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TOSHIAKI J. H A M
Ictalurus) . Many theories have been proposed to explain the mechanisms
of migration of anadromous salmon to their spawning grounds (Jones,
1968). It has been suggested that the orientation is mediated through
olfaction, and reference has been made to a home-stream substance; some
investigators have opposed these views (Ramsay and Hasler, 1961).
Since the problems pertaining to the orientation of migrating salmon are
dealt with in Volume VI (also see Hara, 1970), only current topics will
be mentioned here.
Spontaneous electrical potentials ( EEG ) were recorded from several
different parts of the brains of adult spawning salmon (Oncorhynchus
tschawytscha and 0. kisutch) when they arrived at their home pond
(Hara et al., 1965). Activities in the olfactory bulb and in the posterior
cerebellum consistently had a much higher amplitude than those of other
parts; amplitudes of potentials in the optic lobes were especially low;
in some fish the optic lobes were electrically “silent.” In contrast to adult
salmon, young salmon exhibited high activities in the olfactory bulb and
in the optic lobe. Cerebellar electrical activity in these younger fish was
not yet developed. Spontaneous electrical potential records were also
made from brains of adult, nonmigratory rainbow trout, Salmo gairdnerri,
and of goldfish, Carassius auratus. In both of these nonmigratory species,
EEG activity in the olfactory bulbs was relatively low while that of the
optic lobes was much higher than in adult spawning salmon.
Infusion of “home water” into the nasal cavity of migrating adult
salmon produced a clear stimulation in EEG patterns recorded from the
olfactory bulb; various natural waters from other nearby sources produced
virtually no response ( Fig. 9 ) . These findings suggest that olfaction is an
important factor in guidance during the final phases of homeward migration of salmon and that such olfactory discrimination occurs at the level
of either the olfactory bulbs or the olfactory epithelium. However, there
remains the possibility that certain nonspecific odorous substances, such
as foods, were merely in higher concentration in the home water than in
any other water used.
Apparent specificity of the response to home water was confirmed
by treating salmon from three different termini of migration with a series
of natural waters (Ueda et al., 1967; Oshima et al., 1969~). The high
amplitude EEG response of characteristic pattern recorded from the
olfactory bulb by infusion with home water is specific; little or no
response can be evoked by water from spawning sites of other groups of
breeding salmon (Table I). This implies that each spawning area has its
own specific stimulant or specific combination of stimulants recognized
and responded to by the anadromous salmon. Furthermore, weaker but
definite responses could be evoked by waters (1) traversed by the salmon
migrating toward the spawning site, ( 2 ) from the bypassed branch of a
TOSHIAKI J. H A M
Ictalurus) . Many theories have been proposed to explain the mechanisms
of migration of anadromous salmon to their spawning grounds (Jones,
1968). It has been suggested that the orientation is mediated through
olfaction, and reference has been made to a home-stream substance; some
investigators have opposed these views (Ramsay and Hasler, 1961).
Since the problems pertaining to the orientation of migrating salmon are
dealt with in Volume VI (also see Hara, 1970), only current topics will
be mentioned here.
Spontaneous electrical potentials ( EEG ) were recorded from several
different parts of the brains of adult spawning salmon (Oncorhynchus
tschawytscha and 0. kisutch) when they arrived at their home pond
(Hara et al., 1965). Activities in the olfactory bulb and in the posterior
cerebellum consistently had a much higher amplitude than those of other
parts; amplitudes of potentials in the optic lobes were especially low;
in some fish the optic lobes were electrically “silent.” In contrast to adult
salmon, young salmon exhibited high activities in the olfactory bulb and
in the optic lobe. Cerebellar electrical activity in these younger fish was
not yet developed. Spontaneous electrical potential records were also
made from brains of adult, nonmigratory rainbow trout, Salmo gairdnerri,
and of goldfish, Carassius auratus. In both of these nonmigratory species,
EEG activity in the olfactory bulbs was relatively low while that of the
optic lobes was much higher than in adult spawning salmon.
Infusion of “home water” into the nasal cavity of migrating adult
salmon produced a clear stimulation in EEG patterns recorded from the
olfactory bulb; various natural waters from other nearby sources produced
virtually no response ( Fig. 9 ) . These findings suggest that olfaction is an
important factor in guidance during the final phases of homeward migration of salmon and that such olfactory discrimination occurs at the level
of either the olfactory bulbs or the olfactory epithelium. However, there
remains the possibility that certain nonspecific odorous substances, such
as foods, were merely in higher concentration in the home water than in
any other water used.
Apparent specificity of the response to home water was confirmed
by treating salmon from three different termini of migration with a series
of natural waters (Ueda et al., 1967; Oshima et al., 1969~). The high
amplitude EEG response of characteristic pattern recorded from the
olfactory bulb by infusion with home water is specific; little or no
response can be evoked by water from spawning sites of other groups of
breeding salmon (Table I). This implies that each spawning area has its
own specific stimulant or specific combination of stimulants recognized
and responded to by the anadromous salmon. Furthermore, weaker but
definite responses could be evoked by waters (1) traversed by the salmon
migrating toward the spawning site, ( 2 ) from the bypassed branch of a