Of the hydrographical factors, temperature and salinity have been found
to exert a profound influence on the fish throughout its life-cycle. The
tolerance range of the hydrographical factors of fishes is characteristic of the
species. Even within the same species, the degree of tolerance may vary
slightly, depending upon the age of fish; the degree of tolerance, however,
increases with the growth of fish. Chidambaram (1950), has shown that
sardine catches are maximum when the hydrographical factors are within
certain limits, during the season. Pradhan (1956) remarks that the entire
stock of mackerel, impounded near the mouth of Kali River is apt to die
from a sudden fall in salinity, due to the influx of freshwater from the river.
Similar observations were made by Hubbs (1948), Jenson (1930), Radovich
(1961), Tanoue and Enami (1954), Tonoue (1958 and 1960) and Walford
(1946). Hubbs {loc. cit.) showed that seasonal migrations of fish of the west
coast of United States are controlled by temperature
