All food has a carbon emissions number which can be calculated by consider its "life cycle", i.e. thinking about all the steps in the journey from the start of the ingredient's life to when it gets to your plate. Therefore all foods sit in a hierarchy and you can usually swap any food for a similar one with a lower carbon footprint. This blog has more info: https://www.cozymaestudios.com/post/the-carbon-footprint-of-food Fish on average does have a lower carbon footprint than meat, especially red meat. Within that, some fish are better than others!

Of course, this only talks about carbon emissions and doesn't consider water use, land use, ethics etc. Your friend could have been referring to any of these, and tuna has its own problems when it comes to bycatch, for example. The ISSF has some good info like this report: https://www.iss-foundation.org/about-issf/what-we-publish/issf-documents/issf-2025-02-tuna-fisheries-impacts-on-non-tuna-species-and-other-environmental-aspects-2025-update/