The Impact of Fish on Amphibian Metamorphosis a Study Case From Retezat National Park, Romania

Fish introductions are a threat to freshwater biodiversity, particularly in historically fishless alpine lakes. Amphibians, already in global decline, are highly susceptible to disturbances during their aquatic developmental stages. In Romania, the widespread fish stocking with trout during the 1960s of alpine lakes has resulted in the establishment of the common minnow (Phoxinus phoxinus)—a bait species—presently co-occurring with native amphibians. We conducted a mesocosm experiment to assess the impact of minnows during the early developmental stages of common frogs. We quantified: (i) survival to the onset of metamorphosis, and (ii) endurance, in freshly-metamorphosed froglets. While survival to metamorphosis was significantly lower in the presence of fish, no significant differences were detected in size, body condition, or endurance of metamorphs. The absence of observable carry-over effects suggests possible early-stage compensatory responses, though subtle sublethal effects may have been missed. These findings highlight the importance of conserving fish-free alpine habitats and raise concerns about the overlooked impact of bait fish on amphibian recruitment.