Changes in hormone profiles in relation to ultrasound-determined testis maturation during a reproductive cycle in Atlantic salmon (Salmo salar)

At present, fully mature males are identified by checking for running milt by palpitation during gamete production of Atlantic salmon (Salmo salar). Ultrasound (US) technology could provide a cheap and fast method to monitor maturation progression in males at an earlier stage of reproductive cycle and potentially leading to a new technique for sorting males into different cohorts, depending on their maturation status. In this study a US-determined volumetric gonadosomatic index (vGSI) was evaluated in relation to directly measured GSI and plasma steroid hormones concentrations. Sampling was performed monthly from September 2014 to July 2015 and weekly from August 2015 to the first week of September 2015. 20 and 5 Atlantic salmon broodstock males were examined during monthly and weekly sampling respectively. The US technique was used to estimate testis length when testis was detectable and cross-section images were taken. Testis weight (g) and body weight (kg) were also measured and plasma samples were collected from the same individuals. On May 20th 2015, males were transferred to freshwater to induce final maturation by temperature drop in August 2015. In the laboratory, plasma concentrations of testosterone (T), 11-ketotestosterone (11-KT), and 17α, 20β-Dihydroxy-4-pregnen-3-one (17,20βP) or maturation inducing steroid (MIS) were analyzed using enzyme immunoassay (EIA) method. US-based images of testis were analyzed in MyLab Desk/Desk3, an advanced ultrasound image processing software.
In September 2014, the mean GSI was 0.15, showing no significant increases before February 2015 (0.17). A maximum increase in the testis volume was observed between June and July 2015. Positive relationship (p<0.0001 R2 = 0.888) between GSI and vGSI was observed. Plasma steroids levels were at their lowest level in September 2014 (T = 0.36 ng/ml, 11-KT = 0.88 ng/ml and MIS = 0.19 ng/ml). Plasma T, 11-KT and MIS increased with increasing progression of testicular maturation reaching maximum levels in August (T and 11-KT) and September (MIS) 2015. Positive relationships (p<0.001) were observed between plasma androgen levels (T and 11-KT) and both GSI and vGSI. Although with, low R2 value, positive relationship (p<0.0001) between MIS and both GSI and vGSI was also observed.
In general, changes in plasma T, 11-KT and MIS were closely related to the reproductive cycle of male Atlantic salmon. All the steroids increased towards the progression of maturation followed by increased GSI. vGSI could be used as a valuable measure of gonadal development in male Atlantic salmon, at least, five months before stripping.