Not only can fish detect and perceive painful events, but they also show disturbed behaviour.
Although fish are normally silent, Russian scientists (Chervova) found that the wounding of European weather loach caused the fish to vocalize in pain using their swim bladders.
Scientists in the 1930s discovered that wounded minnows released a chemical that caused fear in other minnows. Scientists in Sigapore (Jesuthasan, 2012) have isolated the chemical in zebrafish, who froze in place.
Fish will often not swim close to an object that they have not encountered before. Researchers in Scotland (Sneddon, 2003) found that trout, that had vinegar (acetic acid) injected under their skin, were less wary than fish who had not had the injection, when a lego tower was placed in their tank. The fish showed a normal cautious response if they were given morphine later.
The researchers said that the painful vinegar distracted the attention of the fish. For this to be happening, the fish must be consciously aware of the negative experience of the pain. The scientist concluded that, "the criteria for animal pain have been fulfilled for the trout."
Researchers, again in Scotland (Sneddon, 2003, 2), injected into the lips of rainbow trout either bee venom or vinegar. A control group was handled but received no injection.
The effects on the fish was a near doubling of respiration, stopping of feeding for 3 hours, and reduced swimming. They rocked back and forth, balancing on either pectoral fin, while resting on the gravel, and they also rubbed their lips into the gravel and tank walls.
Researchers at Liverpool and Manchester Universities (Sneddon, 2003, 3), in a separate experiment, injected vinegar once more into the lips of trout.
After their painful experience, when individual fish were returned to a familiar social group they showed reduced aggression. When the fish were returned to an unfamiliar social group, they showed the usual level of aggression, suggesting that maintaining dominance status took priority over showing signs of pain.
The scientists concluded that fish are considerably affected by pain, and that the perception is not just a simple reflex. The experiment showed that fish are able to perceive and manage the pain felt.
CHEMICAL INDICATION OF STRESS
Two well known indicators of stress in animals are
"Stress in fish, as in other animals, results in two types of endocrine response, the adrenergic response, resulting in increased plasma concentrations of adrenaline and noradrenaline, and the hypothalamo-pituitary-interrenal response, culminating in an increased plasma cortisol concentration."
FISH SPECIES VARY IN THEIR RESPONSE TO A PAIN