The change in catches and mean size of one-year-old perch (Perca fluviatilis) in Lake Geneva was studied between 1957 and 2005, and related to trophic changes and
average temperature. The status of Lake Geneva, assessed on the basis of total phosphorus concentrations, switched from being oligotrophic before 1960 to eutrophic, with total phosphorus concentration reaching nearly 90 µg L−1 in 1976-1979. In response to management measures, the
trophic status then returned to mesotrophy in the early 2000s. Zooplankton
is the main food consumed by young perch, and quantities present (annual
biovolumes of zooplankton) were recorded throughout the study period. The
lake water temperature was also recorded. Data obtained from perch catches
and perch spawns enabled eleven strong cohorts of perch to be identified.
Strong cohorts dominated the stock for three years, until they were
massively caught by fishermen. Yields have fluctuated widely, increasing
rapidly until 1975, and subsequently decreasing, with an exceptional decline
between 1976 and 1981. During the period 1977-1981, unfavourable climatic
conditions in spring were probably the main cause of this fall in perch
yields. For the other periods, perch yields and the mean size of
one-year-old perch were significantly correlated with trophic parameters,
total phosphorus concentration and zooplankton biovolume. Correlations are
higher if only strong cohorts are taken into account. As a result of the
re-oligotrophication process, perch growth has been progressively reduced,
age at first maturity delayed and strong cohorts become less frequent; the
mean size of 0+ individuals in strong cohorts is significantly smaller than
in the other cohorts, suggesting a population density effect in the context
of a limited supply of zooplankton. During the study period, trophic changes
in Lake Geneva have had more impact on perch growth and yield than has
temperature, since no significant correlation could be detected between
water temperature and perch growth.