Why a Freshwater Fish Cannot Survive in Saltwater

Freshwater fish are fish that spend some or all of their lives in fresh water, with a salinity of less than 0.05%. Freshwater fish is hypotonic to saltwater. Therefore, they have low ion concentration within their body cells than saltwater. When they move saltwater, body water of freshwater fish moves out of the body, making the fish dehydrated and causing their death. Therefore, osmolality difference is the main reason why a freshwater fish cannot survive in saltwater.

Key Areas Covered

1. What is Osmolality
     – Definition, Facts, Tonicity
2. Why a Freshwater Fish Cannot Survive in Saltwater
     – Osmosis Path

Key Terms: Hypertonic Solutions, Hypotonic Solutions, Osmolality, Osmosis, Saltwater, Tonicity

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What is Osmolality

Osmolality is the measure of the solute concentration of a particular solution. Plasma osmolality is the measure of body’s electrolyte-water balance. It is proportional to the number of particles per kilogram of solvent. Therefore, the solutions with different osmolality have different ion concentrations. The passive diffusion of water molecules occurs between solutions with different osmolality, across a semi-permeable membrane. This is known as osmosis. Osmosis is shown in figure 1.

Why a Freshwater Fish Cannot Survive in Saltwater_Figure 2

Figure 1: Osmosis

Solutions with high ion concentration are known as hypertonic solutions while solutions with low ion concentrations are known as hypotonic solutions. Water moves from hypotonic solutions to hypertonic solutions. If the osmolality of two solutions is similar, they are known as isotonic solutions. The effective osmotic pressure gradient is known as tonicity.

Why a Freshwater Fish Cannot Survive in Saltwater

Freshwater fish are isotonic to freshwater. This means their body cells contain similar concentrations of ions as freshwater. However, saltwater contains a high concentration of ions than freshwater. Hence, the cytoplasm of the cells of freshwater fish’s body is hypotonic to the saltwater. Then, water from the cytoplasm moves into the saltwater through the plasma membrane. This process occurs until the ion concentrations of the cytoplasm, and the ion concentration of saltwater become equal. Therefore, freshwater fish lose their body water to the saltwater. This dehydrates the freshwater fish in saltwater. Hence, they may eventually die. Osmotic flow in hypertonic solutions is shown in figure 2.

Why a Freshwater Fish Cannot Survive in Saltwater

Figure 2: Osmotic Flow in Hypertonic Solutions

This is true for saltwater fish in the freshwater as well. Since saltwater contains high ion concentrations, the bodies of the saltwater fish also contain high ion concentrations. When a saltwater fish are thrown into freshwater, the body of the saltwater fish is hypertonic to the freshwater. Hence, water moves into the body of saltwater fish through osmosis, swelling the saltwater fish.

However, some fish are euryhaline, i.e., they are adapted to live in both freshwater and saltwater. They have unique osmoregulation features that enable them to survive in different salinities. 


Freshwater fish is hypotonic to the saltwater. Hence, bodily water moves out when they are thrown into the saltwater. They become dehydrated and eventually die in the saltwater.


1. Osmosis. BioNinja, .

Image Courtesy:

1. “Osmosis experiment” By Rlawson at English Wikibooks via
2. “Blausen 0683 OsmoticFlow Hypertonic” By Blausen.com staff (2014). “Medical gallery of Blausen Medical 2014″. WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. – Own work via

About the Author: Lakna

Lakna, a graduate in Molecular Biology & Biochemistry, is a Molecular Biologist and has a broad and keen interest in the discovery of nature related things

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