The "homing" hypothesis, according to which
young migrating salmon (smolts) infallibly return as adults
to the river of their birth in order to repro-duce, often
serves as a basis for dynamic models of Atlantic salmon
populations. Researchers have shown that the instinct
of salmon to return to their river of origin may not be
as strict in the case of geographically neighbouring rivers.
This may have major consequences with respect to biology
and species evolution, and also in terms of population
dynamics and fishery management.
Under the homing hypothesis, the functioning of Atlantic
salmon populations (Salmo salar) can be conceived separately
for each river. Models are often used for population management
purposes, notably to determine the limits for line fishing
in each water course.
However, some individuals may not follow the homing rule,
when rivers are geographically close and have similar
environmental, geophysical, chemical characteristics,
etc. Exchanges of salmon may thus occur between each population
when adults return to the spawning grounds. The flux of
individuals between water courses may indeed be significant
in many small coastal water systems of Europe. These small
populations, which interact by exchanging individuals,
constitute a meta-population.
30% of adults lose their memory
Researchers applied this model to the hydrographical
network comprising two rivers, the Sée and the
Sélune (Lower Normandy), which have a common estuary
in the Baie du Mont St Michel. They used a chronological
series of very detailed demographic data, obtained by
following the migrations of adults and juveniles in the
Oir, an affluent of the Sélune, as well as a series
of captures by line fishing in the Sée and Sélune.
These data made it possible to estimate exchanges within
this hydrographic network. Their estimates indicated that
on average, more than 30% of adult fish had forgotten
in which branch of the network they had been born (Sée
or Sélune). In addition, because of the higher
production capacity for juveniles in the Sée and
a limited capacity in the Sélune, despite a larger
drainage basin, these exchanges were not balanced; the
Sélune received many more fish coming from the
Sée than those moving in the opposite direction.
As for the management of salmon stocks in rivers, this
work shows in particular that spatial approaches should
no longer be envisaged at the level of a single drainage
basin but at a much larger scale, thus requiring a review
of the fishing quotas defined for each river.
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A new model for population dynamics has been developed
by researchers, and takes account of exchanges of individuals
between neighbouring water courses when salmon return
to fresh water. It enables the testing of numerous hypotheses
be-tween two extremes: strict homing and a total loss
of memory concerning the site of birth by all fish in
the network, corre-sponding to a totally random distribution
at their return. The model proposed corresponds to a ramified
hydrographic network. Each branch of the network is characterised
by two variables: the sur-face area of habitat favourable
for the production of young salmon and the ac-commodation
capacity, defined as the number of smolts produced per
unit of favourable habitat surface area. The model describes
the distribution of adults in different branches of the
network when they return to the hydrographic network to
reproduce.
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