This study investigated the relationships between semen fertilization capacity and sperm motility, seminal plasma composition and sperm metabolism in the rainbow trout, Oncorhynchus mykiss, to find out biomarkers for semen quality. Variations in semen fertilization rate could be best described by three multiple regression models: Firstly, a model including the seminal plasma pH (x1), β-D-glucuronidase activity (x2), total lipid levels (x3) (y=894.77x1-53.13x1/26.58x2- 0.0006x1x2- 0.62x3+ 0.008x3/2- 3666.19, F-value = 10.35, R2= 0.805, P < 0.001), secondly, a model including the sperm metabolism parameters malate dehydrogenase activity (x1), respiration activity (x2) aspartate aminotransferase activity (x3), total lipid levels (x4) (Y - 2.06x1- 1.63x2+ 0.073x1x2- 0.049x3+ 0.029X44- 0.0031X4/2+ 97.96, F-value = 12.41, R2= 0.754, P < 0.001) and thirdly, a model including sperm motility rate (x1) and total swimming velocity (x2) (y = 0.44x1- 0.38 x2+ 0.011 x2/2- 0.00006x2/3+ 32.87, F-value = 51.31, R2= 650, P < 0.001). For practical application, the utilization of simple regression models is of value, too, as the technical effort for parameter determination is lower. Fertilization rate can be best described by sperm motility rate (y=0.72x + 25.99, R2=0.594, P < 0.001), seminal plasma pH (y= 1460.2 x - 89.41 x2- 5881.2, R2= 0.525, P < 0.001) and spermatozoal respiration activity (y = -1.85x + 84.59, R2= 0.554, P 80%) is characterised by high sperm motility rate ≤ 75%, medium sperm swimming velocities of 100-120 μm/s, optimal seminal fluid protonic composition (pH of 8.0-8.2), balanced energy metabolism (spermatozoal respiratory activity ≤ 5 μmol/min/100 mg protein, spermatozoal malate dehydrogenase activity ≤ 2.5 μmol/min/100 mg protein, medium seminal fluid total lipid levels of 20-60 mg/100 ml and spermatozoal total lipid levels of 100-400 μmol/100 mg protein), and low seminal fluid lytic activity (β-D-glucuronidase ≤ 0.4 μmol/min/l [≤ 0.5 μmol/min/100 mg protein]).
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