October 11, 2022

“Fish are fascinating in a way that hasn’t quite filtered to the public,” says Raf Freire, a professor at Charles Sturt University in Australia. “There’s evidence that they use tools in the wild, there are fabulous examples of their capacity for long-term memory and learning, and in the past few years, we’ve seen evidence for their behavior being controlled by emotional processes, much like us.”

A growing body of evidence shows that fish have affective states, otherwise described as emotions or moods, that influence their behavior. Recent work by Freire and other animal behaviorists indicates that when fish have negative experiences — being housed with a larger, predator-like fish, for example — it can induce pessimistic decision-making.

With Wild Animal Initiative funding, Freire and PhD student Leia Rogers plan to study affective state in juvenile Murray cod (Maccullochella peelii) to better understand why so many young fish don’t make it to adulthood. Out of thousands of larval fish, the vast majority of them die before reaching maturity. 

Freire’s project will explore the relationship between welfare and recruitment — the rate at which juveniles survive to join the ranks of reproductive adults — by focusing on juvenile Murray cod in Australia’s longest river system, investigating how poor water conditions might impact a fish’s affective state.

Recruitment and welfare

The Murray-Darling Basin is a river system in Australia that spans almost 14% of the continent’s total area and is just over 2,000 miles in length. The basin provides habitat for some of Australia’s most famous species, such as the duck-billed platypus (Ornithorhynchus anatinus), but also hosts some lesser-known inhabitants, including turtles, amphibians, and sleek, aquatic rodents called rakali (Hydromys chrysogaster).

The Murray-Darling Basin is also home to the Murray cod, Australia’s largest freshwater fish. Murray cod can live for decades and grow up to six feet long. However, juvenile fish of this species begin life much smaller, around a third of an inch long. 

“In certain ways, fish reproduction is similar to plant reproduction in that some species produce hundreds of thousands of offspring,” Freire says. “But very few of them survive to adulthood. The capacity to go from juvenile to adult is a real bottleneck for fish populations.”

Freire has dedicated his career to studying animal behavior, noting that he’s always been fascinated by “working out what’s happening in the minds of animals.” He responded to Wild Animal Initiative’s call for proposals, he says, because it seemed like an excellent opportunity to investigate recruitment from the perspective of individual juvenile fish. 

As young wild fish grow from larvae to adults, they face a complex series of challenges — predation, water quality, food scarcity, and more — that in many cases lead to death. Most research on recruitment looks at how entire populations fare, but Freire wants to study individual fish to see if welfare and recruitment are linked. 

“Murray cod produce very few eggs and have parental care, which we call slow-pace living,” Freire explains. “They’re more sensitive to variations in the environment because they don’t have such a fast turnover, so if we can understand what impacts welfare in this species, we might also be able to help the other species in this river system that are less sensitive.”

The judgment bias test

Freire and his team plan to use an experimental design called the judgment bias test to assess affective state in wild juvenile fish. Scientists have used judgment bias tests to assess domesticated and farmed animal welfare for years, and Freire has first-hand experience with the test in his previous work.

“This is a rapidly changing field of research, but at the moment, this test is the most convincing at showing that animals have affective states,” Freire notes.

The judgment bias test typically starts with a training phase in the lab, in which the animal is introduced to positive and negative stimuli. Over time, the animal learns that one stimulus will lead to a good outcome, and the other will lead to a bad outcome. A positive stimulus might be similarly sized fish for a social species, and a negative stimulus might be a large fish that resembles a predator.

Once an animal is familiar with the two training scenarios, researchers introduce a neutral stimulus, such as a medium-sized fish. If the animal is fresh from experiencing negative events, they might interpret the neutral stimulus in a pessimistic way and retreat. But animals who have positive experiences might be in a more optimistic mood and choose to investigate. 

“I like to tell my students that it’s like receiving a bank letter,” Freire says. “If you’re in a good mood, you might be thinking it’s a refund, but if you’re in a bad mood, you might expect that your check bounced. Mood influences how we interpret ambiguous situations, and these interpretations can reveal affective states.”

It’s one thing to perform this test under lab conditions, but quite another to attempt it in the wild. To carry out this experiment on wild fish, Freire and his team will modify the test to take advantage of natural behaviors in juvenile Murray cod, such as the way they approach light sources at night, attracting them into the field experiment. The research team will present the juvenile Murray cod with three artificial fish intended to serve as positive, negative, and neutral stimuli — a same-size fish resembling a fellow juvenile cod, an intermediate-size fish, and a large fish, which will mimic a predator. 

The researchers will record how the young fish respond to each stimulus. They’ll also keep track of water quality metrics at each test site, measuring temperature, pH, salinity, dissolved oxygen, turbidity, conductivity, and water flow rate at various depths. The test is minimally invasive, with fish being studied at night and released the following morning.

If juvenile Murray cod respond more pessimistically in low-quality water conditions, as Freire’s team predicts, it supports the hypothesis that water quality can impact affective state. In turn, it models a way to measure welfare in wild juvenile fish. 

Working together for welfare

The results of this study could help improve millions of lives. The vast majority of wild fish are juveniles, given the enormous number of larvae that fish can create across their lifetimes. By better understanding affective states, Freire imagines this study could prove useful as an early warning system for conservation, and that individual fish welfare could signal larger problems on the population level before mass deaths occur. Since Murray cod is a species of conservation concern, improving survival rates could help populations rebound.

It’s important to note that from a wild animal welfare perspective, improving the lives of wild fish serves as the ultimate goal. Wild fish welfare is so understudied that it’s hard to know which interventions might help the most, but looking at survival is a good place to start.

Freire says he’s pleased to see the field of wild animal welfare gain traction and bring together scientists from a variety of backgrounds. “Science tends to work in silos, but we gain amazing advances when we get the cross-fertilization of ideas,” he says. 

Freire’s proposed study is funded by Wild Animal Initiative’s Grants Program. This is the sixth story in a series of features on our spring 2022 cohort of grantees. Please subscribe to our newsletter to get future stories delivered straight to your inbox.