Factors Influencing the Stocking of Juvenile Pikeperch ( <i>Sander lucioperca</i> )

ABSTRACT Predator species of fish play crucial roles in ecosystem balance through top‐down control, a mechanism frequently exploited in biomanipulation to stock predator species. Juvenile pikeperch ( Sander lucioperca ) were stocked ( n = 52,887) in spring 2023 and 2024 and autumn 2023 to compare stocking efficiency between Recirculation Aquaculture Systems (RAS) and pond culture systems. Post‐stocking mortality was high and differed among rearing methods and stocking seasons (pond‐reared: 93.3% in spring 2023, 100% in autumn 2023, and 98.9% in spring 2024; RAS‐reared: 100% in spring 2023, 99.5% in autumn 2023, and 99.1% in spring 2024). Recapture rates of pond‐reared pikeperch stocked in spring 2023 were highest, despite being smaller than corresponding RAS‐reared pikeperch when stocked. Recapture rates of autumn‐stocked pikeperch were low for both rearing methods in the single release of 2023. Pond‐reared pikeperch were smaller when stocked (91.56 mm in spring 2023; 69.26 mm in spring 2024) but grew faster and often reached a similar size as RAS‐reared fish that were larger when stocked (139.40 mm in spring 2023; 162.80 mm in spring 2024). When recaptured in spring 2023, both groups reached a similar size (~183 mm; at recapture in spring 2024 they had nearly similar sizes pond: 169 mm, RAS: 197.25 mm). Overall, survival of pond‐reared pikeperch was significantly higher than RAS‐reared fish, even after accounting for size at stocking and season. Rearing system, rather than initial size alone, strongly influenced post‐stocking success. High density of adult European perch ( Perca fluviatilis ) preyed heavily on juvenile pikeperch, thereby limiting natural recruitment. For pond‐reared pikeperch, size was crucial due to limited energy reserves for overwintering, whereas RAS‐reared pikeperch were challenged by a delayed shift to natural prey, poor shelter‐seeking behavior, and predator naïveté. Overall, we found that stocking success depended on factors beyond size at stocking, and high mortality prevented stocking alone from achieving desired ecosystem management goals.