How does biomass change as you move from lower to higher trophic levels in an ecosystem?

Due to biomagnification, there are certain species with a high concentration of heavy metals (e.g., mercury), leading to dietary advisories for vulnerable demographics (e.g., pregnant women and young children) to avoid these species. Given that these advisories are often most critical for species at the top of long marine food chains, which species would this advice most likely apply to?