I would suggest you survey (sample) the pond, fishing it many times over six months, may not be the best way to survey but will be the cheapest. Or running a net, or electroshocking. I think I am a good fisherman, so I would try that first. Running a net can be hard work and it's hard to find good help, so I would lean toward electrofishing it through someone who knows how to survey and make suggestions for corrections if needed.
Tracy
