Mercury is only a problem when the body assimilates a lot of mercury. Most of us could never eat enough fish with mercury to cause illness. I suggest we are given warnings because as a society we expect to be able to control everything. Scientists do research and give us what information they have at the time. They suggest limiting mercury ingestion as a safeguard. I suggest that scientists haven't figured out how much will cause what effect.
Remember when we were advised to only drink one or two cups of coffee per day. Now scientists tell as to drink as much as six cups because coffee is now considered to have positive effects for health.
I suggest that many things are safe when eaten/used in moderation. In my early years most lived by common sense because science knew very little. Common sense invcluded the idea of moderation. Remember that scientists, in the 50's and 60's' said heart difficulties mostly happens only to men. We know differently now. All so recently it is considered OK to eat a limited number of eggs. Science isn't all knowing. Scientists are rapidly finding new things.
However, if you are concerned about mercury you can buy mercury free tuna.