Introduction
Freshwater ecosystems are biodiversity hotspots but are highly threatened by anthropogenic pressures, leading to rapid species decline [1]. The Baltic sturgeon (Acipenser oxyrinchus), once widespread in Northern European rivers and coastal waters, became extinct in the region due to overfishing, pollution, and habitat alterations [2]. Current reintroduction programs rely on hatchery-reared juveniles. However, their success is limited by poor post-release survival. Traditional rearing environments are often homogeneous and may impair behavioral plasticity and adaptability. Environmental enrichment has been proposed to enhance cognitive and behavioral development [3,4]. This study investigated how different early-life rearing conditions affect behavior, neuroplasticity, and post-release performance in Baltic sturgeon.
Materials & Methods
Juvenile sturgeons were reared under three conditions: natural (NAT), semi-natural (CONT), and conventional recirculating aquaculture systems (RAS). Behavioral performance was assessed in a Y-maze (n = 27) by measuring activity, exploration, and latency to novel environments. Post-release movement of NAT and CONT fish (n = 12) was monitored using acoustic telemetry in the River Drawa (Poland). Additionally, brain tissue (n = 27) was analyzed for expression of neuroplasticity markers (BDNF, neuroD, PCNA) using RT-qPCR. Statistical analyses were conducted with significance set at p < 0.05.
Results & Discussion
Clear differences in behavioral performance were observed in the different rearing treatments. Fish from RAS conditions exhibited the highest swimming activity, exploration rates, and use of open areas in the maze. While this indicates increased boldness, it likely reflects maladaptive behavior, as excessive activity and risk-prone exploration can increase exposure to predators in natural environments. In contrast, NAT fish showed significantly lower activity, fewer entries into novel compartments, and longer latencies before exploration, suggesting more cautious and energy-efficient behavioral strategies. CONT fish generally displayed intermediate responses.
Performance under flow conditions further emphasized these differences. At the same current velocity NAT and CONT fish maintained position significantly longer than RAS fish, indicating superior swimming endurance and better adaptation to hydraulic challenges. This suggests that exposure to variable flow regimes during rearing enhances physiological performance relevant for riverine environments.
Gene expression analyses supported the behavioral findings. RAS fish showed the lowest expression of neuroplasticity markers (BDNF, neuroD, PCNA), indicating reduced neural development under highly controlled conditions. In contrast, CONT fish exhibited the highest expression levels, while NAT fish showed intermediate values. This pattern suggests that moderate environmental variability—rather than extreme stability—may provide optimal conditions for neural development.
Field telemetry revealed consistent differences in post-release performance. NAT fish initiated movement more rapidly and migrated significantly faster (10.76 km day-1) than CONT fish (7.46 km day-1), indicating more efficient downstream dispersal. Faster migration likely reduces the duration of vulnerability immediately after release, thereby lowering predation risk and increasing the likelihood of locating suitable habitats. In contrast, CONT fish displayed slower, more variable movement patterns, including temporary residency, which may prolong exposure to threats. Despite these differences in movement dynamics, both groups exhibited similar habitat preferences, primarily occupying mid-channel areas. This suggests that fundamental habitat selection cues are retained across rearing conditions, whereas differences are more pronounced in behavioral flexibility and movement efficiency.
Overall, the results demonstrate that rearing environment strongly shapes behavioral traits, physiological performance, and neuroplasticity in Baltic sturgeon. Traditional RAS conditions promote high activity but are associated with reduced cognitive flexibility, endurance, and neural development. In contrast, enriched and semi-natural environments enhance cautious behavior, spatial awareness, and migration efficiency—traits that are likely critical for survival in the wild.
These findings highlight the importance of incorporating environmental complexity and moderate variability into hatchery practices for conservation purposes. Optimizing rearing strategies to promote adaptive behavior and neural plasticity may substantially improve post-release survival and contribute to the long-term success of Baltic sturgeon restoration efforts.
Acknowledgement
The study was funded by the project "Development of innovative rearing techniques for the production of site-adapted stocking fish taking the Baltic sturgeon as an example [Fish fit] (EMFF BB-II.1-140)
References
[1] Haase, P., Bowler, D.E., Baker, N.J., Bonada, N., Domisch, S., Marquez, J.G.R., et al., 2023. The recovery of European freshwater biodiversity has come to a halt. Nature. 620, 582.
[2] Gessner, J., Arndt, G.-M., Kapusta, A., Shibayev, S., Gushin, A., Pilinkovskij, A., Povliunas, J., Medne, R., Purvina, S., Tambets, M., Möller, P.R., 2019. HELCOM-Action Plan for the protection and recovery of Baltic sturgeon A. oxyrinchus in the Baltic Sea area.
[3] Yebra-Pimentel, E.S., Reis, B., Gessner, J., Wuertz, S., Dirks, R.P.H., 2020. Temperature training improves transcriptional homeostasis after heat shock in juvenile Atlantic sturgeon (Acipenser oxyrinchus). Fish Physiol Biochem. 46.
[4] McAdam, S.O., 2011. Effects of substrate condition on habitat use and survival by white sturgeon (Acipenser transmontanus) larvae and potential implications for recruitment. Can J Fish Aquat Sci. 68, 812-822.