eral growth rate, rather than any sudden metamorphic transformation. HOAR
(1951) found that the branchial 'chloride secreting cells' in Pacific salmon
underwent a period of rapid development before migration. This is consistent with
the recent observations of CONTE and LIN (1967), who found an increased rate
of branchial cell renewal during salt water adaptation in young salmonids (see page
221). The levels of N a-K activated ATPase in the gills of developing salmonids
has not been reported, but such measurements would be most interesting.
Transformation of the salmonid parr into a smolt, prior to its seaward migra -
tion, is associated with many body changes, including endocrine ones. Thus, there
is an increased activity of the thyroid gland (FONTAINE, 1954; CONTE and WAGNER,
1965); the 17-hydroxysteroids in the plasma have an elevated concentration (FONTAINE and HATEY, 1954) and morphological changes take place in the hypothalamohypophysial tract (ARVY, FONTAINE, and GABE, 1959). The ability to osmoregulate
in sea-water, however, does not reflect any single factor. As HOAR (1963) has
warned, any effects of endocrines are probably rather general and it is unlikely that
there is any specific hormone concerned with triggering such migratory behaviour
and the associated adaptations to life in the sea.
The population of steelhead trout studied by CONTE and WAGNER (1965) attain the ability to regulate in sea-water in March, but if they fail to migrate at this
time, they lose their ability to osmoregulate in sea-water and this does not return
until the following year. Pacific salmon, on the other hand, retain their ability to
adapt to sea-w ater (CONTE et al., 1966). Why such an ability should be kept by one
salmonid and not another is unknown, but such cyclical changes in the ability to
withstand transfers from fresh water to sea-water are noteworthy. On their terminal
migration to breed in fresh water lampreys and salmon lose their ability to regulate
in sea-water, the physiological condition of the fish apparently being inadequate
to such a task . Clearly the ability, and predisposition, to osmoregulate in either
fresh water or sea-water is at least partly subject to physiological changes which
could involve the endocrine system.
As described above, migration into the sea and the ability to osmoregulate in
such a solution may temporarily coincide, but these two factors are evidently not
causally related. Thus physiological and endocrine changes that are associated with
migration of fish do not necessarily directly influence osmoregulation. The increased th yroid activ ity that occurs in salmonids prior to migration cannot be
strictly correlated with their ability to osmoregulate in the sea (D . SMITH, 1956;
CONTE and WAGNER, 1965). The starry flounder, Platichthys stellatus, has an increased rate of oxygen consumption wh en it is placed in sea-water and this may
be related to the added osmoregulatory needs of such fish (HICKMAN, 1959). The
thyroid gland of the flounder also has an increased activ ity at this time, but the relationship of th is to oxygen consumption (or osmoregulation) is not clear, as attempts
to demonstrate such a causal relationship have not usually been successful. Removal
of the thyroid gland thus, does not alter the metabolic rate in the parrot fish
(MATTY, 1957). Changes in the hypothalamo-hypophysial tract of salmonids during their transformation to a smolt most probably occur subsequently to the ability to osmoregulate in sea-water.
Numerous attempts have been made to demonstrate changes in the ability of
fish to adapt to altered salinity by injecting them with various thyroid hormone
230
(1951) found that the branchial 'chloride secreting cells' in Pacific salmon
underwent a period of rapid development before migration. This is consistent with
the recent observations of CONTE and LIN (1967), who found an increased rate
of branchial cell renewal during salt water adaptation in young salmonids (see page
221). The levels of N a-K activated ATPase in the gills of developing salmonids
has not been reported, but such measurements would be most interesting.
Transformation of the salmonid parr into a smolt, prior to its seaward migra -
tion, is associated with many body changes, including endocrine ones. Thus, there
is an increased activity of the thyroid gland (FONTAINE, 1954; CONTE and WAGNER,
1965); the 17-hydroxysteroids in the plasma have an elevated concentration (FONTAINE and HATEY, 1954) and morphological changes take place in the hypothalamohypophysial tract (ARVY, FONTAINE, and GABE, 1959). The ability to osmoregulate
in sea-water, however, does not reflect any single factor. As HOAR (1963) has
warned, any effects of endocrines are probably rather general and it is unlikely that
there is any specific hormone concerned with triggering such migratory behaviour
and the associated adaptations to life in the sea.
The population of steelhead trout studied by CONTE and WAGNER (1965) attain the ability to regulate in sea-water in March, but if they fail to migrate at this
time, they lose their ability to osmoregulate in sea-water and this does not return
until the following year. Pacific salmon, on the other hand, retain their ability to
adapt to sea-w ater (CONTE et al., 1966). Why such an ability should be kept by one
salmonid and not another is unknown, but such cyclical changes in the ability to
withstand transfers from fresh water to sea-water are noteworthy. On their terminal
migration to breed in fresh water lampreys and salmon lose their ability to regulate
in sea-water, the physiological condition of the fish apparently being inadequate
to such a task . Clearly the ability, and predisposition, to osmoregulate in either
fresh water or sea-water is at least partly subject to physiological changes which
could involve the endocrine system.
As described above, migration into the sea and the ability to osmoregulate in
such a solution may temporarily coincide, but these two factors are evidently not
causally related. Thus physiological and endocrine changes that are associated with
migration of fish do not necessarily directly influence osmoregulation. The increased th yroid activ ity that occurs in salmonids prior to migration cannot be
strictly correlated with their ability to osmoregulate in the sea (D . SMITH, 1956;
CONTE and WAGNER, 1965). The starry flounder, Platichthys stellatus, has an increased rate of oxygen consumption wh en it is placed in sea-water and this may
be related to the added osmoregulatory needs of such fish (HICKMAN, 1959). The
thyroid gland of the flounder also has an increased activ ity at this time, but the relationship of th is to oxygen consumption (or osmoregulation) is not clear, as attempts
to demonstrate such a causal relationship have not usually been successful. Removal
of the thyroid gland thus, does not alter the metabolic rate in the parrot fish
(MATTY, 1957). Changes in the hypothalamo-hypophysial tract of salmonids during their transformation to a smolt most probably occur subsequently to the ability to osmoregulate in sea-water.
Numerous attempts have been made to demonstrate changes in the ability of
fish to adapt to altered salinity by injecting them with various thyroid hormone
230