I would set my tolerance to the lower level period. I would infer from below, that testing should be routine for each lot/fish (given that tuna and swordfish are huge animals one would equal a lot) or demand a CoA from your vendor
Some guidance
https://inspection.c...5/1671460542870
https://www.canada.c...ns-answers.html
Why are there two different standards for mercury in fish?
Health Canada and CFIA's scientists continue to find that the total mercury levels in the majority of commercial fish species, including canned tuna, are below the 0.5 ppm standard for total mercury in commercial fish, which was first established in 1970. However, certain varieties of piscivorous (fish-eating) fish tend to contain more than 0.5 ppm total mercury. Although these types of fish are higher in mercury they are normally consumed less frequently than other types of fish and therefore are not considered to be a significant source of mercury to the average diet. Rather than prevent the sale of these piscivorous fish, fresh/frozen tuna (not including canned tuna), shark, swordfish, escolar, marlin, and orange roughy are permitted to be sold as long as they contain less than 1.0 ppm total mercury. These fish are also subject to consumption advice.
Why does the standard cover total mercury when methylmercury is of concern?
Total mercury is the sum of all chemical forms of mercury, including methylmercury. Analyzing fish tissues for total mercury is less expensive than analyzing for the individual chemical forms of mercury, such as methylmercury. Since fish tissues are commonly analyzed for total mercury, Health Canada has developed standards based on total mercury levels. This approach is acceptable given that most of the mercury present in fish tissue is in the form of methylmercury.
What happens to fish if its mercury level exceeds the standards?
The Canadian Food Inspection Agency (CFIA) regularly tests fish and shellfish from fish processing plants to determine if they meet the Canadian standards for total mercury. If a fish contains higher mercury levels than what the Canadian standards permit, more testing is done on fish from the same lot or batch. Fish that are found to violate the Canadian standards for total mercury are not permitted to be sold in Canada.