Is diel activity determined by social rank in a drift-feeding stream fish dominance hierarchy?

Dominance hierarchies among pool-dwelling drift-feeding fish can determine spatial and temporal segregation of feeding territories. We tested the hypothesis that primarily nocturnal, dominant giant kokopu, Galaxias argenteus, force smaller, primarily diurnal, subordinate giant kokopu to feed by day. To test whether subordinate fish would become more nocturnal in the absence of dominant fish, we temporarily removed dominant fish from stream pools in which stable dominance hierarchies operated. Removal of the dominant fish resulted in a significant increase in the number of smaller giant kokopu active in stream pools at night; however, no change in activity was detected by day. The results indicate that large dominant fish control diel activity of subordinate fish, excluding them from stream pools at times (night) when invertebrate drift densities are at their highest.