I'm a little late to this, but The Economist published a great special report on the state of the world's oceans in late 2008. The series includes articles on declining fish stocks, ocean acidification, and promising ocean management techniques. The introductory article by John Grimond explains the problem quite well:
"Each of these phenomena would be bad enough on its own, but all appear to be linked, usually synergistically. Slaughter one species in the food web and you set off a chain of alterations above or below. Thus the near extinction of sea otters in the northern Pacific led to a proliferation of sea urchins, which then laid waste an entire kelp forest that had hitherto sustained its own ecosystem. If acidification kills tiny sea snails known as pteropods, as it is likely to, the Pacific salmon that feed upon these planktonic creatures may also die. Then other fish may move in, preventing the salmon from coming back, just as other species did when cod were all but fished out in Georges Bank, off New England."
The report doesn't paint a very pleasant picture - nor should it. At the risk of overwhelming the public with a cascade of complex problems, it is time for people to get serious about the problems faced by the ocean. Overfishing has become a commonly used word, yet politicians routinely ignore the recommendations of fisheries scientists because fishing restrictions would be unpopular. The problems caused by runoff from land are well-documented and have devastated once-productive regions, yet polluters protest the expense of preventing damage. What's really expensive is continuing to believe against all evidence that when it comes to the ocean we can toss in or take out whatever we please with no ramifications for human life.