Induced spermiation of rainbow trout, Oncorhynchus mykiss, using a GnRh analogue

In this study, the benefits of using the first Iranian made GnRHa[D-Ala6 des Gly10] mGnRH ethylamide, to induce spermiation in male rainbow trout, Oncorhynchus mykiss, were evaluated, In addition, its effect on acceleration and synchronization, quality and quantity of milt and the plasma Testosterone (T) fluctuations were examined. For these purposes, 40 non-spermiating male rainbow trouts were injected with a mammalian gonadotropin releasing hormone analog (GnRHa) preparations of 0 (control), 30, 40 and 80 &mu;g/kg B.W. or vehicle (propylene glycol). Spermiation was very synchronous and accelerated in treated groups. Six days after first injection, the cumulative spermiation rates reached respectively 40, 49 and 79% in injected groups (i.e. 30 to 80 &mu;g/kg B.W.) while none of the control fish was spermiated. GnRHa injections advanced spermiation and reduced the average time to spermiation from 16 3.67 days for control group to 14.3&plusmn; 2.2, 9.2 &plusmn;0.75 and 6.6&plusmn; 0.29 days for treated groups, respectively (p<0.05). The average volume of total expressible milt of male fish increased significantly after treatment with GnRHa from 8.29&plusmn;1.59 ml/kg B.W. for control to 11.42&plusmn;1.55, 14.39&plusmn;1.55 and 17.14&plusmn;1.55 ml/kg B.W. in groups 2 to 4, respectively (p<0.05). The fertilization and survival rates to the eyed stage did not show any significant difference among the groups (p>0.05). Circulating levels of testosterone (T) prior to the GnRHa treatment were relatively low in all groups. Treatment with GnRHa induced significant increase in plasma T after 12 h, increasing it to 43.43&plusmn;5.82, 38.66&plusmn;5.63, 39.72&plusmn;5.07 ng/ml in groups 2 to 4, respectively, which were higher than T levels for control (i.e. 24.58&plusmn;7.13 ng/ml) p<0.05. These levels remained high up until 48 h in treated groups; but after this time, T levels reduced to the basal levels of time 0, except in 4 which had received its second GnRHa injection at time 48.