358
The 2012 study by the ONEMA (Olivier 2012 ) shows:- a numerical distribution dominated by three species: European
perch (42 %), roach (19.5 %), and Arctic char (18.2 %) - a more even weight distribution, with roach accounting
for the largest share of biomass (25 %), then perch
(19.2 %), Arctic char (16.4 %) and chub (15 %).
While the share of perch and char was as expected and in
line with numbers found in many deep lakes at high altitude,
the share of cyprinids is cause for concern. These fi sh fi nd cur-
rent thermal and trophic conditions in the lake favorable to
their development. Note too that these species are ecologically
very fl exible and can adapt to many mesological conditions.
Regarding the comparison of distributions between 2005 and
2012, while number of species increased slightly with the appear-
ance of rudd and tench, the number of individuals captured is
identical (234 in 2005 vs. 236 in 2012., Total weight, however,
increased signifi cantly (24975 g in 2005 vs. 33153 g in 2012).
The evolution in number and weight of different species
clearly shows cyprinids thriving at the expense of perch. The
situation of the Arctic char remains stable, but remember that
the char population is propped up by regular stocking. Note
too that no juvenile char were caught, which confi rms our
doubts over the natural reproductive capacity of this species.
21.4.3 Modifi cation of Trophic and Physico–
Chemical Conditions
Recent studies show a rapid evolution in the trophic level of the
lake in connection with agricultural practices (Ingé Conseil
2006 ). At the same time, there has a been a concomitant evolu-
tion in the fi sh population and a fairly signifi cant decline in
Arctic char sparking heavy doubt over the success of natural
reproduction. During winter, the weather conditions make it dif-
fi cult to follow the evolution of oxygenation in the breeding
zones, which is an essential component of breeding success for
Arctic char (Guillard 1992 ). During the breeding season, oxygen
measurements at spawning sites show signifi cant deoxygenation
of the water at greater depths (at the bottom and 1 m above) (Fig.
21.14 ). Given the mode of reproduction of Arctic char (eggs
deposited on the lake bottom), the chances of success seem low.y = 5472.5x - 256.73
R² = 0.7131010002000300040005000600070000 0.2 0.4 0.6 0.8 1 1.2Fig. 21.13 Relationship between
number of catchable fi sh from
stocking and CPUE values
0
0
-1
-2
-3
-4
-5
-6
-7
-8
-9
-10
-11
-12
-13
-14
-15
-16
-17
-17.32
T°c 02 mg/l4681012Fig. 21.14 Oxygen/temperature profi le of an S. alpinus breeding site
(East cliff) on January 21st, 2014F. D e s m o l l e s