Isnt there a few fishes that are also high in mercury?
I know in humans, some of em are not recommended for pregnant women. I just am fuzzy on which.
Mackerel, I wanna say?
Correct, mercury poisoning is a concern when it comes to certain species of fish. I've read pretty far into Oishinbo and I haven't encountered a chapter where this issue is addressed, so I suppose I'll explain it here.
Mercury often enters marine ecosystems through water pollution, as runoff from power plants or goldmines. They don't always occur as elemental mercury, often times they will exist as organic mercury (like methylmercury).
When mercury compounds enter the ocean, they get 'eaten' by plankton and other microorganisms, collecting in their body. Small fish eat the plankton, then big fish eat the small fish, and through this process of predation, the mercury collects more and more in predator species higher up the food chain. This process is called bioaccumulation.
Following this process, we can intuit that the species with the highest concentration of mercury in their bodies would be large, predatory fish. There are apparently several other factors that can complicate things somewhat, but this works well enough as a general rule. The amount of mercury ingested in this way is toxic enough to cause developmental abnormalities in fetuses, so pregnant women are often advised not to eat too much seafood.
In particular, the US FDA has highlighted 4 specific species for pregnant women to avoid: King mackerel (that's the one you remembered), swordfish, tilefish, and shark.