Not as much as you think. There was a lot more sewage and pollution dumping into the rivers back in the haydays when you "could walk across the herring in the river". Hundreds of millions of dollars have been put into eliminating those discharges - there is more work to do but our environment and our waters are so much cleaner than back in the day.

I believe that commercial fishing efficiencies over time along with the restoration of predators (seals, striped bass, cormorants, eagles, ospreys, etc) are to blame. Spend an hour watching river cormorants eat - its amazing how much they catch and consume. There has been a man moratorium for a long time (maybe ten years now) and there hasn't been much change (don't know if its considered statistically insignificant - like the salmon effort). The increases in herring populations here in CT probably have more to do with the State's efforts to restore the population by transplanting fish than the man moratorium. They say it takes four years for the fry that make it out to sea to return and spawn, so you figure the first four or five years of the effort have little return on investment. Now is the time when we would be reaping the investment. Maybe the relaxation of regulation proves that. Still haven't heard for sure that the moratorium is over. If the moratorium is over and the population crashes further, I guess we will know that man is at fault.