Seasonal Testicular Cyclicity in Rainbow Trout (Oncorhynchus mykiss)

Studies on circaannual changes in reproduction of Rainbow trout (Oncorhynchus mykiss), an economically important fish of Kashmir have been carried out. The variations in testicular structure and function of rainbow trout were studied during the present investigation. Morphometric and gravimetric data revealed four distinct reproductive phases viz. (I) Resting or Spent stage (II) Maturation stage (III) Mature stage and (IV) Regression stage. Maximum GSI was recorded during the mature stage and minimum in resting or spent phase (0.747). Histologically, testis revealed different germ cell types which were categorized into primary spermatogonia, secondary spermatogonia, spermatocytes, spermatids and spermatozoa. The primary spermatogonia were the largest cells and were present on the periphery of seminiferous lobules. These cells divide meiotically and form secondary spermatogonia. The secondary spermatogonia were smaller in dimensions than primary spermatogonia. The primary spermatocytes develop from secondary spermatogonia by meiotic division. Second Meiotic division of the secondary spermatocytes produces the spermatids. Finally spermatozoa are formed from the spermatids within the lobule lumen. All these stages were visible at the maturing phase whereas in the mature stage the testicular lobules were filled with spermatozoa only.