Up to half of ocean species lost to overfishing
By Steve Connor, Science Editor
Published: 29 July 2005
Half of all sea fish species have disappeared from the major fishing
grounds of the world, according to a study that shows how ocean life
has declined rapidly in the past 50 years.
The dramatic fall in the diversity of fish is blamed on overfishing
rather than pollution or climate change, the scientists behind the
study said yesterday.
The study, which examined fishing logbooks dating back to the 1950s,
also found that the size of ocean "hot spots", which were
traditionally rich in a diverse array of fish species, had shrunk
significantly over the same period.
A research team led by Boris Worm and Ransom Myers of Dalhousie
University in Halifax, Nova Scotia, calculated that species diversity
has declined by between 10 and 50 per cent in all oceans, with the
most important predators such as sharks, tuna, swordfish and marlin
Dr Worm said there was now a clear link between overfishing and the
shrinking of the ocean regions where most fish tended to congregate in
what the scientists call species-diversity hot spots.
"Everywhere you go, in every ocean basin, our hot spots today are
only relics of what was once there. It really hurts to see this,"
Dr Worm said.
Dr Myers said that where fishermen might have caught 10 species of
fish on average in any one area of the sea five decades ago, today
they could only catch five. "It's not yet extinction. It's local
fishing-out of species. Where you once had a range of a species in
dense numbers, now you might catch one or two of a certain species,"
Dr Myers said.
Both scientists took part in an earlier study that showed a 90 per
cent decline in some of the top predators of the open ocean. The
latest study confirms that the decline has affected diversity as well
as overall numbers.
Comparisons between the hot spots of ocean diversity in the 1960s and
the 1990s clearly show the decline. Tuna and marlin, for instance,
were once abundant off the north-west coast of Australia, but now the
region is no richer than other areas.
The study, published in the journal Science, used data from the
logbooks of Japanese fishermen, who have the biggest longline fleet of
trawlers in the world and have collected continuous data on the size
and diversity of catches for many decades.
Longlines can be more than 60 miles long and are baited with thousands
of hooks which attract many species of fish, as well as snagging
turtles and seabirds as by-catch.
The scientists believe the longline data is an accurate sample of the
fish living in any one region at any one time. They also
cross-referenced this information with data on fish diversity gathered
between 1990 and 1999 by the government agencies in Australia and the
United States, which collated data on more than 140 fish species.
Nathan Mantua of the University of Washington in Seattle said that
changes in the weather patterns of the oceans over a decade - such as
the El Niņo phenomenon in the Pacific Ocean - could also affect fish
"If this were the only factor, we might expect declines to be
quickly reversible. What they've shown is that we're on a curvy
one-way street, with clear trends towards a reduction in biodiversity.
There is real cause for alarm here," Dr Mantua said.
Daniel Pauly, a fish biologist from the University of British
Columbia in Vancouver, said the study highlighted a decline that
sometimes could be obscured by other environmental factors. "This
study brings to the surface something that was buried. The long-term
trend of decline is not discernible at first because there are lots of
things happening, like the short-term effects of El Niņo," he