Effects of coelomic and seminal fluids and various saline diluents on the fertilizing ability of spermatozoa in the rainbow trout, Salmo gairdneri.

When trout (Salmo gairdneri) spermatozoa were diluted in coelomic fluid or saline diluents at high dilution rates of 10(-3) and 10(-2) for increasing periods of time before insemination, there was a rapid decline and loss of fertilizing ability. At a lower dilution rate of 10(-1), there was partial or no loss of fertility. Dilution in a KCl-enriched saline diluent to inhibit sperm motility produced a slight decrease in fertility at a 10(-3) dilution rate, indicating that the spermatozoa, although sensitive to dilution, were less so when they were kept immotile. A partial loss of fertility was observed after the spermatozoa or eggs had been washed with saline diluents. The loss of fertility was total when both gametes were washed. Removing the seminal fluid by centrifugation led to a significant decrease in the fertilizing ability of the spermatozoa when insemination was carried out in saline diluent but not in coelomic fluid. Adding BSA at high doses (10 mg BSA/ml) into the diluent led to longer survival of the diluted spermatozoa. We conclude that (1) sperm dilution rate is a major factor in the maintenance of fertilizing ability of diluted salmonid spermatozoa, (2) as reported in the literature, coelomic fluid is superior to mineral diluents only when the gametes have been washed, and (3) some substances (possibly proteins) present in seminal and coelomic fluids play a role in gamete protection. These findings may explain the discrepancies in the literature concerning the duration of motility and fertilizing ability of salmonid spermatozoa.