Some fish harvesting uses controversial methods like suffocation in air, carbon-dioxide stunning, or ice chilling that have been called inhumane by many organizations such as the World Organisation for Animal Health.
According to the Food and Agriculture Organization (FAO), a total of 156.2 million tons of fish, crustaceans, molluscs, and other aquatic animals were captured in 2011.
Some indicators used by welfare studies include[9] Following electric stunning, as fish gradually resume consciousness, they begin to make rhythmic gill-cover movements.
Based on EEG correlations, it is believed that stunned fish remain insensible until they have resumed rhythmic gill patterns.
[11] In 2004, the European Food Safety Authority observed that "Many existing commercial killing methods expose fish to substantial suffering over a prolonged period of time.
Because chilling slows metabolic rate and oxygen needs, it may prolong the duration until death in some instances, with some cold adapted species taking more than an hour to die.
[9] Research in 2009 showed that ice water is faster and less stressful than anaesthetics for killing tropical ornamental fishes like zebrafish.
[18] Most often applied for salmon and trout, CO2 narcosis involves filling the fish water with CO2 to produce acidic pH, which injures the brain.
Fish are cut in highly vascular body regions, and the process is stressful unless the animals are unconscious.
Also known as knocking, percussive stunning involves hitting the fish's head with a wooden or plastic club, called a priest.
One or two appropriate blows can disrupt the brain sufficiently to render the fish insensible and potentially even kill it directly.
[9] However, building an automated machine to process, orient, percussively stun, and bleed bulk quantities of small fish would be difficult.
As with percussive stunning, spiking is used to kill one fish at a time and so is mainly used for large species such as tuna and salmon.
[23] In addition to potentially producing unconsciousness quickly, stunning reduces the stress of restraint and being removed from water.
[23] In June 2024, the Centre for Aquaculture Progress, a non-profit organization dedicated to fish welfare in Europe, oversaw a comprehensive study of European consumer attitudes to electrical stunning.
A paper published by Jeff Lines and his collaborators in 2003 announced that stunning trout for 60 seconds in an electric field of 250 V/m r.m.s.