Physiology of Finfish and Shellfish (2+1)

2.4.10. Preparation for life in a changed environment

One of the important changes is an alteration of osmoregulatory mechanisms which leads to an increase in the osmotic pressure of blood to suit marine conditions. Since the osmoregulatory process must function actively, the fishes utilize most of their energy for this purpose. Their downstream migration is strictly passive to conserve energy for the purpose. Preparation of marine life is apparent in the thickening of the layers of skin in eels which helps them to survive in the new marine environment. Also there is appreciable increase in the number of gill tissue cells which secrete chlorides. These cells multiply several fold a few days after the fish enters into seawater, enabling it to resist salt invasion that threatens it in the marine environment. Silver eel, in its catadromous migration shows a higher rate of intestinal water flux and a lower rate of sodium penetration through the gill than that observed in the yellow eel in freshwater. The silver eel, with its increased aptitude to excrete out salts, successfully regulates salinity. Water loss is compensated for by taking in water. Factors that control the rate of water intake in fish are unknown. Perhaps they just feel thirsty. With all these adaptive physiological changes, which are invariably controlled and monitored by endocrine gland, eels migrate towards the direction of increasing salinity and successfully spawn in the warmest part of the deeper oceans.

A profound modification of the physiological mechanism of osmoregulation is necessary depending on whether the salmon or hilsa is living in salt or freshwater. This modification appearing at a precise stage of life cycle is one of the determining factors for migration. While approaching coastal waters to begin its upstream migration, a clear increase in the concentration of salmon’s blood occurs, indicating a change in the osmoregulatory mechanism that is generally suitable for marine environment. It is possible same in the case of hilsa.

The young salmon, which is born in freshwater spends several years in these surroundings and undergoes important morphological, biochemical and physiological modifications before undertaking downstream migration to marine feeding grounds. The smolt develops a more pronounced tolerance to salinity than the parr because of the increase in the number of chloride cells in the gills. The activity of 2 enzymes, namely succino-degydrogenase and salt linked ATPase associated with chloride cells permits elimination of excess salt through digestive tract when fish enters the sea. Salmon thus gives hyperactivity to lose chlorine and sodium ions in marine water.

(Salmon eggs→ fry or alevins→ parr →smolt→ begins to migrate to the sea)

Hatch ↓freshwater sps