We place fish on the hierarchy of sentience somewhere above slugs, and feel better about eating them than we do birds and mammals. But research on fish behaviour and learning suggests we've seriously underestimated them.
Fish have static, expressionless faces; their skin is cold; they have no obvious pulse and they don't call out when they're in distress (not so that we can hear, anyway).
They don't even have much variation in body posture: they thrash about when they're hooked and jerked into an environment where they can't breathe, but they don't flinch or cringe to show they're afraid or in pain.
All of which makes it easier to conclude that they're nothing like us, and feel nothing. Or nothing much.
But fish have nervous systems similar to higher vertebrates, and research into their cognition and behaviour has revealed surprising results.
Does it matter what fish feel?
Lack of injury and disease, and signs of normal growth and behaviour indicate that an animal's welfare is fairly good, but that's not the whole story. Welfare should also involve freedom from physical and emotional discomfort, and a sense of wellbeing. At the most basic level, this means freedom from pain and fear.
Millions of fish are killed (and injured) in order that some can end up on our plates. Fish are now also one of the most commonly used research animals. If they are capable of feeling pain and distress in ways similar to other vertebrates, then their welfare, as it is with chickens and cows and zoo animals, and all other animals we use for our purposes, should be a matter of moral concern.
The greater their capacity for suffering at our hands, the greater our concern should be.
But do fish really suffer and enjoy a sense of wellbeing in ways we can recognise? Are they capable of learning and memory, evidence of which places animals higher on the hierarchy which humans rule?
We don't know for sure (and may never), but research findings, such as the examples cited here, strongly suggest that not only do fish feel pain and distress, but their cognitive powers may rival some birds and mammals:
Fish, like humans among other animals, have pain receptors called nociceptors. These have been shown to be especially abundant on the lips of fish like trout. When fish nociceptors are stimulated with irritant substances, the fish feed less, their gills beat faster and they rub the affected areas - responses suggestive of discomfort.
Fish, like humans, produce endorphins. One of the main functions of these neurotransmitters is to relieve pain.
In experiments similar to those on rats and mice, fish show that they feel fear, or at least unease, and can learn to avoid negative stimuli. Trout learned that a blue light signalled that a net was about to descend into the water to catch them, and knew to swim to the safety of an adjacent chamber when it was switched on. They also learned to press a suspended pendulum that released a food pellet, much like a rat pressing a lever.
Some fish use sound to communicate distress when they are threatened e.g. when nets are dipped into their tanks. In one experiment, fish grunted when they received an electric shock; what's more, the fish then began to grunt as soon as they saw the electrode.
Blind Mexican cave fish, which rely on pressure changes in the water to detect objects, form a mental map of their surroundings, memorising obstacles within a few hours.
Australian crimson spotted rainbow fish, which learned to escape from a net in their tank, could remember how to do it again months later.
Carp that have been stressed by angling activities become harder to catch in future. Similarly, paradise fish that experienced a predator attack avoided where this had occurred for months.
- Olivia Rose-Innes, EnviroHealth Editor, Health24, October 2012
Burt de Perera, T. 2004. A study of spatial parameters encoded in the spatial map of the blind Mexican cave fish (Astyanax fasciatus). Animal Behaviour 68, 291-295.
Sneddon LU et al. 2003. Do fish have nociceptors: Evidence for the evolution of a vertebrate sensory system. Proceedings of the Royal Society: Biological Sciences.
Sneddon LU. 2009. Pain and Distress in Fish. ILAR J.
Yue, S. 2005. What fish feel. AWI Quarterly.
Eating fish: a slippery choice