360
- This tagging program will also allow us to assess the
annual growth of released individuals.
21.5.3 Implementation of Incubators
The goal is to determine whether environmental conditions
permit the normal egg development in char breeding
grounds.
Concentration of dissolved oxygen and the amount of
silt covering the eggs are key parameters for survival, as is
length of the development period. The longer the eggs take
to develop, the greater the chances that they will be
exposed to conditions adverse to their survival. The big-
gest diffi culty is to continuously monitor the physico-
chemical conditions of the Arctic char breeding sites
throughout the winter to determine whether conditions
remain favorable for hatching. We have therefore chosen
to monitor egg development via an immersion method.
Again, iceover of the lake makes it very diffi cult to moni-
tor physico–chemical conditions.
When catches of spawners are completed, reproduction
will be hosted at the nearby national salmon farm. There are
two windows in which when eggs can be manipulated for
immersion—two hours after laying, or at embryonic stage.
The embryonic stage is later (early February), so if the lake
freezes over, it will be impossible to immerse the eggs.
Immersion will therefore be effectuated immediately after
fertilization.
The experimental incubators are PVC boxes used in
salmon farming (Vibert boxes). The boxes have the openings
that help keep the eggs inside the incubator while maintain-
ing an exchange with the external environment, allowing the
fry to develop to the swimming stage. Side vents allow the
fry to swim out of the box. Each incubation line is located on
a known char breeding ground. There will be three incuba-
tion lines, and each line will have three levels of incubators
(bottom, 1 m above bottom, and 8 m above bottom) with
three incubators per level. Each incubator will be seeded
with 30 eggs. The lines will be plotted by GPS and equipped
with submerged buoys. The incubation line will be removed
as soon as the lake becomes accessible. This experimental
protocol will enable us to assess whether the char eggs can
still develop naturally, and at what depth. This experiment
must not be considered has a substitution of natural repro-
duction. During all this incubation period, six oxygen record-
ers will be immerged at 12,14,16,18 m depth on the bottom
and two others at 1 m(15 m) and 5 m (11 m) over the bottom
on the 16 m incubation line. One value of oxygen is record
every half hour for each recorder between December to May.
This monitoring will allow us to demonstrate the relation
between dissolved oxygen and success of incubation.
21.6 Conclusion
The piscicultural history of Pavin Lake is highly infl uenced
by man, fi rst by the initial introduction after 1859 of several
key species,trouts ( Salmo trutta ), common salmons ( Salmo
salar ), Heusch salmons ( Hucho hucho ), chars ( Salvelinus
Umbla ) and some coregons ( Coregonus Fera ) (See Sect.
1.4.2 ).The Arctic Char introduction has been the most suc-
cessful on the long term and this species became emblematic
throughout the twentieth century.
Over the last 20 years another marked change occurred in
the lake watershed where its groundwaters are coming from. It
is a shift of land use and agricultural practices, from extentive
pasture to intensive harvested pasture, requiring numerous
inputs of mineral and organic nutrients. In the mid-1990s the
increased occurrence of blooms of Anabaena spiroïdes raised
the concern of some limnologists, but the quantifi cation and
origin of the nutrient inputs to Pavin were only established in
2005 (IngéConseil 2006 ). The resulting modifi cation of the
fi sh population,which was studied at the same period, revealed
mass mortality of the crayfi sh population and a decrease of the
Arctic Char landings. This disruption in the lake ecology can
be multifactorial, inappropriate fi sh management, excessive
fi shing pressure, impact of additional fi sh species introduc-
tion, climate warming..., however we believe that trophic
state degradation by nutrient excess is the prominent factor.
In order to restore the original oligotrophic of Pavin Lake
which made it chosen as a typical oligotrophic lake in several
international projects in the 1960s (See Sect. 1.5.1 ), a reduc-
tion of nutrients inputs from agriculture practices is needed.
All local authorities together with the farmers agreed to
establish an operational land management within Pavin
watershed, named Territorial Contract including two steps:- Realize an initial survey of the situation with all the stake-
holders ( LMGE , ONEMA , IRSTEA , FDPPMA63, and
ATHOS ecological engineers), under the supervision of
the Auvergne Regional Park (PNRVA) and the fi nancial
support of the Loire-Bretagne Water Authority (AELB),
the Puy de Dôme department and the Auvergne region.
During 3 years the physico-chemical quality, the chloro-
phyll, the algal counts, the dissolved oxygen and tempera-
ture continuous monitoring by in situ probes should be
surveyed and complemented by the study of the Arctic
Char population. - Modify the present land use to gradually return to exten-
sive pasture without external nutrients inputs, by exchange
or purchase of the concerned agricultural plots.
The fi rst plot exchange have started but the land property
rights complexity for the 120 ha concerned make this second
point more slow and diffi cult than expected.F. D e s m o l l e s