A salmon fish moves from freshwater to the ocean at different times of its life.

Question

A salmon fish moves from freshwater to the ocean at different times of its life. Despite moving, from sea water to fresh water it maintains its blood osmolality at 350mOsm. Explain the problems this fish faces in maintaining body water and solute balance in each environment. (Group A) Describe the most likely mechanisms by which the fish maintains its water and solute balance in fresh water. (Group A) Describe the most likely mechanisms by which the fish maintains its water and solute balance in sea water. (Group B) As the salmon move from freshwater to sea water they encounter water of varying Explain how its water and solute balance problems would be altered during its (Group B) What hormonal and morphological changes occur when the salmon migrates from water to sea water and compare these to when the salmon moves from the ocean fresh water. (Group A)

Explanation / Answer

The transition by salmon from fresh water to sea water is best studied of all fish migrations and the process is known as smolting where the nutrients are rich in ocean compared to fresh water which are limited. The smoltification process is stressful and risky where the fish undergoes physiological, morphological, hormonal and behavioural changes and the mortality rate is high. Hormones such as growth hormones, thyroxine, cortisol and other hormones increase during smoltification and contribute to the development of gill chloride cells which helps in ion reguation in sea water. Importantly cortisol concentration increases widely which helps in physiological functions such as growth, stress removal, immune function and osmo regulation.

Salmon is an excellent osmoregulator. In oceans or sea water salmon is bathed in fluids which is three times concentrated as its body fluids which means it tends to lose water to the surroundings all the time. In short due to high NaCl concentrations salmon suufers dehydration and salt loading. But the case is reverse in fresh water which is devoid of salts, and the salmon is in such a medium where the outside concentration is less than its body fluids concentration and hence suffers from salt loss and water loading when moves back to fresh waters.

In sea water the Na+ Cl- ATPase pumps the Na+, Cl- ions out of the salmon body into sea water through the gills whereas in sea water the NaCl ATPase molecules pump the ions into the salmon from fresh water through the gills thus maintaining the osmotic balance.

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