written by: Terrie Schultz•edited by: Laurie Patsalides•updated: 8/16/2010
Overfishing of small prey fish species has reduced their populations to such a degree that predators that depend on them for food are suffering malnutrition or starving.
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Increase in Aquaculture is Causing Overfishing of Prey Fish
The vast size of the oceans would lead us to believe that they contain an endless supply of fish for the taking, but nothing is limitless. The increasing demand for commercially valuable fish species such as tuna and salmon has spurred an explosive upsurge in the fish farming, or aquaculture, industry.
Enormous quantities of small prey fish, also known as pelagic or schooling fish, are needed to feed the larger farmed fish. Most of the prey fish are ground up into fish meal or fish oil, while some farmed species, such as tuna, eat live anchovies, mackerel or sardines. It requires 20 kg (44 pounds) of prey fish for every 1 kg (2.2 pounds) of tuna that ends up on the dinner table or sushi bar.
The life span of small schooling fish is short, and they are susceptible to population crashes when spawning adults or juveniles are removed before they can reproduce, preventing recovery of overfished populations.
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Loss of Prey Fish is Impeding Recovery of Overfished Predator Fish
Populations of these pelagic prey fish, once plentiful, are now severely diminished, and the effect of their decline is being felt throughout the ocean's ecosystems.
Large carnivorous fish such as bluefin tuna, Pacific salmon, and striped bass that have been overfished need plentiful food in order for their populations to recover. These predator fish often have only a limited number of preferred prey species, and if the prey is not available, they have no other options.
Juvenile Coho and Chinook salmon depend on surf smelt, Pacific herring, and Pacific sand lance when they arrive at the intertidal zone on their journey from the rivers to the sea. Northern Atlantic bluefin tuna and striped bass move up and down the east coast of North America in search of schools of Atlantic menhaden, Atlantic herring and sand lance. The survival of these predator fish is compromised by the decline of prey species that are the staples of their diet. Emaciated striped bass found in the Chesapeake Bay is an example of the effect of the decline of the Atlantic menhaden populations in the area.
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Loss of Prey Fish is Impacting Other Predator Species
Declining numbers of prey fish is also affecting other species, including dolphins, seals, sea lions, birds and whales. When the food supply is inadequate, these animals may fail to reproduce, or abandon their young to starvation. Malnourishment also weakens them and causes them to become more vulnerable to diseases.
Overfishing of anchovies and sardines has led to declining populations of Mediterranean bottlenose dolphins, and 40% of the dolphins in the Ionian Sea have been found to be emaciated.
Depletion of local food sources dramatically impacts nesting seabirds that must find food within a limited range to bring back to their chicks. The diet of the endangered marbled murrelet in California has been altered by the sardine industry, and birds in Patagonia, including penguins, cormorants and terns, are threatened by the anchovy industry.
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To address this catastrophic problem, the ocean conservation group Oceana recommends banning new fisheries for prey fish, setting conservative catch limits on existing fisheries, establishing preserves for predator species, prioritizing uses for prey fish, and protecting breeding hotspots.