Assessing the potential for introducing resident brackish-water pike ( <i>Esox lucius</i> ) to restore depleted Baltic Sea populations

Abstract The northern pike ( Esox lucius L.) inhabits the Baltic Sea and has two reproductive strategies: spawning in freshwater wetlands (anadromous populations) and direct spawning in coastal marine waters (resident brackish populations). Resident brackish populations do not occur in Polish marine waters, and anadromous pike populations in Puck Bay are virtually extinct. This study assessed whether reproductive material from resident brackish pike populations could be used to obtain larvae under hatchery conditions for population restoration. Eggs were collected from resident brackish pike spawners in the coastal waters of Rügen (Germany) and transported 600 km (9 h) to a hatchery in Poland. The eggs were incubated in artificially prepared saline water at 0, 5, 7, and 8.5 PSU. In the 2023 trial, hatching success was 0% at 0 PSU, 0.7% at 5 PSU, 8.3% at 7 PSU, and 8.0% at 8.5 PSU. Larval survival to 18 days post hatching in salinities of 5–8.5 PSU was high (approximately 90%), and growth rates at 14°C ranged from 0.26 to 0.31 mm d −1 .No eggs hatched in 2024. The results indicate that this strategy is feasible, particularly given the high larval survival; however, the low and variable hatching success requires further optimization before application in restoration programs.