Association of annual hormonal profile with gonad maturity of mahseer (Tor tambroides) in captivity

Annual gonad hormonal profile of wild, matured mahseer (29 males and 23 female) averaging in weight between 0.95 ± 0.26 and 1.19 ± 0.23 kg for males and females, respectively, were investigated from November 2007 to November 2008 using enzyme-linked immunosorbent assay (ELISA) technique. Blood was collected from caudal vein, monthly and plasma separation by centrifugation was done to measure reproductive hormones: 17β-estradiol (E2), testosterone (T), and 11-keto-testosterone (11KT). Gonads were sampled for histology processing to observe their maturity. Highest T level in females and males was recorded at 0.22 ± 0.016 and 0.88 ± 0.014 ng/ml, respectively. The 11KT showed several peaks and the highest value was noted at 0.7 ± 0.018 ng/ml in November 2008. The female E2 initially was at 1.48 ± 0.16 ng/ml and significantly increased (P < 0.05) to 1.53 ± 0.39 ng/ml in November 2008. Ovaries were laden with oocytes in several stages in all the samples while testes gonad showed a high level of spermatids throughout the year. Changes in plasma level of the gonadal hormones were correlated with the ovarian and testes maturities. In conclusion, the study suggests that mahseer can be categorized as asynchronized and multiple spawner. The information gathered is important for appropriate breeding and conservation programs of the Malaysian mahseer.

Research highlights
► The association of changes in hormone levels of gonadal steroid with environmental factors may be a valuable tool in understanding the breeding cycle of fishes. The levels of gonadal hormones produced in the plasma have also been proven to affect gonad development in several fishes. In female fishes, 17β-estradiol (E2) is known to stimulate the hepatic synthesis and secretion of vitellogenin, which is accumulated in the oocytes. Increase in 11-keto-testosterone (11KT) secretion will stimulate spermatogenesis; while testosterone (T) is known to assist male gonad development and is a precursor for 17β-estradiol.
► The mahseer, also known as Tor tambroides, is one of the highly sought after freshwater fish in Malaysia. This is due to their aggressive behavior favored by the sports anglers. These species can only be found in clean freshwater rivers such Royal Hutan Belum in Perak, Taman Negara in Pahang, and Sungai Andang in Sarawak (Ingram et al., 2005). There are three reported species of mahseer that can be found throughout Malaysia namely: T. tambroides (Bleeker), Tor douronensis (Valenciennes), and Tor tambra. However, their presence in natural habitat is being threatened due to several reasons including habitat destruction, deterioration of water quality, and over harvesting. Most of the mahseer fries are obtained from the wild, though attempts have been made to induce maturation and ovulation by hormone treatment in captivity, but the results seemed to be unpredictable since mahseer do not reproduce naturally under cultured conditions.
► The information gathered will be valuable for aquaculture development in a non-seasonal region such as Malaysia. Thus, this research was undertaken to investigate the reproductive endocrinological aspect that may trigger the regulation of gonadal development, spermiation, ovulation, and hormones secretion of Malaysia mahseer (T. tambroides) in captivity.