Preference by juvenile Chinese sucker Myxocyprinus asiaticus, for substrate colour in zero versus slow velocity regimes suggest a change in habitat preference of wild juveniles after damming the Yangtze river

Habitat preference of benthic riverine juvenile Chinese sucker, Myxocyprinus asiaticus, a protected species in China, is poorly understood. This situation makes it difficult to evaluate the potential impacts on juveniles after river alteration. To better understand the impact of damming, we studied preference of cultured juvenile Chinese sucker (7.90-8.55cm, standard length) for dark versus light substrate colours in 2 velocity regimes: zero velocity (like on a reservoir bottom) and in a slow water current of 0.15-0.19cm/s (like on the bottom of a slow flowing stream). In the slow current regime, juveniles chose black substrate during the day and night. This likely represents the natural substrate preference by wild suckers in a river. However, in the zero velocity regime, juveniles chose a black substrate during the day, but switched preference to a white substrate at night. The results suggest wild juveniles in the natural Yangtze River have evolved a preference for a black substrate during the day and night. However, after the river was impounded by the Three Gorges Dam and bottom velocity changed to a static regime, juveniles in the reservoir changed substrate colour preference and now prefer a light-coloured substrate colour at night. Thus, damming likely caused a major behavioural change by juveniles. The adaptive significance and effect on fitness of this change in habitat preference is not known but should be investigated as wild Chinese sucker continue to decline in abundance.