Environmental Water Allocations in regulated lowland rivers may encourage offstream movements and spawning by common carp, Cyprinus carpio: implications for wetland rehabilitation

Abstract

Environmental Water Allocations (EWAs) are used to enhance native flora and fauna in regulated rivers, but may also benefit alien invasive species like common carp (Cyprinus carpio). We examined the invasion and spawning risk posed by adult common carp during an EWA delivered from the River Murray to a flow-through wetland in South Australia from June to December 2008. Offstream movements of fish and turtles were monitored continuously via the inlet and outlet creeks. Long-necked turtles (Chelodina longicollis, n = 129) dominated at the inlet where few fish were collected (n = 24), whereas much larger numbers of common carp in prime spawning condition (n = 4709), alien goldfish (Carassius auratus, n = 1201) and native bony herring (Nematalosa erebi, n = 93) were attracted to the outlet and displayed distinct movements. Adult common carp movements began in August, in response to increasing water temperatures, peaked in mid-September before spawning, then declined and were close to zero by December. The timing of EWA deliveries potentially could be manipulated to reduce adult carp invasion and spawning potential while providing some advantage to native fish, but the benefits may be short-lived without additional carp management interventions such as wetland drying.