A population, $P$, of a certain species of fish in a lake begins with $250$ members and one-half disappear every $6$ years. Give the constants $a$, $b$, and $T$ so that the population is represented by a function of the form $P = ab^{-t/T}$, where $t$ is the time in years since the population was first measured.

$a =$ help (numbers)
$b =$ help (numbers)
$T =$ help (numbers)