The population of a certain species of fish in a lake is modeled by the function $P(t)=5000\cdot (0.81{)}^{\frac{t}{2}}$, where $P(t)$ is the population after $t$ years. This model can be rewritten in the form $P(t)=5000\cdot (1-r{)}^t$, where $r$ is the annual decay rate. What is the value of $r$ as a percentage?