Perhaps SCiO's near infrared spectral analyzer could provide some info about fish? Possibly the species, maybe the freshness or adulteration. I think mine cos

Perhaps SCiO's near infrared spectral analyzer could provide some info about fish? Possibly the species, maybe the freshness or adulteration. I think mine cost $400 many years ago, not sure if they are selling to consumers any more. They seem to be selling to farmers for testing corn and feed: https://www.consumerphysics.com/ https://www.consumerphysics.com/ The development kit to develop your own IR profiles is listed on Amazon for $1300. This list of research papers from the "9th International Symposium on RECENT ADVANCES IN FOOD ANALYSIS": http://www.rafa2019.eu/pdf/Program%20RAFA%202019_FINAL_web.pdf http://www.rafa2019.eu/pdf/Program%20RAFA%202019_FINAL_web.p... suggests that an IR spectrometer can tell if water is added to tuna (search for "tuna", it's the second occurrence). If you also search on "fish" in that document you'll find a variety of methods being explored to find the presence of various things in fish (toxins, using PCR to test for DNA to detect fraud, histamine, arsenic, dioxin, microplastics, Bisphenol). PCR tests for Covid-19 seem to cost around $100. Seems like spot checking of tuna using PCR to see if it is really tuna would be doable and would bring the market under control, eliminating fraud. Fishing interests would likely be aligned against that, though maybe not if it leveled the playing field.