Long‐term crucian carp ( Carassius carassius ) resilience to duckweed (Lemnaceae) dominance in a small eastern English pond

Successful scientifically informed fish conservation requires a good knowledge of how fishes have and will be affected by multiple stresses over the decadal-centennial timescales relevant to real-life fish-environment responses. By combining palaeoecological and monitoring data (regular sampling over 16 years), we studied the long-term response of an English pond population of crucian carp (Carassius carassius) to environmental stress (likely anoxia) posed by episodic periods of duckweed (Lemnaceae) dominance. Despite at least three major phases of prolonged high duckweed cover, in addition to historically recorded low water events, crucian carp persisted in the study pond for at least two centuries until the present day. During the most recent duckweed phase (spanning 6 years - 1999-2005), the crucian carp population edged close to extinction, with a few large individuals surviving and an elimination of recruitment (or at least severely reduced recruitment) over 4 years. Crucian carp recruitment resumed 1 year post-duckweed, however, and occurred at regular intervals subsequently leading to a population structure dominated by individuals of age 2-4 years. After 2002, a broader and more even population size and age structure was evident, suggestive of full recovery. The remarkable resilience of crucian carp to dense duckweed cover and likely associated anoxia is deserving of future research. Clearly, duckweed dominance in small hydrologically isolated ponds, as may be more prevalent under climate change, is one of the major threats to the crucian carp. Pond management aimed at decreasing tree canopy cover may help mitigate this by increasing surface wind movement of duckweed. Given multiple pressures on crucian carp populations throughout its range, we call for more active local-scale conservation projects to help reverse its current decline.