This study investigated diel and seasonal variations in the population dynamics of Hemimysis anomala, a non-indigenous mysid originating from the Ponto-Caspian region. There were strong diel variations in the abundance (catch-per-unit-effort; CPUE) of H. anomala, with CPUE being low during the day but increasing by up to an order of magnitude at night. Abundance also varied seasonally, with CPUE increasing from mid-May to a peak in late June/early July, followed by a crash in mid-July. However,

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... the seasonal variations in abundance were apparent only in nocturnal surveys, with CPUE during the day being low throughout the study. There were also seasonal variations in population structure (life stages, length distributions, sex ratios, reproductive state, fecundity). The nocturnal behaviour of H. anomala renders it difficult to assess its geographical distribution or population dynamics using only diurnal surveys or inappropriate sampling methods, which could underestimate the size, structure, reproductive potential and sustainability of non-indigenous populations of this invasive species. Surveillance programmes for H. anomala should therefore be conducted at night, at intervals throughout the year and targeting likely habitats using site-specific methods of proven effectiveness.