I agree that true hyposalinity is an effective method for removing some parasites. No doubt the author of this article is very knowledgeable, but he also seems to be mixing science with anecdotal concepts:
There may be some concern that hyposaline conditions could be stressful to marine teleost fish, or otherwise potentially harmful. While this is true in extreme salinities, studies indicate that this is not the case in more moderate salinities that would be employed in hyposalinity therapy (Wu & Woo, 1983. Woo & Chung, 1995. McDonald & Grosell, 2006).
This doesn't indicate that lower salinities are "better" only that in a lot of circumstances, hyposaline conditions aren't stressful or harmful, at least for relatively short periods of time.
Natural Sea Water is much more saline than the internal fluids of marine fish. Because of this, they expend a considerable amount of energy to reduce the excessive salt load through the process of osmoregulation.
No doubt osmoregulation expends energy. However, everything I could find pointed back to what I posted earlier: Morgan and Iwama (1991) reviewed the studies on the metabolic responses of several species of fishes from fresh-, brackish-, and saltwater. They concluded that low metabolic rates are most often associated to the water salinity in which the species are most commonly found and, therefore, most physiologically adapted to, at a particular life stage.
The kidneys are not the primary site of electrolyte management in marine teleost fish (Stoskopf, 1993). Chloride cells in the gills excrete excess chloride and sodium. “The kidneys of marine fish do play a role in electrolyte excretion; however, there function is more important in the balance of magnesium and sulfate levels and not, as might be assumed, in sodium and chloride elimination” (Stoskopf, 1993).
Though interesting, I don't see where this addresses that lower salinities are "better" for saltwater fish.
The effects of stress caused by capture, transport and handling is a major concern when acclimating fish, especially when they have been bagged for a prolonged period. Stress affects fish in two ways: it produces effects that disrupt or threaten homoestatic equilibrium and it induces adaptive behavioral and physiological responses (Wendelaar Bonga,1997).
The first sentence is anecdotal. Though regarded as true, we can't measure how stressful these conditions are to a fish or how long that stress lasts. The second sentence, I will concede, has scientific basis.
Osmoregulatory dysfunction is closely associated with stress in fish. This is recognized by an increase in osmolarity in saltwater species (Carmicheal et. al, 1984. Robertson et. al, 1988.). This can manifest in the loss of up to ten percent of body weight due to dehydration in one or two days (Sleet & Weber, 1982.). Reducing the salinity gradient between the water and the internal fluids of fish is effective in counteracting osmoregulatory dysfunction and other physiological responses to stress (Johnson & Metcalf, 1982. McDonald & Milligan, 1997.) With marine teleost species, this is accomplished by reducing the salinity of their environment.
Everything but the last sentence has scientific support... following that logic, the last sentence makes sense. However, the "scientific" piece is talking about osmoregulatory dysfunction, not normal osmoregulatory processes. Hence, the argument to use lower salinities to combat stress definitely has merit. The question is, how do we measure the level of stress in a fish or how long that stress lasts? This also ignores the fact that stress might be induced by putting a fish in lower salinity water in the first place... at least if not acclimated to it properly. In other words, if we knew that a fish would maintain a certain level of stress for a certain amount of time, we could conclude that a lower salinity during that time would likely be beneficial to combat dehydration. I don't think we really know the level of stress that shipping causes or how long it lasts. That said, shipping at lower salinities very well might be as safe as shipping at normal salinities.
My point is this... I could take an alligator from a Florida lake and put it in a Canadian lake and say, "The alligator will be better off because its metabolic processes will slow due to the colder climate, making it need less food and enabling it to conserve more energy." This might be true, but knowing that alligators have adapted to and thrive in a Florida environment and climate, there would have to be a LOT of scientific proof that my alligator would be better-off by relocating it to Canada. It is the exact same thing here... these fish have adapted and thrive in the salinities they live in and if that isn't the best environment for them, there has to be a lot of scientific proof to support. What I see out there is mostly anecdotal, and nowhere near enough to convince me that saltwater fish are healthier at salinities lower than from where they came, except when treating for specific parasites.
