A REALLY BIG one.
Just kidding, there must be a workable compromise. Here in the states they use rubber bullets for crowd control. How about giving them non-fatal hints?
Incarceration would allow tourism AND the salmon to thrive. Plus they'd get free food for as long as they wanted. Once the runs subside, they could be released. Hopefully like jailbirds they would fly away quickly and be conditioned not to return!