Hatched Late to Be Fed? Identifying the Origin of Late-Summer Pikeperch Cohorts Using Daily Otolith Increments

Effective fisheries management depends on a clear understanding of the recruitment dynamics of young-of-year (YOY) fish. Analysis of two-decade ichthyological data from Czechia and the Netherlands revealed an unexpectedly small cohort of YOY pikeperch (Sander lucioperca) in late summer. The origin of these individuals remained speculative. Building on the long-term patterns documented in these regions, we provide a new empirical contribution by analyzing the daily otolith increments of 29 pikeperch individuals sampled in the Lipno Reservoir (Czechia) in late August 2023. The otolith-based aging revealed that the extremely small pikeperch were not more than 35 days old, indicating they hatched at the turn of July and August. Even with the uncertainty in the exact timing of the first otolith increment formation, this approach disclosed the origin of the extremely small cohort as late-hatched individuals produced by wild late-spawning pikeperch. These extremely small, delayed pikeperch coexist with their normal, earlier-spawned conspecifics, which are often already piscivorous. Although they mostly do not survive the first winter, they still contribute to the recruitment indirectly by serving as prey for conspecifics. Our results highlight the need to revise the current understanding of pikeperch spawning phenology in the temperate zone.