Matura paper: trout in Lake Biel
« Projet de protection et renforcement de la population de truite dans le lac de Bienne »: DNA analysis with Eawag, a field study on trout egg survival, and a ten-year plan to bring the lake trout back.
- fewer trout caught in Switzerland
- 42%
- rivers in my egg-survival field study
- 3
- restoration projects compared
- 5
- fry per year for ten years: my proposal
- 10 × 10,000
The question
“We used to catch so many more trout in the seventies.” Anyone who fishes on Lake Biel hears this sooner or later. Catching a lake trout there has become a rare event, but it still happens, which means the population is not gone. My paper asks why the trout have declined and what concrete steps could help the lake trout of Lake Biel recover.
What I did
- Background research: Swiss catch statistics, the biology of brown and lake trout, how they reproduce, and what they need to survive: cold, oxygen-rich water, loose gravel and the right current.
- DNA analysis at Eawag: I extracted DNA from river trout in the tributaries of Lake Biel and compared genetic markers (microsatellites) to see which streams hold distinct populations, and therefore which streams the lake trout most likely come from.
- Egg-survival field study with the Maison de la Rivière in three rivers in Valais: tubes with 100 trout eggs each had been buried in the riverbed. We dug them up, counted the surviving eggs and measured oxygen and fine sediment.
- Comparison of five restoration projects in Switzerland, Germany, Scotland, Australia and North America, to see what actually worked.
Results
The DNA analysis showed one clearly separate population, in the Chürzigraben. The other streams overlap strongly. This can mean several small populations, or the effect of decades of stocking with fish that do not return to “their” stream.
The field study showed how much the riverbed matters:
| River | Egg mortality | Oxygen | Fine sediment |
|---|---|---|---|
| Raspille | 1% | good | low |
| Morge | 50% | very good | medium |
| Vispa | 100% | poor | high |
Fine sediment clogs the gravel and suffocates the eggs. Studies report losses of up to 86% from this alone.
My proposal for Lake Biel
- Aerate the deep parts of the lake in late summer, when oxygen below 60 m drops to levels that are dangerous for fish.
- Protect lake trout from fishing for a while, as Switzerland already does for eels, so natural reproduction can recover.
- Breed young fish only from local spawners, to keep the local genetics.
- Collect more DNA samples with the help of anglers (a tiny fin clip), rewarded with a discount on the next fishing licence.
- Release 10,000 fry per year for ten years in the streams that genetics point to, using spawning tubes in the river so the fish imprint on their home stream.
- Open the way to the spawning grounds: add shelter to the canalised Suze in Biel and modernise the old fish ladder at Port, using the new one at Hagneck as a model.
What I learned
My plan changed twice. I first wanted to study microplastics, but nobody could provide the data. Then the detailed spawning data wasn’t available to me. Adapting the question without losing the goal was probably the most useful lesson. Next time I would also analyse the water quality of every tributary and take DNA samples from lake trout caught in Lake Biel itself.
The full paper is in French. If you’re interested, get in touch.