Introduction
Climate change is a major anthropogenic driver of biodiversity loss, intensifying extreme weather events such as floods and heatwaves (Heino et al., 2009). In freshwater systems, rising temperatures threaten cold-water species like Atlantic salmon (Salmo salar) (Buisson et al., 2008; Comte & Grenouillet, 2013). As ectothermic stenotherms, salmon are highly sensitive to temperature, especially during embryogenesis, which is optimal between 7–8 °C (Angilletta et al., 2002; Fraser et al., 2015). Hence, even slight thermal deviations can disrupt biochemical and metabolic processes, affecting growth, reproduction, and behaviour (Jonsson & Jonsson, 2009). Elevated incubation temperatures (>8 °C) have been linked to reduced survival, altered development, and increased abnormalities in salmon larvae (Kamler, 2002).
In Atlantic salmon populations, two male reproductive strategies are observed: migratory (anadromous) males, and precociously maturing males that reproduce in freshwater during their first (age 0+) or second (age 1+) year without seaward migration. This alternative tactic enhances resilience in harsh environments and is thought to result from interactions between genetic predisposition and environmental factors such as temperature-driven growth (Vladic et al., 2010; Martinez et al., 2000). With projected river warming, increased frequencies of precocious maturation may shift population structure toward freshwater residency, potentially altering genetic dynamics. These dynamics are particularly relevant for conservation aquaculture and the design of effective restocking strategies that aim to preserve genetic diversity and ensure the long-term viability of restored populations.
This study aims to evaluate how male genetic background modulates the effects of elevated water temperatures during embryonic development on key traits critical to Atlantic salmon restoration. Specifically, it will assess the impact of increased incubation temperatures on: (i) juvenile stress-related behaviour and (ii) immune system functionality.
Material and Methods
Six wild anadromous male Atlantic salmon (Salmo salar, 5–6 years old) were captured during their 2024 upstream migration using a trapping system at the Lith fish ladder on the Meuse River (Netherlands). They were held in all-male tanks at the CoSMOS (Conservatoire du saumon mosan) hatchery in Érezée, Belgium (50°18′21.78″N, 5°32′33.61″E) until the breeding season. Additionally, six hatchery-reared precocious parr males (age 1+) raised at CoSMOS were used for reproduction.
Eggs were collected from farmed females (age 4+), pooled, and divided into batches of 300 eggs. Each batch was fertilized using fresh sperm from one of the 12 males to obtain a standard sperm-to-egg ratio of 10⁶. Fertilized eggs were incubated at the University of Namur in a dark, temperature-controlled room at 11 °C. After yolk sac resorption, larvae were transferred to larger tanks for free swimming and feeding to apparent satiation. Following two weeks of feeding, the juveniles underwent a standardized startle response test. Subsequently, half of the individuals were exposed to a bath infection with Aeromonas salmonicida achromogenes at a concentration of 1.41 × 10⁹ CFU/mL for 12 hours at 11°C, while the other half served as an uninfected control group. Fish were then euthanized for liver and brain tissue collection.
RNA-seq was performed to identify differentially expressed genes (DEGs) between paternal origins. Selected genes related to immunity and stress were further analysed using RT-qPCR. In addition, RT-qPCR on telomere will be performed to assess the impact of genetic on chronic stress.
Results and discussion
Preliminary results indicate generally low survival rates across all groups, with an average of 37.6 (min: 25.9, max: 57.5) for from anadromous males and 33.5 (min: 25.7, max: 42.8) f or offspring from precociously maturing parr. In the behavioural trials, we anticipate that juveniles fathered by anadromous males will demonstrate faster escape responses under stress. Finally, transcriptomic analyses are still underway, however, we expect to identify distinct gene expression patterns between offspring from the two male types, particularly in pathways related to stress response and immune function. These results would suggest that paternal reproductive strategy influences offspring resilience to early-life thermal stress, with implications for Atlantic salmon adaptation under future climate change scenarios.
References
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