Sperm performance of coastal northern pike ( Esox lucius L. ) from the Baltic Sea shows no impairment between freshwater and brackish conditions

Declining coastal populations of northern pike (Esox lucius) in the Baltic Sea have raised concerns about potential reproductive constraints, including reduced sperm quality linked to changing salinity regimes. This study tested whether sperm performance in coastally collected pike is impaired by activation in freshwater versus brackish water. Sperm velocity and motility were measured from adult males collected at two Baltic Sea locations (Sankt Anna, 2024; Hanöbukten, 2025) and activated in either deionized (0 ppt) or local brackish water (6–7 ppt). We found that sperm velocity and motility declined rapidly over time post-activation, but did not differ significantly between salinity treatments during the critical fertilization window (≤20 s). At later time points (25–45 s), sperm in brackish water maintained higher velocities and motilities than in deionized water, indicating greater longevity under local conditions. Overall, our results show that sperm function in coastal northern pike is not limited by salinity difference, suggesting that current recruitment declines are unlikely to result from impaired male gamete performance. Instead, reproductive limitations in Baltic pike populations may arise from later developmental stages or environmental pressures on egg and larval survival.