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Which is Worse: Himalayan Balsam or Poachers?
Here is a short video that was part of a talk that Mike Clough invited me to give at the conference launching "INNSA" (http://www.innsa.org/). I've added some explanatory voiceover - in place of me talking and pointing at the screen in person :)
The clip explains why invasive plants that die back in winter cause huge increases in "wash-load" sediments (i.e. sediment that is washed into the river from the surrounding land - rather than derived from existing river-bed material).
When you realise that this can bury and suffocate spawning beds, the problem becomes much more obvious than the situation you see in high summer (when growth is lush). The loss of large areas of spawning beds has the potential to be far more serious in terms of reducing the population of fish in your river than occasional (and still serious) poaching. It is just that the fish impacted by silt accumulation never had a chance at life in order to become large enough to be visible victims (unlike poached adult fish).
I know lots of anglers who would be outraged by people illegally netting their rivers, but who "kind of know that invasive plants are not ideal" - but are maybe not especially worried about them (after all they look quite nice, and the bees quite like Himalayan balsam).
This is also the same issue as when fields are ploughed right up to the riverbank (especially when the field is ploughed down the valley slope (instead of along the slope, parallel to the river). These problems are often compounded by the alteration of the natural width of the river channel - which can cause the settling of more of this fine wash-load than would normally be the case. It is a serious issue, and it is time that many more of us get far more agitated about the huge stands of balsam and knotweed that line the banks of many of our rivers (especially on the spawning tributaries - and reaches where main-river spawning takes place).