In most of teleost fishes, the production of gametes (sperm and vitelogenic oocytes) happens in specific periods of the year. For example, tropical fish reproduces mainly in the hot months. Thus, when we study the seminiferous epithelium over the year, we will observe that in the colder months (june-july), the testis will be filled with spermatogonia. As the year goes on, we will observe less spermatogonia, but a lot of spermatocytes. Near or in the hot months (november-february) we will observe spermatids and spermatozoa.

The composition of spermatogenic cell will tells us if the fish could reproduces or if the same fish are on an imature stage of the testis cycle (and a lot more of information regarding hormones, energy metabolism, etc). Of course this subject is far more complex than what i have discussed here.

Please read:

Spermatogenesis in fish - Gen Comp Endocrinol

. 2010 Feb 1;165(3):390-411. doi: 10.1016/j.ygcen.2009.02.013.

Comparative testicular structure and spermatogenesis in bony fishes - Spermatogenesis

. 2015 Feb 23;4(3):e983400. doi: 10.4161/21565562.2014.983400.