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

We still do not know what the consequences of
the fish population of the recent introduction of
a non-indigenous species of freshwater shrimp
(Limnomysis benedeni) will be. It was discovered in
2006, and has subsequently been found throughout
the lake in large numbers. The initial tests carried
out seem to show that this crustacean is only
accepted as prey by the juveniles of perch.
C. CHARACTERISTICS OF FISHERIES EXPLOITATION
(ECKMANN & RÖSCH, 1998; RÖSCH, PERSONAL COMMUNICATION;
ZINTZ ET AL., 2009; WWW.IBKF.ORG)
There is a long tradition of professional fishing on
Lake Constance; in the early 20th century there
were over 400 professionals operating on the lake.
The fishing statistics are based on the annual
reports of the IBKF; the international commission
for the fishing management of Lake Constance.
The fishing yields do not necessarily reflect
the biomass and the composition of the lake’s
populations, because only a few species of
commercial interest are actively exploited: whitefish
and perch and, to a lesser extent, eel, lake trout,
pike, and pikeperch. The other species such as
Cyprinidae (roach, bream, common dace, etc.) only
constitute ancillary catches.
Professional fishing statistics have been available
since 1909. Commercial exploitation can be
divided into 3 periods:
- 1909 to 1955, with the average results being
around 415 tonnes per year, with a maximum of
approximately 700 tonnes;
- In 1956, for the first time, catches exceeded
1,000 tonnes, and up until the late 1990s, they
never fell below 750 tonnes, with an average of
about 1,104 tonnes per year;
- Since the late 1990s, catches have dropped
sharply to around 600 to 800 tonnes per year.
Fishing statistics for Lake
Constance in tonnes caught per year
(Source: www.ibkf.org)
(Barsch = Perch;
Felchen = Whitefish;
Sonstige = other species)
For whitefish, the yields vary from 150 to 1,200
tonnes. A constant characteristic is the major
variability from one year to the next without any
obvious periodicity. The changes that occurred
immediately after 1956 were partly due to an
improvement in the efficiency of the nets due
to the use of nylon. Traditionally, whitefish nets
were made from cotton: back then, the Catches
Per Unit-Effort (CPUE) were much lower with this
material than with nylon. Before 1955, commercial
catches mainly consisted of whitefish, with the
other species constituting less than 30 % of the
catches. Due to the eutrophication of the lake, the
whitefish proportion then dropped sharply to the
extent that it amounted to less than 40% of the
total catch in some years. The stocks exploited
today are characterised by a return to the situation
that existed before the period of eutrophication,
as whitefish constitutes 75 to 80 % of the catches.
However, the quantities of all species caught are
steadily decreasing, as is the average size of the
catches. The quality found in the lake, with its
oligotrophic status, is the main reason for this.
One positive effect of this reoligotrophication that
relates to the Arctic char should be emphasised,
as the current yield is almost 12 tonnes per
year as opposed to a hundred or so kg in the
early 1990s. The real impacts of restocking are
however unknown.