Age and repeated collection interval effects on sperm motility in the endangered delta smelt Hypomesus transpacificus

Understanding the reproductive dynamics of endangered species is crucial for effective conservation strategies. This study examines the impact of age and repeated collection interval on sperm motility in the Critically Endangered delta smelt Hypomesus transpacificus . Morphological data and sperm samples were collected from 1-yr-old and 2-yr-old males as fresh sperm (baseline collection phase, BCP) and as regenerated sperm 7 d after the first collection (repeated collection phase, RCP). Findings revealed that 2-yr-old males had significantly greater fork length (p < 0.001), body weight (p < 0.001), and condition factor (p < 0.001) than 1-yr-old males. Sperm collected during BCP exhibited significantly higher motility than those collected during RCP across both age groups (BCP vs. RCP: amplitude of lateral head displacement, p < 0.01; curvilinear velocity, p < 0.01; and RCP vs. BCP: wobble coefficient, p < 0.01). However, contrary to expectations, age did not significantly affect sperm motility (p > 0.05). The results suggest that a 7-d RCP is insufficient for full sperm recovery and leads to lower sperm quality. This study enhances understanding of delta smelt sperm biology and underscores the importance of strategic sperm collection timing for improving conservation activities. Additionally, these findings provide valuable insights for optimizing breeding protocols in aquaculture and informing conservation strategies for other endangered fish species facing similar reproductive challenges.