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 suffering most.

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 

"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 

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 diversity.

"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 said.