PERSONAL PERSPECTIVE

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TRANSFORMED SEASCAPES AND OCEAN FARMING

by Dr Rosamond Naylor
Julie Wrigley Senior Fellow, Center for Environmental Science and Policy, Stanford University

In addition to overfishing, marine ecosystems and fisheries face serious threats from other sources: run off of land-based pollutants, introductions and invasions of exotic species, coastal development and habitat alteration, unintended by-catch, and climate change.

"Dead zones" associated with excessive nutrient run off and oxygen depletion in marine ecosystems have recently been classified by the United Nations Environment Program as one of the top 20 global environmental problems. Five to ten thousand species are being transported each day from one part of the world to another by ballast water in the shipping industry alone, in many cases invading ecosystems where native species are vulnerable to extinction. Recent climate-related studies show that oceans have absorbed nearly half of the total amount of carbon dioxide released worldwide by human activities, such as fossil fuel burning.

This process is changing water chemistry in ways that threaten corals and other calcifying organisms such as shellfish, with potentially disastrous implications for marine food webs. The impact of any one of these threats is cause enough for concern and policy action. Taken together, they paint a grim picture for the health of ocean ecosystems a7 June, 2005for yet another transformation. Fisheries depletion has been a force in expanding seafood production through fish farming, or aquaculture. During the past decade, global production of farmed finfish and shellfish almost tripled in weight and nearly doubled in value. Roughly 40% of all fish directly consumed by humans worldwide are now farmed.

Although most aquaculture production to date has been of freshwater fish, marine aquaculture has been growing dramatically. Global production of farmed salmon, for example, has roughly quadrupled in weight since the early 1990s. This spectacular increase and the resulting decline in salmon prices have helped prompt aquaculturists to begin farming numerous other threatened marine finfish. New species farmed in open ocean netpens include bluefin tuna, Atlantic cod, Atlantic halibut, Pacific threadfin, barramundi, and mutton snapper.

Most farmed marine finfish are carnivores and therefore dependent on wild fisheries for fishmeal and fish oil used in fish feeds. Roughly 30 million metric tons of anchovies, sardines, capelin, and other small oily fish-close to one third of the current annual global fish catch-have recently been used each year for animal feed production. An increasing proportion of this catch is being used in aquaculture feeds as a result of industry growth and as livestock and poultry operations substitute less expensive ingredients for fishmeal. Farming carnivorous marine fish represents a net loss of fish protein, as two to five kilograms of wild fish are now needed for every 1 kilogram of farmed fish harvested.

If marine aquaculture begins to supplant capture fisheries, impetus will shift from managing the oceans for fisheries to managing them for aquaculture production. Under such a scenario, capturing low trophic level wild fish for aquaculture feeds, with little concern for the effect on higher trophic level wild fish, could become the prevailing credo for economically rational-although ecologically irrational-ocean management.

Marine aquaculture also places other stresses on the marine environment. Like industrial livestock systems, marine netpens contain large densities of fish in confined spaces that pollute surrounding waters. A relatively modest salmon farm of 200,000 fish releases an amount of nitrogen, phosphorous, and faecal matter roughly equivalen7 June, 200565,000 people respectively. The escape of farmed fish from ocean netpen systems-a common occurrence due to storms and human error-can lead to competition and interbreeding with populations of already threatened wild fish. These outcomes add to the underlying environmental degradation already plaguing marine ecosystems from other human activities.

A viable future for marine ecosystems will require integrating management for fisheries, fish farming, and conservation. Despite extensive criticism, fisheries management continues to be based largely on single species models for which there are often inadequate data and which do not reflect interactions in marine ecosystems.

Three additional policy objectives should be promoted domestically and internationally as aquaculture continues to expand. First, the production of aquaculture species that do not rely on fishmeal and fish oil as inputs for feed, such as oysters and other molluscs, should be promoted, and the aquaculture industry should be encouraged to substitute vegetable-based protein for fish-based feeds in the production of carnivorous species like tuna and salmon. Second, domestic programs and international agreements should be developed to control the spread of exotic fish species and pathogens resulting from aquaculture production. Third, regulations should be enforced to reduce nutrient releases from land-based systems and aquaculture.

Without such policies, the aquaculture industry itself stands to lose in the long run, since fish farming depends heavily upon wild fisheries.

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