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The lives of most captive fish are at once boring and bizarre. A zebrafish in a genomics laboratory or an Atlantic salmon in a fish farm generally endures conditions that would be utterly alien to their wild counterparts. The light is blinding, the walls smooth glass or concrete, the floor ungilded by so much as a plant. Even the water lacks the chemical complexity of a river or sea. It’s both a profoundly unnatural existence and, likely, a dull one.
Until recently, those conditions didn’t overly bother too many people. Compared to primates and other lab mammals, fish were perceived as insentient dullards who didn’t require stimulus; as a result, many researchers believed that the logistical challenges of enriching their subjects’ lives outweighed the benefits. But those ways of thinking are being replaced by an appreciation for fishes’ social and cognitive complexity, which raises the question: How do barren environments affect the lives of captive fish? And what might richer ones do for them?
A fish who is chronically bored and anxious may also be a cognitively stunted one.
A rising tide of research suggests that better, more stimulating environments for captive fish could improve scientific research and commercial yields as well as the lives of the fish themselves. “We should give animals the best possible environment that we can,” says Nick Jones, a biologist at the University of Bayreuth who specializes in fish cognition.
An archerfish, stickleback, or gourami in Jones’s laboratory experiences comparatively appealing circumstances. Shade covers block overhead lights and allow fish to take shelter, as they would beneath streamside vegetation in the wild. Aquatic plants and plastic tubes of various dimensions offer additional cover and novelties to explore. They have gravel in which to dig and scratch themselves.
Similar accoutrements can be found in most home aquaria—yet in the worlds of commercial and academic fish husbandry, such adornments have historically been more exception than rule. When, in 2007, scientists reviewed more than 700 studies of methods used by researchers to enrich the lives of captive animals, less than half of 1 percent involved fish. A decade later, many researchers working with zebrafish—whose rapid development and genomic similarity to humans make them useful as so-called model organisms—reported reluctance to enrich their tanks with plants and gravel, in large part because providing even these minimal stimuli would increase the cost and labor of caretaking.
In some ways the situation for captive fish could be compared to that of mice in laboratories several decades ago, says Chloe Stevens, a senior scientist at the Royal Society for the Prevention of Cruelty to Animals. Back then lab-rodent welfare was largely an afterthought; mice were often kept in empty cages. Today researchers commonly offer their rodents climbing structures, spinning wheels, and burrowing material. There’s still plenty of room for improvement, but conditions are far better than before.
Similar progress may be overdue for captive fish. In the past few decades, scientists have amassed a wealth of insight into piscine intelligence; not only do fish almost certainly feel pain,1 but researchers have described some species craving novelty,2 forming partnerships,3 and arguably demonstrating advanced self-awareness.4 A stimulating, safe environment is perhaps no less integral to the well-being of such complex creatures than nourishing diets or clean water.
“Welfare is important because animals have moral value, and we have responsibilities to them,” Stevens says. “But it’s also important for good quality science.”
In a 2021 review, Stevens identified a number of ways in which zebrafish labs could improve their occupants’ welfare.5 Adjusting lighting to mimic natural dawn and dusk phases, swapping out flakes for brine shrimp and other live foods, and even gluing pictures of gravel to tank floors can make zebrafish healthier and more stimulated. Yet such measures, says Stevens, often earn pushback from researchers concerned about confounding their results. Model organisms are supposed to be raised in conditions that other laboratories can easily replicate, in which every variable is standardized; even the texture of gravels or the dimensions of shelter pipes are potentially important. If every lab were to offer fish different enrichment, how much credence could researchers give their data?
But housing fish in spartan conditions poses its own scientific issues. Some biomedical researchers have begun to let their lab mice roam free, on the grounds that captive rodents may not be reliable subjects for pharmaceutical testing. Likewise, a fish who is chronically bored and anxious may also be a cognitively stunted one. In one telling experiment, scientists discovered that zebrafish reared in the presence of plants, shelters, and “novel objects” were subsequently better at navigating mazes and developed bigger brains.6 Tossing a few stones into an otherwise empty tank produces larger cerebella in baby salmon7; exposing rockfish to plastic shelters strengthens their memories.8 A fish whose intellect is challenged by her environment may be a more accurate representative of her species’ aptitudes.
Zebrafish are not the only fish who benefit from enrichment. Take salmonids, perhaps the world’s most widely husbanded family of fish. In stark, unenriched tanks, young trout and salmon nip each other’s dorsal fins like playground bullies, a struggle for dominance that leads to a condition called fin erosion. When researchers added tree branches and shade covers to a tank full of baby steelhead, however, the fish’s fins markedly improved, perhaps because the structure gave the fish places to hide from one another.9
Other studies have shown that the addition of river rocks and PVC tubes reduces the stress levels of juvenile chinook salmon,10 and that the combination of gravel and shelter helps Atlantic salmon survive bacterial epidemics.11 In a 2022 report, the Aquatic Life Institute noted that environmental enrichment in salmon farms and other aquaculture facilities “could directly translate to decreased mortality rates, enhanced growth rates, improved feed conversion ratios, and resistance to disease.”12
Faithfully simulating salmon requires a lot more than a few tree branches and an ersatz heron.
There is, too, a burgeoning conservation case for the enrichment of captive salmon. As salmon populations have dwindled due to overfishing, habitat loss, hydroelectric dams, and other factors, biologists have supplemented wild stocks with legions of hatchery-raised fish. The returns have been mixed, at best—in part, perhaps, because the hatchery’s artificial environment favors traits, like undue boldness or aggression, that leave captive-raised fish or their progeny maladapted for life in the wild.13 When hatchery-reared fish interbreed with wild animals whose genes have been honed by millennia of evolution, entire runs can suffer.
In response to those concerns, hatchery managers have begun to deploy enrichment to more closely emulate natural conditions. In one telling experiment, Norwegian researchers reared two groups of Atlantic salmon parr: one in barren tanks, the other in the presence of plants and rocks. Then they released the salmon into the River Imsa and, months later, intercepted them in traps as they migrated seaward. All told, the enriched salmon were around 50 percent more likely to survive their stint in the wild.14 Likewise, a 2014 study showed that salmon exposed to camouflage shade cloths, meals of aquatic invertebrates, and “a realistic full-size model of a grey heron” eventually settled in better wild habitats and survived at higher rates than their understimulated counterparts.15
“If you think about hatcheries as being like factory farms, then maybe there are more sustainable kinds of ‘farming’ practices that we should be using,” says Seth White, director of the Oregon Hatchery Research Center.
Of course, natural rivers are so immensely complex that faithfully simulating them requires a lot more than a few tree branches and an ersatz heron. “When you talk about enrichment, people tend to think about what you can add to a tank or a raceway”—an artificial channel used in aquaculture—“to make it more like a wild environment,” White says. Yet wild fish inhabit milieus of boundless sensory intricacy, in social hierarchies that are elaborate beyond our understanding.
The hatchery research center is conducting a series of experiments that manipulate variables beyond plants and gravel. Can adding amino acids and other olfactory cues prime the keen sense of smell with which wild salmon navigate rivers? Might pumping flowing water into a once-still tank induce fish to rearrange themselves into more natural schools?
“It kind of expands the definition of enrichment,” White says. “Figuring out which of those things matter is the direction that the research is going.”
Granted, enrichment doesn’t alwaysproduce positive outcomes. One 2009 study, for example, found that adding plants and rocks didn’t make minnows any less anxious.16 Adding enrichment to tanks may sometimes even make zebrafish more aggressive, by introducing limited resources over which to fight.17 “The general level of understanding,” Nick Jones and his colleagues wrote wryly in a 2021 review, “might be summarized as some [enrichment] is better than none, some of the time.”18
To Jones, the inconsistent results bespeak the field’s embryonic stage. Fish clearly seem to benefit from more structurally complex environments—but what kind of structures do they prefer? Made from what materials, and in what dimensions and configurations? The details vary not only among species, but also among life stages and social settings. Juvenile eels need sandy substrates in which to burrow, but adults don’t; a whole school of baby salmon might benefit more from submerged branches than just a few fish. The future of captive-fish husbandry may involve refining these specifics—and transforming enrichment from an ad hoc practice to one grounded in empirical evidence.
“In most cases, enrichment is just what people have done in a lab ever since it started: They had some cheap plastic tubes, they threw them in, and the fish hid in them, so it seemed fine,” Jones says. “Maybe we can push the field forward by focusing on the details of what, why, and how you use enrichment.” 
Lead image: Grigorev Mikhail / Shutterstock
References
1. Sneddon, L.U. & Leach, M.C. Anthropomorphic denial of fish pain. Animal Sentience 3 (2016).
2. Graham, C., von Keyserlingk, M.A.G., & Franks, B. Free-choice exploration increases affiliative behavior in zebrafish. Applied Animal Behaviour Science 203, 103-110 (2018).
3. Bshary, R, Hohner, A., Ait-el-Djoudi, K., & Fricke, H. Interspecies communicative and coordinated hunting between groupers and giant moray eels in the Red Sea. PLoS Biology 4, e431 (2006).
4. de Waal, F.B.M. Fish, mirrors, and a gradualist perspective on self-awareness. PLoS Biology 17, e3000112 (2019).
5. Stevens, C.H., Reed, B.T., & Hawkins, P. Enrichment for laboratory zebrafish—a review of the evidence and the challenges. Animals 11, 698 (2021).
6. DePasquale, C., Neuberger, T., Hirrlinger, A.M., & Braithwaite, V.A. The influence of complex and threatening environments in early life on brain size and behavior. Proceedings of the Royal Society B 283, 20152564 (2016).
7. Kihslinger, R.L. & Nevitt, G.A. Early rearing environment impacts cerebellar growth in juvenile salmon. Journal of Experimental Biology 209, 504-509 (2006).
8. Zhang, Z., et al. Barren environment damages cognitive abilities in fish: Behavioral and transcriptome mechanisms. Science of the Total Environment 794, 148805 (2021).
9. Berejikian, B.A. Rearing in enriched hatchery tanks improves dorsal fin quality of juvenile steelhead. North American Journal of Aquaculture 67, 289-293 (2005).
10. Cogliati, K.M., Herron, C.L., Noakes, D.L.G., & Schreck, C.B. Reduced stress response in juvenile Chinook Salmon reared with structure. Aquaculture 504, 96-101 (2019).
11. Räihä, V., Sundberg, L.-R., Ashrafi, R., Hyvärinen, P., & Karvonen, A. Rearing background and exposure environment together explain higher survival of aquaculture fish during a bacterial outbreak. Journal of Applied Ecology 56, 1741-1750 (2019).
12. Gonzalez, T. An industry shift towards environmental enrichment in aquaculture. Aquatic Life Institute (2022).
13. Blouin, M.S., et al. Offspring of first-generation hatchery steelhead trout (Oncorhynchus mykiss) grow faster in the hatchery than offspring of wild fish, but survive worse in the wild: Possible mechanisms for inadvertent domestication and fitness loss in hatchery salmon. PLOS ONE (2021).
14. Mes, D., et al. Effects of environmental enrichment on forebrain neural plasticity and survival success of stocked Atlantic salmon. Journal of Experimental Biology 222 (2019).
15. Roberts, L.J., Gough, P.J., Forman, D.W., & de Leaniz, C.G. Silver spoons in the rough: Can environmental enrichment improve survival of hatchery Atlantic salmon Salmo salar in the wild? Journal of Fish Biology 85, 1972-1991 (2014).
16. Xu, C., et al. The effect of environmental enrichment on laboratory rare minnows (Gibiocypris rarus): Growth, physiology, and behavior. Animals 12, 514 (2022).
17. Bhat, A., Greulich, M.M., & Martins, E.P. Behavioral plasticity in response to environmental manipulation among zebrafish (Danio rerio) populations. PLOS ONE (2015).
18. Jones, N.A.R., Webster, M.M., & Salvanes, A.G.V. Physical enrichment research for captive fish: Time to focus on the DETAILS. Journal of Fish Biology 99, 704-725 (2021).
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