What is the difference between fresh-water biology and marine biology?

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Fresh water biology is the study of aquatic organisms that occupy water that has 500 parts per million (ppm) of dissolved salts in it.  These organisms basically have a hypotonic system, meaning the salt content in their bodies and cells is lower than that in their salt water relatives.  Fresh water is .05% in terms of dissolved salts, while brackish water, an in-between version of fresh and salt water, has a range of .05% to 3%.  Salt water has 3% to 5% dissolved salts.  Anything above 5% dissolved salts is considered brine, which has a high salt content, and is used in the pickling and preservation processes.  Salt water biology, then, would be the study of aquatic organisms, both plant and animal, that occupy waters that possess a salinity of 3% to 5%.  These organisms have a hypertonic system, meaning there is a higher salt content within their cells and bodies than that of their fresh water relatives.  There are some species of fish, such as eels and salmon, that spawn in fresh water, then migrate to salt water, then migrate back to fresh water to spawn themselves.  This is achieved through the use of hormones, such as cortisol in the salmon.

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