Fish have demonstrated that they remember the circumstances of painful experiences and will afterwards seek to avoid the same situation.
Dutch researchers (Beukenia, 1970) used anglers to fish for pike for three days, using live bait or spinner hooks. The fish had never been fished before.
Pike hooked once by a spinner rarely took it again, and would avoid spinners for the rest of the five days.
The same researchers (Beukenia, 1970, 2) found that when carp were hooked and then lost, that not only did these fish later become nearly uncatchable, but other carp, who were not caught also became so. Even after one year they were three times more difficult to catch.
Sticklebacks receive some protection from predators fish through their spines. In 1957 at Oxford University (Hoogland, 1957), researchers found pike and perch initially snapped up, but then rejected sticklebacks. After a few experiences, the pike and perch learned to avoid the sticklebacks altogether. The scientists found that when spines were removed from sticklebacks their protection disappeared.
After very few experiences both perch and pike become negatively conditioned to the sight of sticklebacks and avoid them before they have made contact.
Goldfish at Missouri University were placed in a shuttlebox (Zerbolio, 1983). They had to respond if light colours at each end were the same by swimming to the other end. When they were wrong, they received an electric shock. Later, they had to respond if the colours of the lights were different. The goldfish learnt and remembered how to avoid the shocks.
In 2006 at Belfast University (Dunlop, 2006) scientists gave goldfish and trout mild electric shocks when they entered a particular part of their tank. The fish swam away when this happened, their respiration went up, and cortisol, an indicator of stress, went up in their blood.
The fish learnt to spend time at one end of the tank, away from the electric shocks, especially on the second day. Trout together in a group were more likely to tolerate the milder voltage.
The researchers said that, "This plasticity in learning suggests that pain responses are not just confined to lower brain reflex actions, but to some degree, involve perception."
They went on to say that, "Current legislation is written under the assumption that fish are incapable of perceiving pain and if there is any possibility that this is incorrect, new legislation will have to be developed to improve aquaculture conditions and change fishing practices."
Goldfish were conditioned to expect an electric shock after a light was shown to them (Yoshida, 2010). Their fear produced a slowing of the heart when the light preceded the shock.
However, when an analgesic, lidocaine, was injected into the brain (cerebellum) of other goldfish, it prevented the goldfish from feeling pain, and so they did not show fear when the light was shone.