Linking movement dynamics to ecosystem structure: Suckers as ecosystem engineers

Suckers are often the most abundant large-bodied native fish in southwestern U.S. streams, yet little is known about their ecosystem impacts, growth, or movement. The species Catostomus insignis and Catostomus clarki act as “ecosystem engineers” modifying the physical structure of these streams. In summers 2008 and 2009, 550 C. insignis and C. clarki were tagged and tracked in the upper Gila River, NM, using coded (PIT) tags. Fish were monitored using antennas to detect either the passage of tagged fish past fixed points in the stream (stationary), or to scan the entire 5 km study reach (portable).  Suckers showed distinct diel activity patterns, moving out of refuge habitats (e.g., undercut banks) into shallow areas at dusk and returning to refuges before dawn. Nighttime movements were positively correlated with increases in suspended sediment load because in shallow habitats, nighttime feeding resulted in intense, but patchy disturbance of soft substrates. Most fish (>70%) moved >100 m away from release sites during the study; however, some fish moved only among adjacent habitats (<300m) while others made much larger movements (2-4 km). Our data suggest that linking the timing and location of movements to ecological processes is key to understanding how fish structure ecosystems.