I have a can of Wild Planet tuna, that says on the label:
Research shows that smaller albacore, such as used by Wild Planet, contains less mercury than larger albacore.
Their web-site has a FAQ that makes a similar claim:
The average mercury content of tuna rises with the age and size of the fish. Wild Planet only sources pole and line caught tuna, which are the younger and smaller migratory tuna that are caught at the surface. These fish have accumulated lower levels of mercury as compared to older and larger tuna and our annual testing protocol, summarized in the accompanying document, verifies that Wild Planet tuna products average 0.076PPM for Skipjack and 0.219PPM for Albacore.
It has a reference that doesn't seem to directly explain this.
Is it true that smaller albacore have a lower levels of mercury by weight/parts-per-million?