Certain fish types such as carnivorous species tend to bioaccumulate contaminants of various types, such as heavy metals. Fish have potential to bioaccumulate, if they are eating contaminated organisms or living in contaminated materials, but not all fish or organisms that become fish food respond the same way to pollutants. In general, it takes a degree of trial and error by scientists, fishery or aquatic biologists to test species for this use. They may test plants, aquatic substrates (sediments), macroinvertebrates, mussels, crayfish, fish or other aquatic organisms. Unfortunately the most useful organisms may succumb to pollutants, and not be available for testing of potentially toxic materials. Aquatic species are useful in estimating past instances or presence of toxins, but their presence or absence or relative abundance, and species sensitivity to pollutants are all helpful indicators. But to actually measure persistent toxicants may involve some degree of sediment sampling, or if the pollution was recent water sampling when designed to capture pollutant. Our Researchgate report on the Spruce Budworm Control Project might give you some ideas, if attempting to address ongoing pollution potential or chemical spill.

Dear, It is necessary to extract the fat, fatty fish accumulate more than other fish.

For the protocol of analysis see this article: Article Health risk assessment of exposure to polychlorinated dibenz...

There are various challenges around using fish to measure bio accumulation and bio concentration, including extrapolation from single tissue to full body burden, and an intention to move away from less reliance on animal testing.

This report from the Environment Agency in England, published in summer 2022 gives a useful overview on some of the current understanding of this topic: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1091336/Guidance_on_interpreting_biota_tissue_concentrations_for_bioaccumulation_assessment_-report.pdf#page5