The presence of pain in an animal, or another human for that matter, cannot be known for sure, but it can be inferred through physical and behavioral reactions. Specialists currently believe that all vertebrates can feel pain, and that certain invertebrates, like the octopus, might too.
Animal protection advocates have raised concerns about the possible suffering of fish caused by angling. In light of recent research, some countries, like Germany, have banned specific types of fishing, and the RSPCA (which has authority in England and Wales) now formally prosecutes individuals who are cruel to fish.
The idea that animals might not feel pain as human beings feel it traces back to the 17th-century French philosopher, René Descartes, who argued that animals do not experience pain and suffering, because they lack consciousness. Bernard Rollin of Colorado State University, the principal author of two U.S. federal laws regulating pain relief for animals, writes that researchers remained unsure into the 1980s as to whether animals experience pain, and veterinarians trained in the U.S. before 1989 were simply taught to ignore animal pain. In his interactions with scientists and other veterinarians, Rollin was regularly asked to "prove" that animals are conscious, and to provide "scientifically acceptable" grounds for claiming that they feel pain. Carbone writes that the view that animals feel pain differently is now a minority one. Academic reviews of the topic are more equivocal, noting that although the argument that animals have at least simple conscious thoughts and feelings has strong support; some critics continue to question how reliably animal mental states can be determined.
Veterinary medicine uses, for actual or potential animal pain, the same analgesics and anesthetics used in humans.
Experiments by William Tavolga provide evidence that fish have pain and fear responses. For instance, in Tavolga’s experiments, toadfish grunted when electrically shocked, and over time they came to grunt at the mere sight of an electrode. Additional tests conducted at both the University of Edinburgh and the Roslin Institute, in which bee venom and acetic acid was injected into the lips of rainbow trout, resulted in fish rubbing their lips along the sides and floors of their tanks, which the researchers believe was an effort to relieve themselves of pain. One researcher argues about the definition of pain used in the studies.
In 2003, Scottish scientists at the University of Edinburgh performing research on rainbow trout concluded that fish exhibit behaviors often associated with pain, and the brains of fish fire neurons in the same way human brains do when experiencing pain. Professor James D. Rose of the University of Wyoming criticized the study, claiming it was flawed, mainly since it did not provide proof that fish possess "conscious awareness, particularly a kind of awareness that is meaningfully like ours". Rose argues that, since the fish brain is different from ours, fish are probably not conscious in the manner humans are, and while fish may react in a way similar to the way humans react to pain, the reactions in the case of fish have other causes. Rose had published his own opinion a year earlier arguing that fish cannot feel pain because they lack the appropriate neocortex in the brain. Studies indicating that fish can feel pain were confusing nociception (responding to threatening stimulus) with feeling pain, says Rose. "Pain is predicated on awareness. The key issue is the distinction between nociception and pain. A person who is anaesthetised in an operating theatre will still respond physically to an external stimulus, but he or she will not feel pain." However, animal behaviourist Temple Grandin argues that fish could still have consciousness without a neocortex because "different species can use different brain structures and systems to handle the same functions."
In a 2009 paper, Janicke Nordgreen from the Norwegian School of Veterinary Science, Joseph Garner from Purdue University, and others, published research which concluded that goldfish do feel pain, and that their reactions to pain are much like those of humans. "There has been an effort by some to argue that a fish's response to a noxious stimuli is merely a reflexive action, but that it didn't really feel pain," Garner said. "We wanted to see if fish responded to potentially painful stimuli in a reflexive way or a more clever way." The fish were divided into two groups, one given morphine and the other saline. They were then subjected to unpleasant temperatures. The fish that were given saline subsequently acted with defensive behaviours, indicating anxiety, wariness and fear, whereas those given morphine did not. Nordgreen said that the behavioural differences they found showed that fish feel both reflexive and cognitive pain. "The experiment shows that fish do not only respond to painful stimuli with reflexes, but change their behavior also after the event," Nordgreen said. "Together with what we know from experiments carried out by other groups, this indicates that the fish consciously perceive the test situation as painful and switch to behaviors indicative of having been through an aversive experience."
The Norwegian Research Council is funding a three-year research project, scheduled to end in December 2011, into whether cod can feel pain. The researchers will use fMRI and EEGs to study how the cod brain works. The aim of the study is to identify the parts of the cod brain that activate when cod are exposed to potentially painful stimuli, and how those signals are processed.
Zebrafish, native to the streams of the south eastern Himalayan region, are commonly used as a model organism in studies of vertebrate development and gene function. Zebrafish are used to study development, toxicology and toxicopathology, because the body of a young zebrafish is nearly transparent, providing unique visual access to their internal anatomy. Another extensively used model organism is the medaka, which is much sturdier than the traditional zebrafish. Medakas are easy to rear in the laboratory because of their prolific reproduction rates and short generation times. The short-lived ram cichlid is also used in laboratory studies because of its ease of breeding and predictable pattern of ageing. Sticklebacks have traditionally been used as model organism in the study of fish behaviour.
The extent to which animal research causes pain to laboratory animals is the subject of much debate. Marian Stamp Dawkins defines "suffering" in laboratory animals as the experience of one of "a wide range of extremely unpleasant subjective (mental) states." The United States Department of Agriculture defines a "painful procedure" in an animal study as one that would "reasonably be expected to cause more than slight or momentary pain or distress in a human being to which that procedure was applied."