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Oh electricity, such a wild beast. If we only could find a way to turn it off again after we've let it go.

Of course they could turn it off, but then the army of carp just waiting outside the parasitic barrier would take its chance and colonise the lake. Hence, no turn-off, at least not for both barriers at the same time.

Clever girls.

They just have multiple barriers with sufficient separation for rescue operations... this would allow them to turn up to n-1 barriers off.

Then it would function like an [air]lock :-(

Open the outer door, enter the middle area, wait for the inner door. (lather, rinse, repeat for n > 2)

If the electrodes were controllable at a sufficiently fine granularity, they could be activated in a sort of creeping barrage:

http://en.wikipedia.org/wiki/Barrage_%28artillery%29#Moving_...

So there would be a number of active zones at a time, each one slowly moving downriver, pushing back any invading carp in front of them.

You could switch off the electrodes in any section of the river, and then when you switch them back on again later, the pattern sweeps any occupying invaders back out.

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