Sustainable management of leisure and professional fisheries at the ...

atch of scales from at least 100 individuals every
month. This sampling effort, which was particularly
comprehensive, could only be carried out with
the voluntary, diligent collaboration of the
professional fisherman.
Different pieces of knowledge were also monitored
over one or more of these years:
- The diet of the whitefish, by analysing the contents
of at least 20 stomachs per month in 2008 and
2009 and, on a more one-off basis, the diet of the
pikes, Arctic chars, roaches, perches and burbots
caught by the professional fisherman;
- The sex and level of maturity of all sizes of
whitefish, sampled thanks to scientific sampling
using nets with different mesh sizes, in September
2008 and July 2009;
- The growth of pikes (2008) and perches (2009),
based on scales and otoliths.
At the same time, other studies were also
conducted over this period. The ONEMA carried
out a sampling operation using electric fishing
in the lake’s littoral area, and in October 2010
repeated a scientific fishing sampling using nets
compliant with European standard EN 14757.
The monthly diet of whitefish in Lake Annecy in 2008
and 2009 split into fishes caught in the pelagic zone
and in the benthic zone (Source: GERDEAUX, 2010)
The existence of a substantial stock of scales
archived at the INRA station at Thonon-les-Bains
enabled a history of the growth of whitefish in
Lake Annecy to be drawn up. It is clear from this
that there has been a downward growth trend for
sizes at the ages of 2 and 3 years, even though
a shift has been observed for individuals born in
2005 and 2006. The overall trend may be related
to the oligotrophication of the lake. The resumption
of growth for those individuals born more recently
seems to be correlated with a great abundance in
2005 and 2006 of the zooplankton taxons, which
are the preferred food consumed by the whitefish
(Leptodora, Bytotrephes and Daphnia genuses).
Consequently, there was major progress in
terms of knowledge about the fish population of
Lake Annecy thanks to these 3 years’ worth of
investigations. Unfortunately, it is not possible to
make reference to all of the results obtained in this
document, and only the main conclusions relating
to whitefish are briefly presented.
In relation to the diet of the whitefish, the 2008
and 2009 results were relatively close, and
clearly demonstrated that the distribution of prey
greatly depends on the habitat frequented by
the individuals. Thus, the whitefish caught in the
pelagic zone (using a pelagic net) preferred to
feed on zooplankton, while the individuals caught
near the bottom and in the more littoral zone
(using a “spider” net) basically consumed benthic
invertebrates: Chironomidae nymphs and larvae,
and molluscs.
Total length back-calculated at 1 (“L1”), 2 (“L2”) and
3 years (“L3”) for Lake Annecy whitefish depending
on the year of birth of the fishes (abscissas axis).
(Source: GERDEAUX, 2010)
The shape of the distribution histograms according
to the length of the whitefish caught by the
professional fisherman changed over the 3 years
monitored. The mode increased in 2008 from
spring to summer; to the contrary it still remained
centred on 42 cm in 2009, then 39 cm in 2010.
Although you could distinguish the arrival of a
new generation at the end of the 2008 and 2009
seasons (with a peak of 39-40 cm), it is impossible
to distinguish any change in the distribution of the
sizes in 2010.