Seasonal changes in the quality of rainbow trout (Salmo gairdneri) semen: Effect of a delay in stripping on spermatocrit, motility, volume and seminal plasma constituents

Seasonal changes in ejaculate characteristics were monitored for groups of male rainbow trout (Salmo gairdneri) which were stripped weekly, beginning at various periods after the onset of the spermiation cycle. The duration of stripping varied from 14 to 28 weeks for the different groups, and the minimum duration of milt release was longer for groups in which semen collections wewe begun earlier in the season. A delay in initiating milt collections caused an increase in volume, or an increase in volume and spermatocrit at first collection. A delay of 14 weeks before stripping resulted in the release of 98.3% of their seasonal output of spermatozoa over the first 2 months of stripping, while the control group released only 34.7%, and groups in which stripping was initiated 3,6 or 10 weeks later released 63.7%, 64.5% and 67.5%, respectively. The control group released a mean of 416.3 × 109 spermatozoa per fish in 85.9 ml of semen during the season, while groups initiated 3, 6, 10 or 14 weeks later released 445.7 × 109 in 119.8 ml, 330.8 × 109 in 133.4 ml, 297.2 × 109 in 50.1 ml and 96.5 × 109 in 24.9 ml, respectively. Spermatocrit, motility and seminal plasma ion concentrations declined as the season progressed, but volume was independent of time. Late in the season both motility and seminal plasma ion concentrations were highest in groups which were stripped the fewest times. Motility was lower in samples with low spermatocrits or large volumes.