I can’t put it much better than the article, Marine reserve sets new standard for recovery:

No-fishing reserves can restore marine ecosystems better than previously thought and can turn a heavily degraded site into an international model for conservation, according to a decade-long study led by researchers at the Scripps Institution of Oceanography in La Jolla. The report showed the amount of fish in an undersea wildlife park near the southern tip of Baja California soared 463 percent between 1999 and 2009.