Pain in fish: Weighing the evidence

This is the second of a two-part series exploring the question, “Do fish feel conscious pain?” Part One appeared on Monday, 29 January.

The topic of humane treatment of fish has bubbled to the surface in various international news outlets recently, including reporting on stories as varied as a ban on boiling lobsters in Switzerland, aquaculture net pen crowding, and the humane killing of fish for human consumption. 

While the seafood-eating public may just be pondering the important moral question of whether fish feel pain, scientists have been debating the question for years. Two sides have emerged in the discussion; One, led by ethologist Jonathan Balcombe, believes fish can feel pain, pointing to the pallium, a part of the fish’s brain, as having similarities to a mammal’s neocortex, which governs sensory perception.

On the other hand, University of Wyoming Professor Emeritus James D. Rose believes the research supporting Balcombe’s supposition is at best tenuous.

“Some who argue for pain in fish seem to realize the necessity of finding a plausible neural mechanism for it,” Rose wrote in his 2016 paper, “Pain in fish: Weighing the evidence.” 

For the pallium to function the same way a vertebrate's cortex does, “then the mechanisms essential to both pain and consciousness must be identified and characterized in the pallium, a very challenging undertaking that remains to be attempted...”  

Rose also cites published research that shows fish palliums are very simple and function very differently when compared to human and mammalian neocortexes. 

Rose proposes that, in keeping with the scientific principle that the simpler of two competing theories should be preferred, “an adaptive unconscious” for fish is the better theory.

“Why propose the existence of a more complex process (consciousness) when a less complex one accounts for the data, not to mention the lack of a plausible mechanism for the more complex process?” Rose said.

Another major point of debate surrounds the trigeminal nerve – the largest of the cranial nerves, which is found in all vertebrates and is responsible for sensation in the face and motor-functions such as biting and chewing.

Balcombe's book states trout heads were examined and “found to contain both A-delta and C fibers [in the trigeminal nerve]. In humans and other mammals these fibers are associated with two types of pain sensation: A-delta fibers signal the sharp initial pain of an injury, whereas C fibers signal the duller, throbbing pain that follows. Interestingly, the researchers found that C fibers were present in a much lower proportion in trout (about four percent) than has been found in other vertebrates studied (50 to 60 percent).” 

In his book, Balcombe describes a study in which goldfish are subjected to intense heat. Some of the goldfish in the study were given morphine prior to the experiment while others were not. The goldfish that did not receive morphine injections were still showing signs of trauma 30 minutes after the ones that had received morphine had returned to normal functioning. 

“[Their] response can be likened to our reaction to putting our hand on a hot stove,” Balcombe wrote. “First, we have an immediate, reflexive response: we involuntarily jerk our hand away from the heat without pausing to think about it. It is only a second or so later that we feel the true brunt of the pain. Then we may endure hours or days of discomfort while our bodies protect the offended limb and remind us not to do it again. This result suggests to me that goldfishes might have more of  those C fibers – the ones associated with lasting, throbbing pain – that trout were found to have in short supply.””

Rose' study points out, however, that certain species of fish, including those commonly fished for food like tuna, marlin, salmon, and halibut, rarely have C fibers that are associated with long-term pain or extended suffering. However, they commonly have nociceptors that respond to an initial experience of pain or discomfort. Rose deduces from this that the prevalence of these short-term nerve endings in fish makes sense since “A rapid response to potentially injurious stimuli...would likely be selectively advantageous to fishes.”

“However, preoccupation with conscious suffering [as would occur with the C fibers], especially when little can be done to minister to injuries, would be selected against as it would be unlikely to benefit fishes, which must survive in an environment where they can ill afford to be debilitated by conscious suffering,” Rose wrote.

Rose also questions whether scientific research will ever be able to understand how fish consciousness compares to human consciousness. 

“If fish have consciousness, their consciousness must be so different from ours, as deduced from their brains and their behavior, that we have no idea what it would be like,” he wrote. “We really only know the consciousness of our own species, and that is hard enough to describe. Furthermore, fishes are highly diverse organisms and there are tens of thousands of species of them. Would the consciousness of a basking shark be like that of a barracuda?”

Balcombe acknowledged that much of the research on fish sentience cited in his book involves only a few species. 

“Only a fraction of bony fish species (of which there are over 30,000) have been well studied. But we can extrapolate from a representative species. The rainbow trout and zebrafish are widely used in research and viewed as representative,” he told SeafoodSource. “I think that what applies to them regarding pain can safely be extended to most if not all bony fish types. We do not have to test for pain in every species. We can study it in a handful of representative examples and then we can extrapolate to include the other species.”

In the light of the conflicting research findings, a statement by the American Veterinary Medical Association in their 2013 Guidelines for the Euthanasia of Animals summarizes the current state of the debate.

“While there is ongoing debate about finfishes’, amphibians’, reptiles’, and invertebrate animals’ ability to feel pain or otherwise experience compromised welfare, they do respond to noxious stimuli. Consequently, the guidelines assume that a conservative and humane approach to the care of any creature is warranted, justifiable, and expected by society."

Balcombe remains convinced that, no matter where anyone’s personal opinion falls on whether fish are conscious and able to feel pain, they must be treated with respect. He has said he chooses to eat a vegan diet in respect of that belief.

“If we sum all this up in one package, we come up with the reality that fish are a very diverse and rich group,” Balcombe said. “Having spent the last five years delving into them, I’m now convinced that they are just as deserving of our respect, compassion, and consideration as are any other groups of animals, including those beloved mammals.”

For Rose, however, the proper care of fish means using “function and nature-based approaches over a feelings-based approach to the welfare of fishes and aquatic invertebrates, because the former two definitions do not contradict reality, do not invite use of double standards, and do not contravene basic scientific principles.”

“Function-based welfare does not depend upon assumptions of awareness or resolution of the scientific debate about whether fishes and aquatic invertebrates experience pain, suffering, and emotional feelings,” Rose said.