[CAluvdubya]

Sardines contain virtually no mercury and among the safest fish you can eat (with regard to mercury level).

Sardines do not contain all the nutrients necessary for a well balanced cat diet. They're a good supplement, but not suitable as a complete food.

Also, to the best of my knowledge, a long term, steady diet of fish tends to cause urinary problems in male cats.

[SamAdams76]

All of this makes me wonder how cats survived the millions of years before humans made them pets and started formulating food for them.