Experimental tests of intraspecific competition in stream riffles between juvenile and adult longnose dace (Rhinichthys cataractae)

This study was conducted to examine the role of intraspecific competition in size-specific habitat segregation by longnose dace (Rhinichthys cataractae). Previous studies indicate that adults use faster velocity areas and larger substrates than juveniles. An instream maze containing fast- (40-50 cm/s), medium- (25-35 cm/s), and slow-velocity (0-10 cm/s) habitats was used to examine the habitat preferences of both size classes. Cages provided with a single shelter were used to examine the effect of the presence of adults on shelter use by juveniles, and adults were removed from an enclosed riffle to examine the response of juveniles. Both adults and juveniles used the fast- and medium-velocity habitats within the maze and avoided the slow-velocity habitat. Juveniles reduced their use of the shelters in the presence of adults, and expanded their niches to include faster velocity areas of the riffle when adults were removed from the enclosed riffle. However, juveniles did not increase their use of larger substrates in the absence of adults. These results suggest that segregation of juvenile and adult longnose dace according to velocity is due to intraspecific competition for faster velocity areas within the riffle, while segregation according to substrate size may be due simply to an increasing preference for larger substrates as fish grow.