DO INTRODUCED FISH ESCAPE INFESTATION COSTS? Sub-lethal Effects of Freshwater Pearl Mussel (Margaritifera margaritifera) Glochidia on Invasive Brook Trout (Salvelinus fontinalis)

Sammanfattning

Biological invasions threaten freshwater biodiversity, especially when introduced species disrupt native species and ecosystem processes. In Sweden, invasive brook trout (Salvelinus fontinalis) often outcompete native brown trout (Salmo trutta) and act as a reproductive sink for the endangered freshwater pearl mussel (Margaritifera margaritifera), whose larval stage (glochidia) depends on salmonid hosts. Brook trout are non-functional hosts that prematurely shed glochidia and are thus thought to gain a competitive advantage by avoiding the costs of infestation, according to the enemy release hypothesis. This study investigated whether brook trout nevertheless incur sub-lethal physiological or behavioural costs following glochidial infestation. 0+ juvenile brook trout were artificially infested, and their physiological condition, including hepatosomatic index (HSI), specific growth rate, and Fulton’s condition factor, was measured 94 days after the infestation. Activity, habitat use, and competitive interactions were assessed in an artificial stream flume, testing individuals both alone and paired with a native brown trout competitor. Results confirmed that all glochidia detached prematurely, verifying brook trout as unsuitable hosts. Despite this, previously infested brook trout exhibited a significantly reduced HSI, indicating a physiological cost through depleted internal energy reserves. Behaviourally, spatial habitat use was driven primarily by interspecific competition rather than infestation history. When paired, brook trout faced competitive displacement, spending significantly more time hiding on the substrate and less time in shelters. Furthermore, agonistic interactions were relatively infrequent and driven entirely by body size, with smaller individuals receiving more attacks. Integrating these outcomes reveals a potential dual-stressor scenario in which previously infested brook trout must navigate competitive displacement while simultaneously experiencing reduced energy reserves. These findings demonstrate that even short, unsuccessful infestations impose hidden energetic costs, providing nuance to the enemy release hypothesis and indicating that glochidia exposure may contribute to reduced competitive performance in healthy ecosystems with robust native populations.