Tires in lakes. Are they harmful?

I am a free-diver from Canada, and I dive in a nearby lake, with one of my friend. In that lake, there is a lot of tires, old ones. We recently had the idea to go to that lake with a third friend, and bring them out of the said lake, but before doing so, I verified online if tires are actually harmful to the environment. I’ve been surprised to discover that they don’t seem to be as harmful as i thought, according to some websites, and some other sources tend to believe that the heavy metals that they contain can be really dangerous for the ecosystems.

I’ve never seen any fishes inside those tires, nor around them, so they are probably not used as “structures” for the fishes of said lake.

So before doing anything, let me ask, should tires be removed from lakes? Are they harmful?