Predatory Fish Levels Reduced by 90%

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

After spending a decade assembling data sets representing all major
fisheries in the world, the study's authors constructed trajectories of
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, the researchers 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 observe that the tendency in fisheries biology to use only
the most recent data increases the problem of shifting baselines. These
fish are not only declining in numbers, the authors say, but with
intense fishing pressure they can never attain 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
be. 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, they never even have the chance to reproduce," says
Dr. Ransom Myers of Dalhousie University, the paper's lead author.

"Numbers dropped fastest during the first years, as fisheries moved
into new areas," adds 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, says Worm, most scientists and
managers are hardly aware of the profound magnitude of change that took
place at the beginning of almost every major fishery. As a result, he
continues, managers today are working hard to stabilize the last
10%—often unaware that the virgin biomass of their fishery was once ten
times greater. But, he adds, there is some good news too: "In most
regions we saw increases in faster-growing species which seemed to fill
in for overfished stocks. This points to the recovery potential for 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, the authors say, is drastic but essential. "A minimum
reduction of 50% of fishing mortality may be necessary to avoid further
declines of particularly sensitive species," asserts Myers. "If stocks
were restored to higher abundance, we could get just as much fish out
of the ocean by putting in only 1/3 to 1/10 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," emphasizes Worm. "These are the megafauna, the big
predators of the sea, and the species we most value. Their depletion
not only threatens the future of these fish and the fishers that depend
on them, it could also bring about a complete re-organization of ocean
ecosystems, with unknown global consequences."

Concludes Myers: "We are in massive denial and continue to bicker over
the last 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."

Contact: Ransom A. Myers, Killam Chair in Ocean Studies, Dept. of
Biology, Dalhousie University,

Halifax, Nova Scotia, B3H 3J5 Canada. Tel: (902) 494-1755; Fax: (902)
494-3736. E-mail: Ransom.Myers@Dal.Ca