Hormonal signatures of gonad maturity and seasonality of spawning in migrating hilsa, Tenualosa ilisha.

Identifying the hormonal signature of gonad maturity and spawning seasonality is essential for upgrading the forecast of recruitment, environmental impacts, and captive maturation. The study evaluated the pattern of gonad development and associated plasma levels of sex steroids and thyroid hormones in both sexes to identify gonadal stage markers and reproductive seasonality in an Indian shad, Tenualosa ilisha. Various stages depicted the cellular organization of ovarian and testicular developments. The ovarian histology revealed hilsa as group synchronous total spawners. The seasonal distribution of distinct maturity stages indicated unsynchronized gonad development within the population. Hilsa exhibited a differential preference of environmental cues in spring and late autumn spawning. However, peak spawning activity took place at similar photoperiod and temperature. The appearance of post-ovulatory follicles (POFs) and late spermiating testicular lobules (LS) marked active spawning in different sexes. The gonadosomatic index (I_G) showed temporal, stage-specific variations with a positive correlation with plasma 11-KT (11-ketotestosterone), estradiol (E2) in females or with 17α, 20β-dihydroxy-4-pregnene-3-one (DHP) in males, although the relationship altered temporally for each variable. The positively correlated average plasma 11-KT and E2 values with the number of pre- and post-vitellogenic females serve as hormonal signatures for identifying initiation and completion of gonad growth, respectively. Higher circulating levels of 11-KT compared to T during annual gonad cycles indicate 11-KT as the predominant androgen that controls spermatogenesis in this species. Plasma DHP showed a minor peak in March and a significant elevation in October, synchronized with active spawning in females and males marked as a hormonal signature of spawning irrespective of sex. Plasma T4 showed a positive correlation with I_G and spawning females, whereas plasma T3 had a similar correlation with 11-KT and the number of early spermiating males in the population. The results demonstrate hormonal signatures of gonadal recrudescence, maturation, and spawning at the population level for promoting recruitment and artificial propagation of hilsa. [ABSTRACT FROM AUTHOR]