Biology Reference
In-Depth Information
6.2.3
Olfactory system
Salmon have long been known to imprint (olfactory imprinting) and
home to natal stream odors. Olfactory imprinting in salmonids occurs
during the smolt stage. Evidence came from studies in which coho salmon,
brown trout (
Salmo trutta
) and steelhead trout (
Salmo gairdneri
) exposed to
synthetic chemicals, either morpholine or phenethyl alcohol, during the
smolt stage were attracted as adults to rivers scented with the chemicals
(Cooper et al., 1976; Scholz et al., 1976; Johnsen and Hasler, 1980; Hasler
and Scholz, 1983). If exposure to the chemicals occurs during the parr stage,
no imprinting is observed (Hasler and Scholz, 1983). A sensitive period
for olfactory imprinting (SPOI) has been identifi ed in Atlantic salmon
undergoing smoltifi cation (Morin et al., 1989a), using measurement of the
magnitude of a conditioned cardiac response to L-cysteine in fi sh during
six 8-d intervals (Morin et al., 1989b). They concluded that there are two
distinct optimal intervals for olfactory learning during smoltifi cation but
only one « sensitive » period for olfactory imprinting, occurring between
21 and 28 days after the onset of smoltifi cation induced in the laboratory;
fi sh tested for odor recognition exhibited a greater unconditioned cardiac
deceleration to L-cysteine if they had been exposed during the third age
interval of smoltifi cation.
During induced smoltifi cation (by a regime of increasing temperature
and photoperiod), two peaks of olfactory activity occurred: the fi rst peak
occurred at the beginning of smoltifi cation (the « acute phase ») coinciding
with the previously described sensitive period for olfactory imprinting; the
second peak occurring after the acute phase of smoltifi cation coincided with
a free thyroxine peak (Morin et al., 1994).
Studies in Atlantic salmon suggest a quadrupling of olfactory receptor
cell number, as well as specifi c changes in the relative composition of the
olfactory bulb neuropil during the parr-smolt transition (Bowers, 1988).
More extensive investigation of Chinook salmon (
O. tshawytscha
) confi rmed
these fi ndings suggesting growth in the input layer of the olfactory bulb
coincident with smolting (Jarrard, 1997).
6.2.4
Osmoregulatory organs
6.2.4.1
Gills
Apart from its role in respiration, nitrogenous excretion, thermal exchange
and mucus production, the gill plays a major role in ionic and osmotic
regulation through the action of chloride cells (mitochondria-rich cells
or ionocytes). These specifi c cells increase considerably in number and
activity in SW compared with FW fi sh (Conte and Lin, 1967) and during
