Flow rate and temperature are amongst the most crucial
factors structuring the aquatic environment of running
water and the most likely to be affected by climate change.
These evolutions have an impact upon fish populations
because of their ecological demands: oxygen requirements,
tolerance of variations in temperature, etc. The climate
changes observed to date seem to have been sufficiently
minor not to cause any profound modifications, such as
species invasions or extinctions. Nevertheless, researchers
at INRA1 have already observed some changes in the Atlantic
salmon, such as a shorter life expectancy, the more rapid
renewal of populations or an increase in the number of
early-maturing males, leading to a reproduction strategy
which excludes the marine phase.
Evolving biological characteristics...
The biological characteristics of Atlantic salmon populations
in North-Western France were studied over a period of
more than 30 years (1972-2002) by researchers from INRA,
in collaboration with the Conseil Supérieur de
la Pêche (National Council for Fisheries) and the
fishing federations, based on a sample of approximately
21,000 individuals comprising 98% of adult salmons captured
by line fishing. Evolutions in this fish population were
analysed by grouping rivers in three regions: Lower Normandy,
Northern Brittany and Southern Brittany.
............................................
The Atlantic salmon (Salmo salar) is a mi-gratory fish
which breeds in fresh water and develops in the sea. The
female buries her eggs in river gravel. After one or two
years of life in fresh water, the young arising from these
eggs, or parrs, migrate towards the sea ("smolt"
stage). The marine phase lasts for 1 to 3 years before
the fish return to their original river to reproduce.
The Atlantic salmon is a stenothermal cold wa-ter fish
(which does not like temperature variations), highly sensitive
to water qual-ity. It continues to frequent some thirty
rivers in France.
............................................
Very marked similar trends were observed in the populations
of the three regions:
• a lowering in the age of individuals in fresh water,
with an increase in grilses (1 year at sea), a marked
reduction in spring salmon (two or more years at sea),
and the virtual disappearance of individuals living for
long periods in the sea (3 years or more),
• a strengthening of their semelparous nature (single
reproduction), linked to the very marked reduction in
the number of fish returning to rivers after a second
period at sea.
Female on its redd with a small male. Males maturing
early, as from the second year of life in fresh water,
can fertilise up to 65% of eggs.
In contrast, no changes in the size of fish was demonstrated.
Salmon in Lower Normandy were always larger than those
in Brittany, at an equivalent sea age. Today, the evolution
of these characteristics is resulting in a shorter life
expectancy for individuals and a more rapid renewal of
populations (one to two years). This evolution, which
is linked to changes in both the continental and marine
environment (temperature and eutrophication) appears to
reflect the adaptation of a species to an environment
which has become more unstable and unfavourable towards
the ecological requirements of Atlantic salmon.
... and modified reproduction strategies
The temperature range favourable to salmon spawning at
the end of autumn is between 3 and 12°C, at our latitudes.
In addition, mature females must be able to expel their
eggs within 8 to 10 days of ovulation, otherwise their
fecundity or the survival of eggs will be considerably
impaired. Over the past 20 years, the percentage of time
during which water temperatures exceeded the tolerable
threshold fixed at 11.5°C has risen from 4 to 11%.
Furthermore, daily maximum temperatures of 11.5°C
or higher, causing inhibiting discomfort or physiological
stress, are increasingly frequent, involving up to 60%
of days during the reproductive period. During such periods,
the females cannot reproduce, but the eggs continue to
mature. Thus, a reduction in the laying window may lead
to winters without effective reproduction if the water
temperature continues to rise. Such an eventuality may
endanger the population or even cause it to become extinct,
if the phenomenon continues for more than three consecutive
years.
While practically all females reproduce after marine migration
(1 or 2 years), a certain proportion of male juveniles,
or early-maturing males, become mature as from their first
year of life in fresh water, before leaving for the sea.
They participate in spawning alongside large anadromous
males (having lived at sea) and may fertilise up to 65%
of the eggs spawned, according to paternity analyses.
It seems at present that their numbers are rising and
that a reproduction strategy which excludes the phase
of marine growth is increasingly common. At the same time,
narrowing of the laying window may lead females to be
less demanding about their choice of males under favourable
thermal conditions, allowing the greater reproductive
success of these early-maturing males, with as yet unknown
consequences for the future of the population.
