I don't know if what you are saying is true. Still, if it is, it's still important to know what is toxic (damaging) to the human body. Ultra-violet light is toxic, as is lead, arsenic and not being silly - water. I suggest the question becomes, is the form of mercury found in sea animals toxic when these animals are eaten by people. As I am informed at this point, the consensus is no. Doctors and scientists who are not doctors have a common comment on poison, "What is poison? That's not the question because anything can be toxic, even water. It's about levels." In other words, anything in the proper amount, is toxic.
Also, in some food chains, toxins are in fact made safe as they are broken down in to non-toxic forms or are kept in parts such as only the fatty tissue or liver that keep the remaining flesh of the animal safe to eat. Consider the blowfish used in specialized sushi.
In desert training some Special Forces operators are known to concoct a disgusting beverage to prevent cramping - an entire paper packet of salt poured in to a bottle of water. Terrible to taste. The results are remarkable however as those who have done it do attest.
Podophyllin resin is a deadly poison if ingested. It will kill you. Used topically it safely treats genital warts. Aspirin certainly can kill but in lower doses ..
I hate the dolphin slaughter. It's brutal. I am not against it. I have seen animals killed much more humanely, in fact with the animal seeming to have no awareness of even stress in the moment. Pre-historic peoples ran buffalo off cliffs and harvested their carcasses. Dolphins herd fish in to tight groups to harvest them. In the United Kingdom, present day, an animal must be unconscious for several seconds during the killing. This is accomplished by electric shock causing grand mal seizures. The animal is processed so quickly that it needs to be cooled right away because of the remaining body heat.
I don't like seeing anything die, I do eat meat. As I understand the current best science regarding mercury in fish is that it is not a danger to humans.
I have seen humane butchering. The skill and quickness of those butchers is remarkable. I think the dolphin slaughter just makes us sad and we want a reason to stop it. It makes me terribly distressed to see those intelligent animals killed this way but, I do support harvesting animals. What I really want, and I hope this does not sound silly because I mean it, is for the food replicators used in Star Trek TNG to exist. I do understand that the growing of meat is something happening now and I would be glad to eat it. If it is safe and tastes as good, I would eat it. If it was safe and didn't taste as good I would gladly give up slaughtered meat and still eat the grown. But I don't think the world would follow me and in any nation that depends on harvesting animals from the sea or land, they would think me crazy for not simply taking the animals near them that for generations their people have eaten.
Whatever the food chain, toxin concentrations increase the higher you go. This is true in the sea as well as on land. There is a huge difference between eating dolphins at the top of the food chain to filter feeders or even fish at the low end!