Here's my wag on it:
I would figure not because to the best of my knowledge
tuna are slower growing fish who eat and accumulate merc.
by digesting the smaller fish over a course of time. The sea plants, which I think are fast growing, and while probably affected by ocean pollution, true, shouldn't have the same concentrations. I can't imagine the plants
just absorbing merc? I could be way off base.
On a wild whim, why don't you email that question to either Scripps
Oceanography Institute or Woods Hole out on the East Coast.
If you can get the email forwarded to some expert there, at
least they can give you an idea if this problem has been looked into?
I would be curious to know as well.