Do Fish Feel Pain? Part II

Do Fish Feel Pain? Part II

Disclaimers: The study of whether or not fish feel pain is a divisive issue that certainly can't be thoroughly addressed in a single article. Furthermore, the neurobiology in this matter is complex, and I'm not qualified to tell if any of it is correct, though I accept that explanations of neurobiology presented in peer-reviewed studies are accurate.

...story continued from the April issue.

The Fish Feel No Pain camp also harbors studies, but additionally, likes to point out errors and inconsistencies in the studies and assertions of the Fish Feel Pain camp. Anthropomorphism is, it seems, the leading error. It's an intuitive and emotionally powerful argument. A hook in your mouth would hurt; why wouldn't a fish feel the same? There is a simple argument to refute this, though. When you pull on a hook embedded in the lip of a fish, the fish pulls in the opposite direction. If I were to put a hook through your nose and pull, would you pull away from me? But wait - didn't I say we should not infer anything about the psychology of fishes based on what we observe because we are humans, not fishes? Indeed. But let's, for the sake of argument, anthropomorphize If we say the hooked fish feels pain the same way the hooked angler does, then we must conclude from the fish's behavior that fish seek to increase the pain of being hooked by pulling against the hook. Therefore, fish are masochistic creatures because they induce this extra pain repeatedly and on purpose.

Okay, so maybe fish do not experience human pain, but what about fishy pain? Here is where the neurobiology comes in. Suffice to say, fish lack the neuroanatomical structures necessary for translating nociceptive signals into pain. (If you want all the details, check out the article on Scientia Salon in the list of resources. Here's a preview: "Fish lack the distinct topographical coding of spatiotemporal integration of different somatosensory modalities; they lack the higher-order integration of somatosensory information with other sensory systems; and they lack a laminated and columnar organization of somatosensory information.")

In humans (and other mammals), the neocortex and mesocortex are responsible for interpreting pain. There is no convincing, empirical evidence that the fish brain has analogous structures that perform the same function. The classic way of determining how different parts of the vertebrate brain function (both in fish and mammals) is to physically remove them, one by one. From such studies, we know that there is no unique part of the fish brain. Birds and reptiles do have neuroanatomical features that would theoretically, maybe, allow them to feel pain. Cells seemingly functioning like mammalian cortical cells were found in some different places in bird brains, and still another place in reptiles. That was in 2012. Fishes? Nothing yet. Some have argued that bony fishes have forebrain structures which are homologous to those involved in human pain, but homology means only that a structure was present in a common ancestor (of fishes and mammals). No functional equivalent has been established.

If fish do not feel pain, what causes the behaviors that resemble what we would interpret as pain (squirming, writhing, gasping, etc.)? The answer is nociception. Studies with decerebrate rats have shown that pain-like behaviors, including avoidance learning and conditioned emotional response, are possible without the part of the brain that interprets pain. Rose et al. (2014) provides more detail on these experiments and results.

Rose et al., the MVP of Fish Feel No Pain camp, is an evaluation and critique of recent research and literature pertaining to fish pain. Foremost is Rose et al.'s assessment of Sneddon et al. The first part under fire is Sneddon's criteria:

(1) to show that the animal has the same apparatus to detect a noxious stimulus that humans have.
This only requires the presence of nociceptors, which are neither necessary nor sufficient for experiencing pain.

(2) to demonstrate that a noxious event has adverse behavioral and physiological effects.
Physiological and behavioral responses to noxious stimuli are fully possible and (even in humans) regularly executed without consciousness, which is an essential requirement for pain.

(3) the animal should learn to avoid this noxious stimulus.
Avoidance learning can involve only unconscious associative conditioning, and thus doesn't require pain.

(4) the behavioral impairments during a noxious event should not be simple reflexes.
Many studies describe pain as a response that is "more than a simple reflex," but no explanation is provided to distinguish between a simple reflex and a complex reflex. Typically, a simple reflex would be something like a knee jerk or a limb withdrawal, and a complex reflex would include vomiting or righting reflexes, which require coordinated action of numerous muscles. According to this criterion, almost any sustained, whole-animal behavior that seems to result from a noxious stimulus would be considered evidence of pain. This practice constitutes the logical fallacy of false dilemma: a situation in which only limited alternatives are considered, when there is, in fact, at least one additional option. This can arise intentionally to force a choice or outcome. Also, using such vague and open-ended language allows investigators to decide what they want behaviors to mean, after the fact.

Then there are inconsistencies in the results. First, sustained pain should have triggered an endocrine stress response, causing increased swimming. However, no change in swimming occurred. Second, not eating is considered a reliable response to stressful or noxious stimuli, but the trout were eating within ninety minutes when the effects of the acid persist three hours. Third, the rocking behavior was likely induced by the anesthesia since no rocking behavior was observed when Newby and Stevens repeated the experiment sans anesthesia in 2008. Fourth, the authors concluded that the mouth rubbing behavior was due to pain, but they also interpreted the ninety minutes to resume feeding as avoiding mouth stimulation (due to pain). If they were avoiding mouth stimulation, why did they rub their mouths on the gravel? The interpretations are contradictory and constitute an example of hypothesizing after the results are known (HARKing).

In another paper on rainbow trout, Sneddon dosed fish with morphine after injecting them with acid, observed reduced mouth rubbing, and concluded this to be proof that mouth rubbing indicated pain. However, the dose of morphine required for this result was ten times the lethal dose for any bird or mammal of similar size, and still did not alter the swimming behavior of the trout. This alone illustrates that the response of trout to morphine is quite different from mammals (Sneddon later stated that the published dose was an error, but the corrected amount still exceeded the lethal dose in mammals by a significant margin).

Moving on from Rose Sylvia Earl's assertions about fish definitely feeling pain are problematic because they reverse the scientific burden of proof, and therefore the hypothesis, from "fish do not feel pain" to "fish feel pain." While the first employs the key principle of scientific reasoning (falsification), the second could not be disproven even if every fish on Earth could, in fact, not feel pain. Imagine trying to disprove the hypothesis "fish do not feel pain." You would only need to find one fish, anywhere, that feels pain. Now imagine trying to disprove the hypothesis "fish feel pain." Finding one fish that doesn't feel pain fails to disprove it. Even if we could, somehow, test every fish species on the planet, some 30,000, it would not disprove the hypothesis. What if there are fish on other planets that feel pain? What if past fish, now extinct, felt pain? What if fish evolve that feel pain? What if there is a single mutated fish, an anomaly within its species, that feels pain? There's simply no way to disprove the hypothesis, "fish feel pain." The reasonable course of action is to choose a hypothesis that can be disproven.

Now, I'd like to focus on the feeding habits of fishes, and to begin, I offer "the bane of reasoned scientific consensus," a story (from an angler): "I was once fishing in the vicinity of a number of other anglers and caught an undersized fish. Unhooking it prior to release, I saw a piece of fishing line running from its mouth. Closer inspection revealed an eye of a hook protruding out of its gullet, deep in its mouth behind the gill rakers. Deciding to attempt a 'good Samaritan' act, I took my long nose pliers and carefully worked the hook out of the fish's esophagus. As the bend of the hook emerged I was amazed to see that there was a minnow, still hooked through the lips. The minnow, while dead, was bright eyed, the fins not frayed, the scales and mucous coating intact despite the gastric acid of the gut. My conclusion was that the minnow had been dead for less than 30 minutes, more likely, less than 15." Put that story in a human context. Suppose, while eating dinner, you swallow your fork, and it impales your esophagus and becomes stuck, with just the tip visible at the back of your mouth. What do you do? Start in on the second course!

Fish often eat things that would be very painful for you or me to consume: urchins, crabs (uncooked, mind you), coral, barnacles, stingrays (stinger included), catfish, etc. Although some predatory fish have evolved particular features for eating tough prey (such as bony-plated mouths for eating coral) and often show preference for softer, squishier foods, they still frequently eat things that cause injury. One great hammerhead, Sphyrna mokarran, was found with 96 stingray barbs embedded in the mouth, throat, and tongue. These feeding habits are difficult to reconcile with anthropomorphic claims that fish feel pain.

A study by Wedemeyer and Wydoski in 2008 focused specifically on angling effects in wild salmonids, including brook trout, Salvelinus fontinalis; brown trout, Salmo trutta; cutthroat trout, Oncorhynchus clarkii; and Arctic grayling, Thymallus arcticus. The fish were hooked and played for 1-5 minutes. Stress-sensitive indicators, including blood glucose, chloride, osmolality, and hemoglobin were measured immediately after capture. All measurements were well within normal physiological tolerance limits. Then, fish of the same species that were hooked and played for 5 minutes were released into net pens and held for up to 72 hours. The confined fish exhibited blood chemistry alterations that appeared to be related to stress, indicating that confinement causes more physiological stress than either hooking or playing. Similar results were found in snapper, Pagrus auratus, and mao mao, Scorpis violaceus. While handling and exposure to air does trigger a fish's physiological stress reaction, successful catch-and-release, (which obviously excludes accidental mortality) seems to cause little physiological stress, and zero past the immediate minutes following release. There's no need to unduly anthropomorphize animals in order to treat them humanely.

Because we so easily reflect on our own behavior, we think of animals as having the same qualities, but this line of thought can camouflage biologically and evolutionarily more probable explanations of animal behavior. Furthermore, much of human behavior is unconsciously enacted, and our survival depends on that fact (or it used to, when immediate reactions meant the difference between life and death). The mind increases operating efficiency by relegating much of the high-level, sophisticated thinking to the unconscious, which does a surprisingly excellent job of sizing up the world, warning you of danger, setting goals, and initiating action hence the 'law of parsimony': why propose the existence of a more complex process (consciousness and pain) when a less complex one (adaptive unconscious and nociception) accounts for the data, especially when there is no plausible mechanism for the more complex process?

Overall, having read about 100 pages of literature on the subject (which is a fraction of what's available), the Fish Feel No Pain camp does seem to be more robust in its evidence and logic, and at least equal in its anthropomorphic comparisons. While I am personally convinced that fish do not feel pain as we conceptualize it, there appears to be much we still don't know (somatic vs. visceral pain in fish, for one). I think our conclusions should reflect our uncertainty, and we should err on the side of limiting harm where it might occur. Practice safe catch and release!

Where I learned about fish pain, and you can too!

"Nociception in Fish: StimulusResponse Properties of Receptors on the head of Trout Oncorhynchus mykiss"
Ashley, Paul J., Sneddon, Lynne U., McCrohan, Catherine R.

"Can Sylvia Earle Save the Oceans"

"Can fish really feel pain?"
Rose, J. D., Arlinghaus, R., Cooke, S. J., Diggles, B. K., Sawynok, W., Stevens, E. D. and Wynne, C. D. L. (2014)

"Physiological and Behavioural Responses to Noxious Stimuli in the Atlantic Cod (Gadus morhua)"
Jared R. Eckroth, yvind Aas-Hansen, Lynne U. Sneddon, Helena Bicho, Kjell B. Dving

CFOOD | Science of Fisheries Sustainability

Scientia Salon


Science Daily

..and many other studies, papers, opinions, etc.