MIGRATION, HOMING AND NAVIGATION IN FISH Flashcards
Many fish undergo migrations. These can occur
within a single type of water (freshwater
only or seawater only) or between freshwater and marine environments
Eels spawn at sea and enter freshwater to feed
(catadromous life cycle).
Salmon and some trout show the reverse pattern, spawning in freshwater and feeding at sea
(anadromous life cycle)
Migration, homing and navigation in salmon-Salmon, both Atlantic and Pacific, spawn in small streams and feed at sea.
Nearly all (95%)
of the fish that survive return to the exact stream in which they hatched.
Young salmon, called parr, remain in their streams where they establish territories and feed
on insects. Some time after 18 months (2 years in Atlantic salmon),
the parr form large
shoals and swim downstream while undergoing a major physiological change called the
smolt transformation. This transformation, under the control of thyroid hormone, prepares
the fish for life at sea: they take on a silver colour and adjust their osmoregulation to cope
with sea water.
In the sea, the salmon grow rapidly and may migrate large distances (up to 5000 km from
the mouth of their natal river). After some years at sea (3 years for coho salmon) they begin
the migration back to their natal river to spawn. When they arrive at the coast they spend
some time preparing for the spawning run up river:
- osmoregulation readjusted to cope with freshwater
- gonads increase in mass (up to 50% body weight)
- stop feeding, mobilise fat reserves
Further changes (development of secondary sexual characteristics) occur as the fish migrate
upstream.
How do salmon find their way back to the stream in which they hatched?
Return to the natal stream requires (1) open sea navigation to coast followed by (2) homing
to the stream in which they hatched
(1) Open sea navigation in salmon
While migrating between the coast and feeding grounds, salmon probably use sun position,
polarised light patterns and the earth’s magnetic field.
(2) Mechanism of homing in salmon
(Homing refers to the return by salmon to the exact stream in which they hatched)
Salmon learn to recognise their natal river based on smell
Hasler’s olfactory imprinting hypothesis proposes that during a sensitive period in the
smolt transformation salmon become imprinted to the smell of their home river and use the
learnt odour pattern to recognise the river when they return as adults.
This olfactory imprinting hypothesis requires that
- every stream has a characteristic and persistent odour;
- salmon are able to discriminate between the odours of different streams
- salmon remember the odour when they return after their time at sea.
What is the source of distinctive odours?
There are two hypotheses as to the source of the odours: (1) rocks, soil and plants in and
around the stream; (2) pheromones, mucus or faeces from closely related salmon in the
stream. Salmon can discriminate their own population and even siblings on the basis of
scent and are attracted to water bearing the scent of their own population. However,
although the smell of other salmon may contribute to the odour bouquet of a stream it is not
the most important element
Sequential imprinting
When swimming upstream, salmon may have to choose which tributary to take at many branch points along the way. When they come to a fork in the river they may swim back and forth across the confluence before making a choice. If they choose the wrong branch they lose the correct scent and drift back down to the fork where they normally make the right
choice.
This suggests
This suggests that a series of odour bouquets characteristic of the different parts of the river
is learnt in sequence as the smolts migrate downstream and followed in reverse order as the
adults return upstream (the sequential imprinting hypothesis).
Role of hormones in migration and imprinting
Thyroid hormone increases 5-10 fold during the smolt transformation. This appears to be
induced by both developmental processes and environmental factors (new environments,
changes in water temperature or flow rate, photoperiod and lunar phase). Elevated thyroid
hormone initiates migration downstream, exposing the salmon to new environments which
further increases hormone level. The increase in thyroid hormone level as the salmon move
downstream increases the tendency to learn local odour features by imprinting.
