Predatory fish levels reduced by 90 percent.

The global biomass of large predatory fish both open-ocean species such as tuna, swordfish, and marlin and large groundfish such as cod, halibut, skates, and flounder--has been reduced to a mere 10 percent of pre-industrial levels, according to a recent study in the journal Nature. The study asserts that industrial fisheries take only 15 years or so to reduce a fish population by roughly 80 percent.

After spending a decade assembling data sets representing all major fisheries in the world, the authors of the study constructed trajectories of the biomass and composition of large predatory-fish communities from four continental shelves and nine oceanic systems, from the beginning of exploitation to the present. For shelf ecosystems, they used data from standardized research trawl surveys to track the decline in the populations of large fish. To measure the decline in open-ocean ecosystems, they gained access to Japanese longlining data. Pelagic longlines are the most widespread fishing gear, and the Japanese fleet the most widespread longline operation, covering all oceans except the circumpolar seas.

The authors of the study observe that the tendency in fisheries biology to use only the most recent data increases the problem of shifting baselines. Not only are the fish declining in numbers, but with intense fishing pressure they are also not attaining the sizes they once did. "Where detailed data are available, we see that the average size of these top predators is only one-fifth to one-half of what it used to he," said Dr. Ransom Myers of Dalhousie University, lead author. "The few blue marlin today reach one-fifth of the weight they once had. In many cases, the fish caught today are under such intense fishing pressure that they never even have the chance to reproduce."

"Numbers dropped fastest during the first years, as fisheries moved into new areas," added co-author Boris Worm of Dalhousie University and the University of Kiel in Germany, "often before any fisheries management protocols were in place--and before anyone was looking." Without this baseline information, Worm said, most scientists and managers are hardly aware of the profound magnitude of the change that took place at the beginning of almost every major fishery. As a result, managers today are working hard to stabilize the last 10 percent--often unaware that the virgin biomass of their fishery was once 10 times greater. There is, however, some good news too, according to Worm: "In most regions we saw increases in faster-growing species, which seemed to fill in for overfished stocks. This points to the recovery potential of the community at large. But unfortunately we often switch fishing pressure to species that are doing well, and drive them down in turn. This sabotages recovery."

The solution is drastic but essential. "A minimum reduction of 50 percent of fishing mortality may be necessary to avoid further declines of particularly sensitive species," Myers said. "If stocks were restored to higher abundance, we could get just as much fish out of the ocean by putting in only one-third to one-tenth of the effort. It would be difficult for fishermen initially--but they will see the gains in the long run."

"The impact we have had on ocean ecosystems has been vastly underestimated," Worm emphasized. "These are the megafauna, the big predators of the sea, and the species we most value. Their depletion not only threatens their future and the fishers that depend on them, it also could bring about a complete re-organization of ocean ecosystems, with unknown global consequences."

Myers concluded: "We are in massive denial and continue to bicker over the shrinking numbers of survivors, employing satellites and sensors to catch the last fish left. We have to understand how close to extinction some of these populations really are. And we must act now, before they have reached the point of no return. I want there to be hammerhead sharks and bluefin tuna around when my five-year-old son grows up. If present fishing levels persist, these great fish will go the way of the dinosaurs."

(Adapted, with permission, from Ocean Update, Vol. 8, No. 5, June 2003.)