WASHINGTON — A magnet belt might spoil the effect of your new swimsuit, but, according to research by a US chemist, it could deter a shark from ruining it forever. Eric Stroud’s “Smart hooks” — magnetized fishing hooks, wrapped with a magnesium alloy foil — are being used by fishermen in several countries, and tests show they reduce the number of sharks accidentally killed by between 60 and 70 percent.