Wild fish welfare: injuries and deformities
A new paper by Wild Animal Initiative’s Strategy Researcher, Simon Eckerström Liedholm, takes a first look at the welfare impacts of injuries and deformities in wild fish.
June 8, 2026
Wild fish are collectively the most numerous group of vertebrates on Earth. Learning about the welfare challenges they face and how we might be able to mitigate them is therefore a priority in wild animal welfare science. But because research on fish welfare has focused mainly on farmed fish, our knowledge at this stage is very limited.
One particularly promising entry point into the study of wild fish welfare is injuries and deformities.
External injuries and morphological deformities likely correlate with reductions in welfare, and can be easily observed and non-invasively measured — making them useful and practical potential indicators of wild fish welfare.
“We know that wild fish experience injuries and deformities from natural threats like parasites and predators, and from anthropogenic harms like pollution and fishing,” says Wild Animal Initiative Strategy Researcher Simon Eckerström Liedholm. “Yet we know surprisingly little about how widespread injuries and deformities are in wild fish. If we hope to understand whether interventions aimed at reducing injuries and deformities could have a meaningful impact on wild fish welfare, we need to establish this foundational knowledge first.”
To begin filling that knowledge gap, Simon reviewed studies from six major fish biology journals to quantify the prevalence of injuries and deformities in wild fish.
A first look at the numbers
After applying a set of selection criteria — empirical studies that reported observational frequency data on externally visible injuries or deformities in free-living fish — Simon retained 30 studies out of 540 initial search hits. The retained data contained 43 prevalence estimates across 22 species of fish. Across all estimates, an average of about 15% of sampled wild fish showed signs of injury or deformity.
Injuries alone had an average prevalence of around 21% (across 27 estimates), while deformities alone averaged about 7% (across 13 estimates). The lower figure for deformities is likely explained at least in part by mortality: Fish with deformities in early life stages tended to have higher mortality rates, so they are likely underrepresented in any snapshot taken at a single point in time. In other words, the share of wild fish who will experience an injury or deformity at any point in their lives is almost certainly higher than the share experiencing those issues at a given time.
But if lifetime prevalence of deformities was underestimated, instantaneous prevalence of injury or deformity was likely overestimated. Many of the studies Simon reviewed seemed to be motivated by suspected disease outbreaks, sudden mortality events, or pollution from nearby industries, meaning that the sampled populations were often a priori likely to show signs of injury or deformity. So while lifetime prevalence would likely be higher than instantaneous prevalence, this bias suggests that instantaneous prevalence in these studies is likely overestimated compared to a randomly selected wild population of fish.
Limitations in the literature
A clear oversight in the literature was bias in species coverage. A phylogenetic signal analysis showed that the fish species included in the reviewed studies were more closely related to each other than would be expected by chance (Pagel's λ = 0.43; p < 0.001). Taxa like salmonids (e.g., salmon, trout, and char) were particularly overrepresented. This makes sense from a fisheries research perspective: “These species have clear economic value, so they get studied more often,” Simon says. “But from a welfare perspective, it’s unhelpful. Many of the fish who are likely to suffer from injuries on an even larger scale — the hundreds of billions of individuals released as bycatch by large fishing operations, for example — have received almost no attention.”
What injuries and deformities can (and cannot) tell us
It’s important to understand that the absence of injuries and deformities does not guarantee good welfare. Externally visible injuries and deformities likely have positive predictive value (their presence is a reasonably good indicator of poor welfare), but relatively poor negative predictive value, in that their absence doesn't necessarily indicate good welfare. Many things that cause poor welfare won't show up in an external examination. Internal parasites, low oxygen levels, high levels of nitrogenous compounds, social isolation, and chronic fear of predators are just a few issues that don’t necessarily have externally visible effects.
But because injuries and deformities have the potential to be measured cost effectively and without unnecessary disturbance, they represent a promising method for assessing wild fish welfare at scale. And underwater camera technologies and automated image recognition will likely continue to improve, making it possible to monitor wild fish welfare even more cheaply on a large scale. There are already tools that can automatically detect fish in underwater footage and even classify species identity, but there's still a lot of work to be done before we can automatically classify the presence or absence of injuries and deformities for many different species of wild fish.
This paper makes the case that wild fish welfare deserves serious scientific attention, and that external injuries and deformities seem like a practical place to start. For a more complete picture of the problem, future work will need to examine a much wider range of species, and compare these external measures with other welfare indicators, including assessments like cognitive bias tests and population-level information like mortality rates.
If you are a researcher interested in exploring those questions, please get in touch.
“Welfare implications of injuries and deformities in wild fish” was published open access in the journal Animal Welfare on April 15, 2026. You can read it in full here.
