Estimation of sperm concentration of wild and reconditioned brown trout, Salmo trutta L.

Sperm concentrations were assessed for milt samples taken from three stocks of brown trout, Salmo trutta L., (wild resident trout, and reconditioned and sea-reared sea trout) using direct counts, spermatocrit and an optical density of 1:1000 dilutions. There were significant linear relationships between spermatocrit and absorbance, and both spermatocrit and absorbance with sperm concentration. The results gave sperm concentrations from 2.2 x 10(9) mL(-1) to 26 x 10(9) mL(-1), Mean concentrations differed significantly between stocks; the reconditioned males had a mean concentration of 9.1 x 10(9) mL(-1), sea-reared males 18.0 x 10(9) mL(-1) and 8.0 x 10(9) mL(-1) in 1994 and 1995, respectively, and wild resident males 13.3 x 10(9) mL(-1). Sperm concentrations from all three stocks of trout were negatively correlated with length, and therefore, age. It was indicated that sufficient milt was being produced to achieve good rates of fertilization, although some of the larger trout did have low sperm counts, suggesting that fertilizing ability may decrease with male size or age within any one stock.