Commercial fish processing ships can affect birds, whales, dolphins, turtles and sharks by their broad reach methods of catching fish.
Purse seine ships, with nets up to two kilometres in circumference, can encircle whole shoals of pelagic fish, such as mackerel, herring and tuna.
A major international scientific study released in November 2006
in the journal Science found that about one-third of all fishing stocks worldwide have collapsed
(with a collapse being defined as a decline to less than 10% of their maximum observed abundance), and that if current trends continue all fish stocks worldwide will collapse within fifty years.
The FAO State of World Fisheries and Aquaculture 2004 report estimates that in 2003, of the main fish stocks or groups of resources for which assessment information is available, "approximately one-quarter were overexploited, depleted or recovering from depletion (16%, 7% and 1% respectively) and needed rebuilding.
The threat of overfishing is not limited to the target species only. As trawlers resort to deeper and deeper waters to fill their nets, they have begun to threaten delicate deep-sea ecosystems and the fish that inhabit them, such as the coelacanth.
In the May 15, 2003 issue of the journal Nature, it is estimated that 10% of large predatory fish remain compared to levels before commercial fishing. Many fisheries experts, however, consider this claim to be exaggerated with respect to tuna populations.
From 1950 (18 million tons) to 1969 (56 million tons) fishfood production grew by about 5% each year; from 1969 onward production has raised 8% annually. It is expected that this demand will continue to rise, and MariCulture Systems estimated in 2002 that, by 2010, seafood production would have to increase by over 15.5 million tonnes to meet the desire of Earth's growing population. This is likely to further aggravate the problem of overfishing, unless aquaculture technology expands to meet the needs of human population.
Overfishing has depleted fish populations to the point that large scale commercial fishing, on average around the world, is not economically viable without government assistance; EU law, however, prohibits subsidies to fleets of its member states.