Industrial fishing proven very efficient
A comprehensive meta-analysis of trawl surveys, Japanese pelagic longlining and other data from four continental shelf and nine open ocean ecosystems indicates a 90% decline in predator fish biomass worldwide since the beginning of industrial fishing. This is a major feat indeed.
In a letter to Nature (15 May 2003), Ransom A. Myers and Boris Worm report the following:
Analysis suggest an average equilibrium1 of 10% of pre-exploitation levels, with 95% of the individual communities2 landing between 5% and 24%.
An 80% decline in biomass typically occurred within 15 years of industrial exploitation. This is usually before scientific monitoring takes place.
As new areas were exploited, they were initially very rich in fish, but catch rates were greatly reduced in just a few years.
Compensatory increases of fast-growing, non-exploited species were observed, but were in most cases subdued by increased bycatch3 mortality or by becoming targets for trawling.
Presently, all major sources of large predatory fish are exploited and severely diminished.
Basis of study
This data was collected from research trawl surveys from northwest Atlantic Ocean, the Gulf of Thailand and the Atlantic Ocean off South Georgia. The surveys included codfishes, flatfishes, skates and rays, among others. For oceanic systems, Japanese pelagic longlining data from 1952 - 1999 were used. It represents complete catch rate data for tuna, billfishes and swordfish.
Data from most of the northern hemisphere was excluded, since modern fishing methods have been in use for decades before scientific surveying began.
To estimate the reduction in biomass, they used "catch per 100 hooks" for each area, not total yield. They assumed that catch rate is proportional to biomass, but added that this probably underestimates the decline, since intensive fishing tends to reduce average age and weight of the preyed fish.
Consequences
Reduced yield leads to changed strategies for the fishing industries. If some species are fished into oblivion, other species are tracked down and in the end severely reduced. There is a global trend towards ever lower mean trophic levels of the catch, as reported by the UN Food and Agriculture Organization (FAO). Also, the management goals of today are to stabilize the fish stock at current levels. But is it really beneficial to stabilize the stock at levels that are some 10% of pre-industrial levels?