I really try to avoid fact checking people in public comment sections on the internet, but sadly this time I couldn't resist.
There was a reel on Facebook explaining the "sad" reason why cetaceans in human care often receive ice. It's because they're given "dead fish" you see, and these frozen thawed fish contain less moisture than the live fish they eat in the wild, so the ice helps prevent dehydration.
First of all, this is half true. The other half is that many cetaceans just love getting ice.
Supplemental hydration can also include gelatin and squid. The latter two are not unique to cetaceans btw. Squid and specially formulated gelatin based foods are also used in many fish diets in part to provide additional moisture. They also count as enrichment, depending on the context.
In any event, this creator thinks it's sad... that caretakers are providing their dolphins additional hydration? What, does he also think it's sad that they get vitamins? Reptile keepers regularly dust their animals' food with calcium to make sure they don't get metabolic bone disease. Is that also sad? These things are standard zoological practices, not signs of neglect. Wouldn't it be sadder if they didn't do that?
(Reason #423 why framing matters).
"But it's not natural!"
Neither is running on a treadmill, but that doesn't mean it's bad for you. Also, I'm pretty sure ICE is one of the most natural things you could use....
"Well why can't they just give them live fish!?"
Because the logistics of sustaining a live colony for such a purpose would be an expensive nightmare. Do you know how many pounds of fish these animals need to eat in a day? Using them as an occasional form of enrichment might be feasible, but on the regular? Just. No! Tell me you don't know how fish care works without telling me you don't know how fish care works!
It's cruel to the fish, for the same reason that it's cruel to feed snakes live food (assuming they aren't picky eaters). Fish are not just props, they are live animals that can experience stress and pain!
Freezing the fish helps keep it safe to eat.
"Why not just release them back into the wild?"
Because that would also be cruel since they wouldn't know how to survive. Next question.
"How do they get dehydrated when they live in water!?"
Oh man. Oh man oh man.
Cetaceans get most of their hydration from their diet. A quick google search indicted that they can drink some salt water, though I don't know how true that is, and to be clear, their kidneys are likely much more efficient at filtering out salt than ours (someone who knows more about this, please feel free to correct me). Regardless, most of their hydration comes from their food. Lack of food leads to dehydration, both in captivity and in the wild.
But there was one guy who said that dolphins in captivity can't drink the water because it's chlorinated. This is not true. The residual amounts of chlorine left after their water passes through filtration is no higher than what is in your tap water.
(Btw, this is another reason why live fish aren't typically used, because you don't house fish in systems that use chlorine, even in small amounts).
When I pointed this out to the guy, his response was that "most of their diet contains water from the exact same source."
To his credit, he did back down when I pointed out that this was wrong. But come my friends! As a fish person, let me explain to you why this is not correct!
Fish blood chemistry is not the same as the ambient water around them. They would not be able to maintain homeostasis otherwise. You see, their gills and kidneys help them osmoregulate.
Freshwater fish blood contains more salts than the surrounding freshwater, which means the via osmosis, water will enter the body, and unless they can compensate for this, the fish will swell up like a balloon, lose necessary salts, and basically drown. So what do they do? Their kidneys produce large amounts of dilute urine get rid of this extra water. Meanwhile, chloride cells in their gills take up ions from the water to transport into their blood.
Saltwater fish, however, have the opposite problem. Their blood has less salts than the surrounding water, so they need to get rid of extra salt. How? Well, they actually do drink the saltwater, and then their kidneys help filter out the extra salt. They basically produce very concentrated urine. But the kidneys cannot do this alone, so the chloride cells in their gills also help by basically working in reverse. They move extra ions out of the blood.
Elasmobranchs, crustaceans, mollusks and other marine mammals that may also make up some cetacean diets will have their own methods of osmoregulation.
All of this is to say, no, eating saltwater animals does not mean you're literally consuming ocean water, as these organisms have specific mechanisms for filtering out the excess salts.
This is, btw, why you can't just put a freshwater fish in saltwater or vice versa. They will die. Nor is this unique to fish, most organisms adapted for one environment don't thrive in the other.* Case in point: we cannot drink salt water, because our kidneys cannot handle that.
But some animals can! Take the salmon for instance. They hatch in freshwater environments, then move to saltwater, and then back when spawning season comes. How do they do this? Well, there are specific environmental/developmental triggers that cause their bodies to start osmoregulating in reverse. This does not happen instantaneously, however. They need time to adjust. They will migrate to brackish water, and allow their bodies a few days to adjust before continuing.
Some species of fish have higher tolerances for wider ranges of salinity/total dissolved solids (TDS) than others. Fluctuating these levels can actually trigger spawning in some species too! Discus, for instance, can be maintained in relatively harder freshwater than what they'd experience in the wild. But for breeding purposes, it's recommended to use softer water with lower a TDS. Given that discus can be more finicky, however, any of these adjustments should be made gradually.
On the other hand, there might be another species where a more sudden change is beneficial. Tropical fish native to floodplains that will experience sudden onsets of flooding from the rainy season, for instance, may need precisely that sudden rush of freshwater with lower pH and TDS to trigger spawning. Even if spawning isn't the end goal, simulating these seasonal fluctuations can be a very effective form of naturalistic enrichment!
....Oh dear, I went on a very nerdy tangent, because SCIENCE!!!! Enjoy.
*this can actually be taken advantage of in certain treatment contexts! Saltwater aquarists will sometimes do what is called a "freshwater dip", where the saltwater fish is very briefly immersed in freshwater to help rid them of harmful external parasites or bacteria. Usually, it's used when fish are first brought into quarantine. It's not 100% effective on its own, but it's one of the many tools in an aquarist's arsenal. Oh! This can also be used on sea turtles in some rescue situations! Again... SCIENCE!!! >:D
