Guest blog post: Global change, individual behaviour, and trout population persistence

New experiments reinforce that behavioural plasticity can be key for coping with environmental changes

Guest blog post by Daniel Ayllón and Steve Railsback

Early in the morning, Daniel Ayllón and his research mates at the Universidad Complutense de Madrid drive towards the mountains near Madrid. They’re out to survey streams where the endangered Southern Iberian spined-loach and Northern Iberian spined-loach used to coexist. We say “used to,” because once again they fail to find the Northern Iberian spined-loach, probably locally extinct. Such extinctions are not unusual, as freshwater fishes are one of the most threatened groups of animals in the world. There are still many brown trout there, though; the water is still cold enough for them.

Salmonids (trout, salmon and char) are especially challenged by climate change because they need cold, oxygenated and clean water. Trout populations at low altitudes or low latitudes are thus particularly at risk; many in the Iberian Peninsula have been declining for decades as rivers warm and dry. Climate models project a bleak future: such Mediterranean populations will face hotter and drier streams, with more frequent and longer droughts and heat waves, and increasing competition from warm-water fish.

A photo of a brown trout swimming over a bed of smooth pebbles in clear water.
Brown trout (Salmo trutta). Photo by J. R. Pérez (AEMS-Ríos con Vida archive)

Despite these changes, local extinctions of trout are still rare, because salmonids are among the most adaptable and resilient of freshwater fishes. They are changing their physiology and phenology, growth and reproduction patterns, and life-history strategies to adjust to the new environmental conditions, via evolutionary, plastic and behavioural mechanisms. While evolutionary ecologists typically focus on genetic adaptation to forces such as climate change, behavioural plasticity could be even more important, because it is fast, reversible and often predictable.

In fact, thermoregulatory movements seem a ubiquitous behavioural mechanism in salmonids: individuals move up and down river networks to find less-stressful temperatures and better growth potential. Behavioural plasticity in circadian activity and habitat selection (deciding when and where to feed) also help trout resist short-term environmental changes. However, we don’t know how important changes in circadian activity─or behaviours in general─are to long-term population persistence in the face of climate change. So to shed light on this question, in a recent work published in Individual-based Ecology, we ran two virtual experiments using the inSTREAM individual-based model to represent a trout population in northern Spain.

A photo of a river flowing between rocky banks, surrounded by greenery and towering mountains under a clear sky.
The Roncal study site on the River Eska (northern Spain). Photo by Benigno Elvira

Steve Railsback and his colleagues at Cal Poly Humboldt University and the US Forest Service’s Pacific Southwest Research Station in Arcata, California, have been developing, testing, and applying inSTREAM for 25 years. The central idea of individual-based models (IBMs) and of individual-based ecology in general is that a biological system can be described through its individual agents, their environment, and the interactions among agents and between agents and environment. The agents of a system (for example, all fish in a population) are modelled as unique and autonomous individuals with their own properties.

The controlled experiment of Harvey and White to quantify how trout trade off feeding vs. predation risk. The experimenters trained wild trout to feed at this dispenser, and then moved it to increasingly risky habitat. The feeding rate needed to keep the trout from leaving increases with the risk it perceives. IBMs like inSTREAM use knowledge about individual behaviour from experiments like this to predict complex population responses. Video by Jason L. White.

Agents also have behaviours: they make decisions, following simple rules or algorithms, independently of other individuals, and seek objectives such as surviving to reproduce in the future. These behaviours are adaptive: agents’ decisions depend on their state and the state of their environment. In this way, population-level results actually emerge from the behaviour of the individuals. In inSTREAM, model trout decide whether to feed vs. hide from predators at different times of day, assumed a trade-off between the need to feed and the predation risk it poses. Temperature has a strong effect on this trade-off because a fish’s metabolic rates, and thus the amount of food it needs, increase sharply with temperature.

A photo of three people wearing waders engaged in field research in a river.
Three members of the research team at the UCM conducting habitat surveys at the Roncal study site. In IBMs like inSTREAM, modelled populations and their environment are characterised by field data collected in surveys like this. Photo by Benigno Elvira.

What did we learn with our IBM? First, our simulations show what behavioural ecologists know from experiments: that during warm summers trout can meet their metabolic requirements only by feeding at multiple times of day and segregating temporally, so that fish of different size can feed at the same spot but at different times of day. Feeding during daytime is more profitable but riskier, while doing it at night is safer but less efficient, and feeding during twilight provides near-daytime growth and somewhat-reduced risk.

We then analysed how model trout change their circadian foraging behaviour under increasing climate change. As we expected, trout showed great behavioural plasticity: trout of all ages responded to warmer and drier conditions by increasing daytime feeding and overall foraging activity, although there were differences across age classes in the distribution of daily activity. Our second experiment used a great advantage of IBMs as a virtual laboratory: we can run experiments that are impossible in reality. We tested the importance of behavioural plasticity by simply turning the behaviour off. In our simulations, virtual populations of trout capable of flexible circadian feeding were more resistant to climate change─had higher biomass and a more balanced age structure─than were populations of trout that feed only during daytime.

These experiments reinforce that behavioural plasticity can be key for coping with environmental changes, so we shouldn’t minimise its relevance when predicting the persistence of salmonid populations in warming and drying rivers. This conclusion no doubt also applies to other taxa that have powerful adaptive behaviours.

This study epitomises individual-based ecology, the subject of Pensoft’s new journal: we use what we know from empirical research on individual physiology and behaviour, in an individual-based model, to study complex population responses of direct relevance to our changing world.

Research article:

Ayllón D, Railsback SF, Harvey BC, Nicola GG, Elvira B, Almodóvar A (2025) Behavioural plasticity in circadian foraging patterns increases resistance of brown trout populations to environmental change. Individual-based Ecology 1: e139560. https://doi.org/10.3897/ibe.1.e139560

Deep black as midnight: striking new moray eel discovered in Central Indo-Pacific river mouths, named after god of the underworld

This new moray eel is named after the underworld god Hades for its distinctive habitat, unique behaviors, and most notably, its deep, dark coloration.

The Hades’ snake moray (Uropterygius hades), a dark brown, slender snake moray eel, has chosen the road less traveled, thriving in dim and muddy river mouths, unlike most of its marine moray eel relatives. It is widely distributed across the Central Indo-Pacific, and has been found in southern Japan, Taiwan, the Philippines, southern Java, and Fiji. This new moray eel was named after Hades, the god of the underworld, due to its unique habitat, burrowing behavior, high sensitivity to light, and most notably, its deep, dark coloration.

Live photo of Uropterygius hades. Image credit: Dr Wen-Chien Huang

Scientists Dr Wen-Chien Huang, Dr Rodulf Anthony Balisco, Dr Te-Yu Liao, National Sun Yat-sen University, Taiwan, Western Philippines University, the Philippines, and Dr Yusuke Hibino, Kitakyushu Museum of Natural History and Human History, Japan, describe this new species in a paper published in the open-access journal ZooKeys. They named it after Hades, the underworld god, to emphasize its imposing appearance and its habitat in dim, turbid environments. This idea was inspired by Dr. Wen-Chien Huang, who was influenced by Ralph Fiennes’ portrayal of Hades in the movie Clash of the Titans.

Live photo of Uropterygius hades. Image credit: Dr Wen-Chien Huang

There are approximately 230 species of moray eels worldwide, with most inhabiting marine environments. Only one species has been confirmed to spend the majority of its life in freshwater. Some marine species, like the slender giant moray (Strophidon sathete), can tolerate and occasionally enter lower-salinity environments such as river mouths. However, moray eels specifically adapted to estuarine habitats are exceedingly rare.

The discovery of Hades’ snake moray was actually accidental, when the three researchers from National Sun Yat-sen University investigated the cave of the Puerto Princesa Subterranean River, aiming to survey the aquatic fauna and targeting a cave eel species, the bean-eyed snake moray (Uropterygius cyamommatus). This eel, with its highly reduced eye size, is considered an ideal example for studying the evolutionary processes that allow eels to adapt to cave environments. However, the researchers did not find any bean-eyed snake morays in the cave; instead, they collected a slender moray with a conspicuous, uniformly deep dark color.

Fresh specimen of Uropterygius hades. Credit: Dr Wen-Chien Huang

When kept in an aquatic tank, the Hades’ snake moray exhibits tail-first burrowing behavior, which is rarely seen in moray eels. Additionally, it is highly sensitive to light, consistently attempting to hide when exposed to it. Its small eyes—thought to be an adaptation to low-light environments—and its reduced number of head sensory pores—believed to help avoid clogging by the substrate—suggest that this species might be an excellent burrower, relying primarily on chemoreception rather than vision to detect prey or avoid predators.

Original source:

Huang W-C, Hibino Y, Balisco RA, Liao T-Y (2024) Description of a new uniformly brown estuarine moray eel (Anguilliformes, Muraenidae) from the Central Indo-Pacific Ocean. In: Ho H-C, Russell B, Hibino Y, Lee M-Y (Eds) Biodiversity and taxonomy of fishes in Taiwan and adjacent waters. ZooKeys 1220: 15-34. https://doi.org/10.3897/zookeys.1220.129685

Chinese scientists discover a new species of catfish in Myanmar

During a survey of the freshwater fishes of the Mali Hka River drainage in the Hponkanrazi Wildlife Sanctuary, Myanmar, scientists Xiao-Yong Chen, Tao Qin and Zhi-Ying Chen, from the Chinese Academy of Sciences (CAS), identified a new catfish species among the collected specimens. It is distinct with a set of morphological features including its mouthparts and coloration. The discovery is published in the open access journal ZooKeys.

The new catfish belongs to a genus (Oreoglanis) of 22 currently recognised species. They are characterised with unusual teeth. While pointed in the upper and the back of the lower jaw, the teeth at the front of the lower jaw are shorter and broad. The latter are placed in a continuous dent. Out of the 22 species of the genus, there are only two known to live in Myanmar.

The new catfish, scientifically named Oreoglanis hponkanensis, has a moderately broad and strongly depressed head and body, and small eyes. The species is predominantly brown in colour, with light yellow belly and several yellowish patches across the body. Noticeable are also two round, bright orange patches in the middle of the fin.

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Original source:

Chen X-Y, Qin T, Chen Z-Y (2017) Oreoglanis hponkanensis, a new sisorid catfish from north Myanmar (Actinopterygii, Sisoridae). ZooKeys 646: 95-108. https://doi.org/10.3897/zookeys.646.11049