Four fish species recovering in Lake IJsselmeer
- JJJ (Joey) Volwater
- fish ecologist
After years of decline, pikeperch, perch, roach and bream are once again showing clear signs of recovery in Lake IJsselmeer. An analysis of more than thirty years of monitoring data suggests that the fisheries management measures introduced in 2014 have played an important role in this recovery. The results were recently published in the scientific journal Fisheries Management and Ecology.
Around 2012, the stocks of these four commercially important fish species reached a historic low. To prevent further decline, the Dutch government implemented major measures in 2014 to reduce fishing pressure, including substantial restrictions on gillnet and seine fisheries. Researchers subsequently analysed more than thirty years of data from annual fish monitoring programs in Lake IJsselmeer and Lake Markermeer to examine how the populations developed over time.
More and larger fish
The analyses show that the decline of all four fish species has been halted and that their populations are recovering. Pikeperch in particular showed a strong increase. The spawning stock of this species increased more than tenfold between 2014 and 2024. The number of larger fish also increased across all four species.
“Length based indicators are a good index of the age and size structure of a population. The fact that we are now seeing more large fish again is therefore an important sign that these fish stocks are recovering,” says Joey Volwater, researcher at Wageningen Marine Research.
According to the researchers, the results indicate that the reduction in fishing pressure is the main explanation for the recovery. Changes in water temperature and nutrient levels in the IJsselmeer appeared to play only a limited role in the analyses.
Notably, not only did the fish stocks recover, commercial catches also increased over time. Pikeperch catches in particular showed a strong and sustained increase.
Future developments
The researchers emphasise that the development of the fish stocks will continue to require attention. Factors such as climate change, changes in the ecosystem and the introduction of new species may also affect pikeperch, perch, roach and bream in the future.
“In a rapidly changing environment, effective monitoring of fish stocks is essential for an adaptive management and should be continued for a sustainable management of the commercial fish stocks,” says Joey Volwater.
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