Monitoring growth of whitefish (Coregonus cf. suidteri) in Lake Hallwil

Many lakes of the pre-Alpine region suffered from severe eutrophication
that affected the natural reproduction of whitefish (Coregonus spp.) and
necessitated large-scale supportive breeding programs. With the advanced
reoligotrophication, it is now important to evaluate the relevance of
continued artificial breeding for population dynamics. We focused on a
whitefish population of a lake that has reached phase III of the
reoligotrophication, i.e., lake biomass production is declining since 2012
in response to low phosphorus concentrations. We show that most eggs are
naturally spawned, the observed oxygen concentrations would again support
embryo development at all depths, and ready-to-hatch embryos can indeed be
found on spawning grounds. We marked all hatchery-produced eggs of the
2014 cohort with Alizarin red, stocked them as usual (at larval or early
juvenile stages), and recaptured them over a period of five years. Fish
were aged from yearly growth rings on scales, and otoliths were checked
for marks. We found 90.3% of the 2014 cohort to be hatchery-born. This
ratio did not decline with fish age. We also determined juvenile growth of
the cohorts 2012-2020 (based on the first annual ring on scales) and found
that stocking intensity predicted juvenile growth (r2 = 0.67). This strong
density dependence suggest that stocking has largely determined population
size over the first nine years of reoligo­trophication phase III. We
conclude that large areas of spawning grounds allow again for successful
embryogenesis, that large quantities of eggs are naturally spawned, but
that natural recruitment is significantly reduced by ecological or
evolutionary factors, e.g., competition with hatchery-born fish,
desynchronization of trophic interactions, or long-term effects of
fishing- or hatchery-induced evolution