question: why can’t saltwater fishes survive in fresh water?
comment: Their blood and all their cells is naturally much saltier then fresh water fishes. Salt works kind of like a one way valve when letting water through membranes. So when there is lots of salt in the blood then the water will get through the gills into the blood but not back. So the fish is unable to get rid of the water in any way and it will just fill up with water and die. There are some fish that can adapt to both fresh and salt water if they go slowly enough. For example trout and salmon will swim between the sea and fresh water streams and lakes. This is because they are able to regulate their salt content much better then other fish.
passages: Other than their temperature sensitivity, tilapia exist in or can adapt to a very wide range of conditions. An extreme example is the Salton Sea, where tilapia introduced when the water was merely brackish now live in salt concentrations so high that other marine fish cannot survive.
  
  Tilapia are also known to be a mouth-breeding species, which means they carry the fertilized eggs and young fish in their mouths for several days after the yolk sac is absorbed.
  
  Section::::Species.
   Studies suggest that Florida manatees need access to fresh water for proper regulation of water and salts in their bodies.
   In nature, this species often travels to the limits of where salt water becomes fresh water, so they can be kept in fresh water for brief times. For long-term care, however, they should be kept in salt water. A salt concentration equal to 1/4th of ocean levels is suitable for younger fish, but adult fish spend most of their time in the ocean, so it is recommended to raise the salt level to 1/2 of ocean levels. Some also recommend using normal ocean water levels for adult fish. Suitable temperatures are 24-28 °C. A pH level of 8 is recommended.
  
  Section::::Ecology.
   The midwater fish have special adaptations to cope with these conditions—they are small, usually being under ; they have slow metabolisms and unspecialized diets, preferring to sit and wait for food rather than waste energy searching for it. They have elongated bodies with weak, watery muscles and skeletal structures. They often have extendable, hinged jaws with recurved teeth. Because of the sparse distribution and lack of light, finding a partner with which to breed is difficult, and many organisms are hermaphroditic.
   The smolt body chemistry changes, allowing them to live in salt water. While a few species of salmon remain in fresh water throughout their life cycle, the majority are anadromous and migrate to the ocean for maturation: in these species, smolts spend a portion of their out-migration time in brackish water, where their body chemistry becomes accustomed to osmoregulation in the ocean.
   Fish kills can also result from a dramatic or prolonged drop in air (and thus, water) temperature. This kind of fish kill is selective – usually the dead fish are species that cannot tolerate cold. This has been observed in cases where a fish native to a more tropical region has been introduced to cooler waters, such as the introduction of the tilapia to bodies of water in Florida. Native to Africa's Nile River, the tilapia stop feeding when water temperatures drop below and die when it reaches . Thus, tilapia that have survived and successfully reproduced in Florida are occasionally killed by a winter cold front.
   Many species of fish do reproduce in freshwater, but spend most of their adult lives in the sea. These are known as anadromous fish, and include, for instance, salmon, trout, sea lamprey and three-spined stickleback. Some other kinds of fish are, on the contrary, born in salt water, but live most of or parts of their adult lives in fresh water; for instance the eels. These are known as catadromous fish.
   Serious problems can occur when fish originally raised in ponds or aquaria are released into the wild. While tropical fish do not survive in temperate climates, they can thrive in waters that are similar to their native habitat. Non–native species that become established are called exotic species. Freshwater examples include various cichlids in Florida, goldfish in temperate waters, and South American suckermouth catfishes in warm waters around the world. Invasive species can seriously disrupt their new homes by preying on, or competing with, native species. Many marine fish have also been introduced into non-native waters, disrupting the local habitat.
   Species migrating between marine and fresh waters need adaptations for both environments; when in salt water they need to keep the bodily salt concentration on a level lower than the surroundings, and vice versa. Many species solve this problem by associating different habitats with different stages of life. Both eels, anadromous salmoniform fish and the sea lamprey have different tolerances in salinity in different stages of their lives.
  
  Section::::Classification in the United States.
   Fresh water creates a hypotonic environment for aquatic organisms. This is problematic for some organisms with pervious skins or with gill membranes, whose cell membranes may burst if excess water is not excreted. Some protists accomplish this using contractile vacuoles, while freshwater fish excrete excess water via the kidney. Although most aquatic organisms have a limited ability to regulate their osmotic balance and therefore can only live within a narrow range of salinity, diadromous fish have the ability to migrate between fresh water and saline water bodies. During these migrations they undergo changes to adapt to the surroundings of the changed salinities; these processes are hormonally controlled. The eel ("Anguilla anguilla") uses the hormone prolactin, while in salmon ("Salmo salar") the hormone cortisol plays a key role during this process.
   Marine teleosts also use gills to excrete electrolytes. The gills' large surface area tends to create a problem for fish that seek to regulate the osmolarity of their internal fluids. Saltwater is less dilute than these internal fluids, so saltwater fish lose large quantities of water osmotically through their gills. To regain the water, they drink large amounts of seawater and excrete the salt. Freshwater is more dilute than the internal fluids of fish, however, so freshwater fish gain water osmotically through their gills.
   Freshwater fish
  
  Freshwater fish are those that spend some or all of their lives in fresh water, such as rivers and lakes, with a salinity of less than 0.05%. These environments differ from marine conditions in many ways, the most obvious being the difference in levels of salinity. To survive fresh water, the fish need a range of physiological adaptations.
   Most forage fish are filter feeders. For example, the Atlantic menhaden, a type of herring, lives on plankton caught in midwater. Adult menhaden can filter up to four gallons of water a minute and play an important role in clarifying ocean water. They are also a natural check to the deadly red tide.
   The abiotic factors the Atlantic silverside needs to survive varies for populations of fish based on their geographical location. A rule of thumb for the species includes an average temperature of , a salt content of the water ranging from 0 to 37ppt (Tagatz and Dudley 1961), and a well-mixed body of water to prevent hypoxic conditions.
   Marine teleosts also use gills to excrete electrolytes. The gills' large surface area tends to create a problem for fish that seek to regulate the osmolarity of their internal fluids. Salt water is less dilute than these internal fluids, so saltwater fish lose large quantities of water osmotically through their gills. To regain the water, they drink large amounts of sea water and excrete the salt. Fresh water is more dilute than the internal fluids of fish, however, so freshwater fish gain water osmotically through their gills.
   Section::::Aquatic organisms.
  
  Water is a critical issue for the survival of all living organisms. Some can use salt water but many organisms including the great majority of higher plants and most mammals must have access to fresh water to live. Some terrestrial mammals, especially desert rodents, appear to survive without drinking, but they do generate water through the metabolism of cereal seeds, and they also have mechanisms to conserve water to the maximum degree.
   Section::::Habitats.
   Spotted seatrout populations in the coasted Everglades are declining. As the second most commonly caught species of fish in the Florida Bay, spotted seatrout comprise a large portion of the fishing industry and are integral to the ecosystem as well as surrounding economy. Water temperature of less than 80 degrees Fahrenheit and salinity levels below 37.5 parts per thousand (ppt) are ideal for seatrout spawning; however, water management stations in the Everglades and Florida Bay reported salinity levels of 64.4 ppt in July 2015 and recorded water temperatures of up to 92 degrees Fahrenheit. These environmental conditions are far from ideal for the seatrout and add additional difficulties for the survival of juveniles as well as important prey such as larval shrimp and small fish.
   Osmoregulators tightly regulate their body osmolarity, which always stays constant, and are more common in the animal kingdom. Osmoregulators actively control salt concentrations despite the salt concentrations in the environment. An example is freshwater fish. The gills actively uptake salt from the environment by the use of mitochondria-rich cells. Water will diffuse into the fish, so it excretes a very hypotonic (dilute) urine to expel all the excess water. A marine fish has an internal osmotic concentration lower than that of the surrounding seawater, so it tends to lose water and gain salt. It actively excretes salt out from the gills. Most fish are stenohaline, which means they are restricted to either salt or fresh water and cannot survive in water with a different salt concentration than they are adapted to. However, some fish show a tremendous ability to effectively osmoregulate across a broad range of salinities; fish with this ability are known as euryhaline species, e.g., salmon. Salmon has been observed to inhabit two utterly disparate environments — marine and fresh water — and it is inherent to adapt to both by bringing in behavioral and physiological modifications.
   During the initial dive in March 2005, one of the discoverers, Hubert Staudigel (a geologist at San Diego's Scripps Institution of Oceanography), commented "I suppose it's possible they migrate up the water column and feed in the water column and migrate back down to the cracks and crevices to hang out. But it seems odd that a deep-sea fish that would normally be experiencing 2- to 5-degree Celsius (35.6 degrees to 41 degrees Fahrenheit) water would be seeking out water that is warmer."
   Oceanic epipelagic fish can be true residents, partial residents, or accidental residents. True residents live their entire life in the open ocean. Only a few species are true residents, such as tuna, billfish, flying fish, sauries, pilotfish and remoras, dolphin, ocean sharks and ocean sunfish. Most of these species migrate back and forth across open oceans, rarely venturing over continental shelves. Some true residents associate with drifting jellyfish or seaweeds.
   Section::::Physiology.:Osmoregulation.
  
  Section::::Physiology.:Osmoregulation.:Environmental osmotic conditions.
  
  Southern bluefin tuna migrate between a variety of different ocean regions, however the osmotic conditions faced by the tuna stay relatively similar. This species of tuna inhabits ocean areas that are relatively high in salinity compared to the rest of the world’s oceans. Like other marine teleost fish, the southern bluefin tuna maintain a constant ion concentration in both their intracellular and extracellular fluids. This regulation of an internal ion concentration classifies southern bluefin tuna as osmoregulators.
   Osmoregulators tightly regulate their body osmolarity, which always stays constant, and are more common in the animal kingdom. Osmoregulators actively control salt concentrations despite the salt concentrations in the environment. An example is freshwater fish. The gills actively uptake salt from the environment by the use of mitochondria-rich cells. Water will diffuse into the fish, so it excretes a very hypotonic (dilute) urine to expel all the excess water. A marine fish has an internal osmotic concentration lower than that of the surrounding seawater, so it tends to lose water (to the more negative surroundings) and gain salt. It actively excretes salt out from the gills. Most fish are stenohaline, which means they are restricted to either salt or fresh water and cannot survive in water with a different salt concentration than they are adapted to. However, some fish show a tremendous ability to effectively osmoregulate across a broad range of salinities; fish with this ability are known as euryhaline species, e.g., salmon. Salmon has been observed to inhabit two utterly disparate environments — marine and fresh water — and it is inherent to adapt to both by bringing in behavioral and physiological modifications.
   Many pets have escaped or been released into the Everglades from the surrounding urban areas. Some find the conditions quite favorable and have established self-sustaining populations, competing for food and space with native animals. Many tropical fish have been released, but blue tilapias ("Oreochromis aureus") cause damage to shallow waterways by creating large nests and consuming aquatic plants that protect native young fish.
   Lotic systems typically connect to each other, forming a path to the ocean (spring → stream → river → ocean), and many fishes have life cycles that require stages in both fresh and salt water. Salmon, for example, are anadromous species that are born in freshwater but spend most of their adult life in the ocean, returning to fresh water only to spawn. Eels are catadromous species that do the opposite, living in freshwater as adults but migrating to the ocean to spawn.
  
raw_presuppositions: Saltwater fish can't survive in freshwater.
raw_corrections: Some saltwater fish can survive in freshwater if they adapt slowly.