Abstract
Sperm motility of the fifteen‐spined stickleback Spinachia spinachia was investigated in different salinities, with or without the addition of ovarian fluid (25%). Sperm velocity, longevity, linearity and percentage of motile sperm were analysed with the aid of computer‐assisted sperm analysis (CASA). The sperm were found to have the longest duration of motility in sea water (60–90 min in salinities 20 and 30), shorter in brackish water (15–30 min in salinities 5·5 and 10) and were immotile in fresh water. The presence of ovarian fluid did not influence any sperm motility variable in any tested salinity.