A fact-finding tour of troubled waters. Marine scientist and first-time author Safina, founder of the Audubon Society's Living Oceans Program, ranges far afield to substantiate fears that something has gone badly wrong in the oceans. The fault, of course, lies with humans. A commission has determined, for instance, that the number of bluefin tuna in the north Atlantic has declined by 90 percent in recent years. Not every one agrees. ``Fishermen tell me,'' Safina adds, ``that the scientists grossly underestimate the numbers.'' Along the Grand Banks off Canada, the legendary great shoals of cod have been decimated, causing the government of Canada to suspend the once vast cod-fishing industry. On the Pacific coast, salmon are fast disappearing, the victims of silted rivers, dams, and overfishing. And far out in the Pacific, sharks and rays, swordfish and skates are declining in number. Safina visits all these places, giving little lectures on fish ecology along the way (readers might otherwise never have known that in water of 57 degrees Fahrenheit, a swordfish maintains a cranial temperature of 84 degrees). He allows that the decline of fish has multiple causes; as one of his informants, a California farmer, remarks, ``It's not just the farmers or fishermen. It is water transfers, ocean temperatures, toxic pollutants, timbering, all these things.'' On the matter of those toxic pollutants Safina has much to say: He is rightly appalled that in many Pacific nations fish are caught by poisoning the waters with cyanide, a process that kills not only the fish but also fragile coral reefs, among the world's most endangered ecosystems. Industrialized nations, he notes, and especially Japan, aren't doing much to help matters, arguing over quotas and territorial limits instead of recognizing that without severe restrictions on fishing the oceans may not be much of a larder in years to come. A valuable account of the devastation we have wrought on what Safina calls ``planet Ocean''--and, thanks to the author's down-to-earth style, a pleasure to read.