Self-Awareness in Aquarium Fish

ON 08/14/2025 AT 05:30 PM

Fish are intelligent and sensitive creatures, and many have significant self-awareness.

The question of self-awareness in animals, particularly in fish, has been a subject of scientific inquiry for decades. The notion that fish, including those we keep in aquariums, might possess a level of self-awareness comparable to higher mammals has been a topic of debate among ethologists, biologists, and animal behaviorists, but not aquarists who closely observe and truly care for their fish--most of whom recognize the intelligence of their fish.

This article explores the fascinating topic of self-awareness in aquarium fish, drawing on scientific studies and real-world experience to shed light on this intriguing subject.

Understanding Self-Awareness

Before delving into the topic, it is essential to understand what self-awareness means. It refers to the ability of an individual to recognize themselves as separate entities from their environment and other individuals (Gallup, 1970). This concept is often tested through the mirror self-recognition test (MSR), where an animal is marked with a colored dot and exposed to a mirror. If the animal tries to remove or react to the mark on its body, it is considered to exhibit self-awareness. This test is of course not a good measure of self-awareness, but it is what is commonly used. Most non-human beings never encounter a mirror and so have no concept of a reflection. If one has never seen their reflection they don't know what they look like and therefore are unlikely to recognize their own image, at least at first. This would apply to humans as well. Any human who has never seen a mirror or their own reflection would be unlikely to initially recognize themselves, except by reason of deduction due to their proximity to the mirror and noticing the matching movements. Most human children don't recognize themselves in a mirror until after 20 months of age, well after most are able to talk and walk.  

Self-Awareness in Fish

The concept of self-awareness in fish came increasingly under the spotlight following a study conducted by Kohda et al. (2019), which demonstrated that cleaner wrasse could pass the mirror test. The fish displayed unusual behavior when a mirror was introduced into their tank, suggesting that they recognized the image as their reflection. The cleaner wrasse tried to remove the mark on their bodies after seeing their reflection, indicating a level of self-recognition. However, in the wild, a wrasse would never encounter a mirror and wouldn't have any concept of a reflection. In an aquarium they might see their reflection in the tank glass or the water surface. 

The study's findings were met with skepticism. Some researchers argue that the fish were merely exhibiting 'contingency checking' or 'social behavior,' where they react to the mirror image as if it were another fish (De Waal, 2019). Others contend that the mirror test is not a definitive measure of self-awareness, as it assumes that animals perceive and interpret their reflections the same way humans do (Bshary, 2019).

Nonetheless, the study by Kohda sparked interest in further exploring self-awareness in fish. The cleaner wrasse's behavior suggests that fish may possess a more complex cognitive capacity than some scientists imagined. 

Cognitive Abilities in Fish

The potential for self-awareness in fish is supported by studies showing that fish have remarkable cognitive abilities. Brown (2015) found that fish can learn and remember complex tasks, recognize other individuals, and even display long-term memory. Fish also exhibit problem-solving skills, such as navigating mazes or using tools to obtain food (Bshary, Wickler, & Fricke, 2002). 

A review of 200 studies entitled "A Kettle of Fish: A Review of the Scientific Literature for Evidence of Fish Sentience" published in 2022 found "an abundance of evidence for fish sentience in the published scientific literature."

These findings negate the beliefs that fish have a 'three-second memory' or 'don't feel pain.' Instead, they suggest that fish possess a level of cognitive sophistication that would likely extend to self-awareness.

The UK government's Animal Welfare (Sentience) Act 2022 formally recognizes all vertebrates, including fish, as sentient beings. This means they are acknowledged as capable of experiencing feelings such as pain, pleasure, distress, and joy, This recognition also implies self-awareness. 
 
The Scottish Animal Welfare Commission 2025 report titled "Ascribing Sentience to Fish: Potential Policy Implications" found:
  1. Fish brains feature analogous structures to mammalian amygdala (for emotions) and hippocampus (for memory). They possess nociceptors (pain-detecting receptors), opioid systems that modulate pain (responsive to analgesics like morphine), and hormones like oxytocin/isotocin involved in social bonding. Sensory systems for vision, hearing, and chemical detection are advanced, often surpassing those in mammals.
  2. Fish display adaptive responses to threats, including avoidance learning, fear (e.g., fight, flight, freeze, or hide behaviors), and stress coping. They exhibit complex cognition, such as tool use, numeracy, spatial navigation, social cooperation, reciprocity, and learning from others. Emotional states are evident through judgment bias (e.g., pessimism after stress) and motivational trade-offs (e.g., enduring pain for rewards).
  3. Studies on farmed salmon show observers can reliably detect emotional expressions, with behaviors shifting under stress to indicate negative or positive states.

Implications for Aquarium Fish Keeping

The sentience and self-awareness in aquarium fish raise important considerations for fish keeping. Because fish are capable of self-recognition and complex cognition, it is crucial to provide them with an environment that stimulates their mental capacities. 

Aquarium owners should consider incorporating features that encourage exploration and problem-solving, such as mazes, hiding spots, and interactive toys using non-plastic and non-toxic materials. Regularly changing the tank's layout can also provide mental stimulation for the fish (Keenleyside, 1991), but may also cause stress for some. So, changes to layout should be partial and gradual. Better to add a single new rock or plant than to completely change the layout.  

Furthermore, the potential for self-awareness in fish emphasizes the importance of ethical fish keeping. This includes providing adequate space, maintaining optimal water conditions, and ensuring a balanced diet. It also underscores the need for humane treatment of fish, both in aquariums and in the wild.

Conclusion

The topic of self-awareness in aquarium fish is a fascinating and complex topic that needs much more exposure for humans to catch up to and accept reality. These findings have significant implications for aquarium fish keeping, emphasizing the importance of providing a stimulating and humane environment for these intriguing creatures.

References

  • Brown, C. (2015). Fish intelligence, sentience and ethics. Animal Cognition, 18(1), 1-17.
  • Bshary, R., Wickler, W., & Fricke, H. (2002). Fish cognition: a primate's eye view. Animal Cognition, 5(1), 1-13.
  • Bshary, R. (2019). Cleaner wrasse pass the mark test. What are the implications for consciousness and self-awareness testing in animals? PLoS Biology, 17(2), e3000021.
  • De Waal, F. B. M. (2019). Fish, mirrors, and a gradualist perspective on self-awareness. PLoS Biology, 17(2), e3000112.
  • Lambert H, Cornish A, Elwin A, D'Cruze N. A Kettle of Fish: A Review of the Scientific Literature for Evidence of Fish Sentience. Animals (Basel). 2022 May 5;12(9):1182. doi: 10.3390/ani12091182. PMID: 35565608; PMCID: PMC9100576.