"It out-competes our native fish" says Dave Mee, from the Environment Agency.
"Water becomes totally dominated by these small fish."
"They're small, they're a nuisance and they're not viable for anglers to fish for, so they're not a targeted species".
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Sunny spells during the evening with isolated showers easing away, although it will cool off quite quickly.
Welsh soldiers have joined hundreds of others from across the country training in Norfolk.
It'll be a fine day for most, with some good spells of sunshine and light winds, giving a pleasantly warm feel in the sunny spells.