Probably worth reading to the end, that the problem seems to be exacerbated by a breakwater that's been built creating a catch basin. In any case it's not an indication that more harvest is needed to reduce the "waste". These systems have evolved to produce more eggs than are needed to provide insurance against unusual events that may kill off large amounts of the spawn. Our erroneous belief we can accuratly predict, and harvest all this excess capacity is primarily why fisheries always end up overharvesting. We cant predict it very well, miscalculations harm the stock for decades. And when some natural event happens that the excess was needed, the natural insurance policy was cancelled by our harvesting it.
Leave the herring alone, it's not just adult herring that feed salmon, eggs and larvae feed young salmon, and a whole host of other organisms essential to the food chain.