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Article History

Received: 20 August 2025

Revised: 14 October 2025

Accepted: 24 October 2025

First Online: 8 November 2025

Declarations

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: All experiments were approved by the Stockholm Animal Research Ethical Permit Board (permit nr: 17362–2019, 12211–2021 and 17402–2020), followed local laws, and adhered to the ASAB/ABS Guidelines for the use of animals in behavioral research and teaching. Animals were monitored daily for general health and welfare, both during and outside the experiments. Stress during the experimental trials was reduced by videotaping the animals with no human observers in the same room, and focal fish in the trained association experiment were isolated for no longer than 24 h. Two stimulus fish died during the experiments by jumping out of their tank (trained association experiment), and one focal fish after the trials (spontaneous choice experiment). Fish were anaesthetized by immersing them in a water bath with a low dose of benzocaine (67 mg/L) until they fell on their side and then we waited another 10 s to make sure they were unconscious. All fish that were anesthetized during the photography fully recovered. After the experiments, focal fish were placed back in their home tanks in line with the ethical permits. Stimulus fish in the trained association experiment were euthanized after the experiment due to their advanced age, by immersing them in a water bath with a higher dose of benzocaine (400 mg/L) for 10 min. After taking pictures, their brains were destroyed to ensure death.

: The authors declare no competing interests.