I have been working on the Length-weight relationship of a marine invertebrate. In this research, I identified the gender (gonad presence, invasive) of the organisms while counting the length and weight.

There are a number of specimens, which are at the smaller end of the size, has no gender characteristic observed. Hence, categorized as immature/juvenile. However, there are some immature specimens which are larger in size, as large as the mature specimens.

Is there any statistical analysis which can be used to identify the cutoff point of the size, where we can be 95% sure, where the organism is assumed matured without observing the gonad prior?

Thank you very much for your attention. 

Cheers,

Boo