The piece of evidence that BEST supports the author's claim in the article "Say No to Genetically Engineered Salmon" is:
Genetically altered salmon is bad for human health, as it contains more chemicals than wild salmon, and it is bad for the environment, as the genetically altered fish are raised in overcrowded, netted ocean pens.
This evidence directly addresses the concerns about the effects of genetically engineered salmon on both human health and environmental impacts, which are central to the article's argument against genetically modified fish.