The nuclear weapons testing from $1960$ s and Fukushima nuclear power plant accident have elevated the concentration of radioactive isotopes in the oceans. The concentration of Caesium$-137($Cs$-137)$ in fish and shrimp tissue is proportional to the concentration of Cs$137$ in the oceans and their body mass. If we assume that the radioactive decay is the only reaction that cesium is lost from the tissue, calculate the concentration of Cs$-137$ in the fish after $5$ years $($units of $B\frac{q}{K}g).$ Initial concentration right after the accident was $12,000$ Becquerels $($Bq$)$ per kilogram and the half$-$life for this isotope is $3$ years. $($a Becquerel is a measure of radioactivity; $1$ becquerel equals $1$ radioactive disintegration per second$).$