The importance of night-time observations for determining habitat preferences of stream biota

Observations of the presence or density of individuals in specific habitats are often used to develop predictive models of preferential habitat use and habitat suitability curves. However, most studies only use day-time observations to model habitat associations. In this paper, we present day and night observations of habitat use by two abundant species of freshwater shrimp, Atya lanipes and Xiphocaris elongata, in a tropical montane stream. At night, the number of shrimp observed was an order of magnitude greater than during the day. Habitat preferences of Atya changed, with more individuals using low velocity edge sites at night. Xiphocaris were not readily visible during the day, but were very abundant at night. Our results suggest that habitat models cannot be based only on day-time observations. Because the level of nocturnal activity is not known for most species of fish and invertebrates, studies of habitat preferences should include both day and night observations.