be problematic for most marine fishes, but what about 1.021 vs 1.0264?

I moved several fish into my DT today after an extended QT/fallow (remember to QT your snails too kids), and while they were all within spitting distance of 35ppt, it made me wonder.

Would it really be stressful for a fish to go from slightly lowered salinity right up to NSW levels, or vice versa? If the homeostatic mechanisms at play can adjust over the course of 20 minutes, why not 20 seconds?

What sort of scientific research has been done on this topic?

[Betta132]

Well, you can put a saltwater fish in freshwater for a few minutes at a time, to kill parasites and such. It doesn't hurt them, though sometimes they seem stressed by it.

[billsreef]

Fish can handle such minor fluctuations in salinity without any difficulty. Such things are very common on near shore reefs ;)

[GroktheCube]

Fish can handle such minor fluctuations in salinity without any difficulty. Such things are very common on near shore reefs ;)

After actually spending some time on a fringing reef for the first time last year (Ningaloo in Western Australia, absolutely beautiful if you aren't bothered by saltwater showers), I was a bit amused to find how much reefing "common sense" is a radical departure from what you see on a natural, healthy reef. I suppose this (along with temperature stability) is another example.

My line of thinking was just thinking about the human kidney, and how quickly and efficiently it adapts to maintain homeostasis (sg, electrolyte levels, etc). I can't imagine a fish's kidney being THAT inferior to a human's. As long as your water/electrolyte consumption remains within the bounds of sanity (read: within the rate limits of disposal by the kidneys), sudden and dramatic changes from one minute to the next are not really significant stressors.

For example, let's say I drink ~200ml of water over a single 60 minute period (probably just above what I need to maintain total hydration), and then consume 500ml during the next 60 minute period. My kidneys will need to dispose of more than twice as much water, but given that they can dispose of about a liter per hour, that's not biggie.

Following that same line of thinking, I can't really imagine the kidneys of a fish having a difficult time coping with a swing from slightly below normal salinity up to full "normal" 35ppt. While I can't say I'm very knowledgeable about fish endocrinology, I can't imagine the relevant signaling required to tell the kidneys what to do would take more than a few minutes, and unless the salinity swing were so drastic that it could cause substantial sg/electrolyte swings within that brief period, I can't see the salinity swing being a potential threat.

Am I missing something here? Unless there's a huge difference between how well the homeostatic mechanisms respond in a fish vs a mammal, I just can't imagine 1.018 to 1.0264 swing (requiring the fish to dispose of almost twice as much salt, but obviously still well within the limits of normal kidney function) being a substantial stressor.

[gone fishin]

I may be way off base here. It has been known to happen a time or two. From posts and articles I have read I thought the real damage of osmotic shock was at a cellular level. The rapid increase in salinity caused cells to rupture and in turn killing the fish slowly.

[billsreef]

It needs to be fairly large swing to overcome the fishes ability to osmoregulate and prevent that ;)

[GroktheCube]

I may be way off base here. It has been known to happen a time or two. From posts and articles I have read I thought the real damage of osmotic shock was at a cellular level. The rapid increase in salinity caused cells to rupture and in turn killing the fish slowly.

I'm sure it could happen, and I know far more about human A&P than piscine, but it seems unlikely except under extreme conditions.

I'd imagine the vast majority of the cells in a fish's body have some inherent ability to osmoregulate through simple ion pumping, and the kidneys should be able to respond to changes in a few minutes. I imagine it'd take a pretty extreme change for the fish's cells to be unable to keep up with the change until the kidneys caught up.

I could be wrong, but I'd also imagine that all of the exposed permeable/semi-permeable tissue (gills, mucosa, etc) had an inherent ability to accept/reject water and ions on an as needed basis. It would be necessary for them to have that ability to avoid osmotic damage on a day to day basis, given they're almost never exposed to an isotonic external environment in nature, at least for most fish.

A lot of this is just educated guessing on my part from my nonspecific knowledge about general biology, so I could be wrong, but I've never really seen any formal controlled research on the subject.

[gone fishin]

AAHH thanks. That sounds much more reasonable than what I was thinking.

[Mazzy]

Well, this isn't "scientific" by any means, but a local zoo does live feedings once a month for visitors and they throw a whole bunch of trout into the polar bear pools. Those pools are pure salt water. It's obvious the trout aren't happy; they swim pretty slowly. However, they DO swim, and I personally have sat and watched the clumsy (and extremely lazy) polar bears try for an hour or better to catch them while the fish keep right on swimming. So, to me, that says it'll take an awful long time for a fish to actually die of osmotic shock, especially given the small parameter swings you are talking about, because these trout are going from ~0.00 to 1.026 in a matter of seconds and still keep right on doing their thing - AND managing to avoid hungry bears!

[tazdvl]

I did a search on osmotic shock and landed here. I know this tread is old but just wanted to post this in case someone else searched and landed here. I drip acclimated 2 sets of Mollies that were in freshwater at pet store dripped both sets at 3 hour drip at roughly 1-2 drips per sec. and then added them to tank was running 35 ppt on tank. Did not measure the salinity of water they had been in but volume roughly doubled in that time frame all 8 Mollies have been fine for 2 weeks now in saltwater. I have read alot of posts on acclimation of Mollies to saltwater and the recommended time in acclimation is all over the board from temp acclimate then dump in to acclimate fish for weeks. My gut feeling is saltwater fish are much more resilient then believed in most forums. But it is just a opinion, not backed by any evidence. If anyone does have studies that show saltwater fist should only change over x amount of saliinty in x amount of time please post.