Why are we so weird about fish

I’m finding myself writing about our aquatic friends quite a bit recently. While the number of scientific procedures using animals in the UK are decreasing overall, the number of procedures carried out on fishes is increasing. In 2023, over 350,000 of the 2.68 million animal procedures performed were on fishes, with over 300,000 on zebrafish.

But why Zebrafish?

Zebrafish are small freshwater fish, native to the Himalayan region, that grow up to 5 cm and live for around 2 years. They are a social species and naturally form shoals.

At around the time of the Millennium, genome sequencing revealed that zebrafish share 84% of human disease-related genes. Since then, they have become increasingly used to study human disease.

Zebrafish have a short lifespan, they are small, have rapid reproduction cycles, and produce as many as 100 young in a single clutch of eggs, which means they are convenient for researchers to study disease effects over a lifetime.

Their eggs are also comparatively large, and their embryos are translucent, so researchers can manipulate them and study their development in the lab easily.

Partial Replacement

We have written about ‘Partial Replacement’ before (debugging our science and not all replacement is equal), but I am going to again. Because I can and it’s on my mind. In essence, it enables animal use in some capacity, which might include those not considered capable of experiencing suffering, based on current scientific thinking.

Early-stage zebrafish embryos, up to 5 days post-fertilisation, are not considered sentient, and therefore invasive experiments on them are not regulated under ASPA in the UK or the EU Directive. Zebrafish embryos and larvae are therefore being used in increasing numbers as a substitute for adult animals as a ‘partial replacement’. Their transparent bodies, rapid development, and genetic malleability make them attractive to researchers studying developmental biology, toxicology, and drug discovery.

Do people actually care about fish?

Despite their increasing use in science and the alarmingly recent, but unarguable, evidence of their sentience and cognitive abilities, fish welfare has been less of a focus of public interest or regulatory attention. We tend to be more concerned about the welfare of more familiar terrestrial and mammalian animals used in the laboratory. Using fish in experiments has even been viewed as ethically preferable or even neutral.

The historical belief is that fish do not feel pain or suffer because of their different nervous systems and brain structures. Although this notion has been widely refuted by scientific evidence, this bias persists. We know that the more distant or different an animal is from humans in evolutionary terms, the less likely people are to want to protect them. We humans don’t share the same aquatic environment, and we find it difficult to interpret their emotional signalling – fish can’t vocalise distress or make facial expressions familiar to us, so we don’t find it easy to relate to their experiences.

Times are changing, thankfully. Fish welfare is a growing research field, and a recent study confirmed that, within the UK, the majority of participants agreed that fish can feel pain and support better welfare for them.

The onset of sentience

Currently, according to the law, zebrafish larvae younger than 5 days after fertilisation are not sentient animals. They cannot feel pain or any other feeling that matters to them. Just 24 hours later, they are considered sentient, and Home Office Licences must be in place to carry out procedures that just a day ago, could have been done without impunity. Those 24 hours are important.

In this regard, the law sees the onset of sentience as binary. Like a switch. It’s either off or on. On day 4 it’s off. On day 5, it’s on. But with animals like fish, how quickly they develop and grow depends on environmental factors like temperature, water quality, and lighting conditions, as well as biological factors such as genetics, food quality and availability, and population density. This means that small differences between tanks of fish and variation between facilities can result in some fish being much more advanced in their development than their neighbours. A day 4 fish in one tank may be more advanced in their cognitive development than a day 6 fish in another, colder tank.

Several studies have also provided evidence that at 5 days old, Zebrafish larvae do indeed respond to negative stimuli (like temperature changes, toxic chemicals and other stressors) and, in turn, to the effect of pain relief drugs, in the same way as adults.

Babies have to come from somewhere!

In the UK, it is quite common for Universities and Colleges that don’t have Home Office ASPA licenses to pursue research using non-protected animals, including larval fish, but in order to use zebrafish embryos and larvae, you need breeding adults to produce them.

If the intention is to use their offspring in subsequent studies, but only up to 5 days after fertilisation, then the researcher, their work, and their institution do not by current law, need a licence to do so. They will not be inspected, and do not have to submit statistics of their animal use. There is also no legal obligation for the research to undergo review by an Animal Welfare and Ethical Review Body.

Even if the facility is judicious with the conditions of their breeding and husbandry, there is a significant risk that sentient animals may be used, and also that the legal time will be breached inadvertently, leaving the researchers to fall foul of the law.

These animal welfare, ethical and legal issues aren’t unique to zebrafish. Embryonic and foetal birds, reptiles, amphibians, and cephalopods also develop outside their mother’s body and may also be used as a ‘partial replacement’.

But the embryonic and foetal forms of mammals are also not protected by ASPA (but their mothers are). All mammals, birds and reptiles are only protected animals once they have reached the last third of their gestation or incubation period.

Fish and amphibian larvae become ‘protected animals’ once they are capable of feeding independently. Cephalopods like octopuses, squid, and cuttlefish become ‘protected animals’ from the point when they hatch from their eggs.

What can we do about this?

My lovely colleague Amy has already written about how not all Replacement is equal, and that it is time to update the legal definition of replacement to reflect a stronger ambition to fully replace animal use in science and be transparent about it.

We could also ensure better oversight of the use of immature stages of sentient animals in scientific research by bringing them all under the protection of the law. But we’re still waiting for decapod crustaceans to be included in ASPA, despite overwhelming evidence of their sentience now being recognised. Adding animals to ASPA does not seem to be straightforward.

But with vertebrates, surely it should be easier.

If their parents are already protected, do you think we should argue that their developing young should be too?