Movements and habitat use of common carp (Cyprinus carpio) and Murray cod (Maccullochella peelii peelii) juveniles in a large lowland Australian river

Native Murray cod (Maccullochella peelii peelii) are listed as a nationally vulnerable species, whereas non-native common carp (Cyprinus carpio) are widespread and abundant. Understanding key aspects of life history, such as movement patterns and habitat selection by juvenile Murray cod and common carp, might be useful for conserving Murray cod populations and controlling common carp numbers. We used radio-telemetry to track eight juvenile Murray cod and seven juvenile common carp in the Murray River, Australia, between March and July 2001. Common carp occupied a significantly greater total linear range (mean ± SD: 1721 ± 1118 m) than Murray cod (mean ± SD: 318 ± 345 m) and the average daily movement was significantly greater for common carp (mean ± SD: 147 ± 238 m) than for Murray cod (mean ± SD: 15 ± 55 m). All Murray cod and five of the seven common carp displayed site fidelity or residency to one, two or three locations. Murray cod were found only in the mainstream Murray River among submerged woody habitats, whereas common carp occurred equally in mainstream and offstream areas, and among submerged wood and aquatic vegetation. Murray cod were found in deeper (mean ± SD: 2.3 ± 0.78 m) and faster waters (mean ± SD: 0.56 ± 0.25 m·s-1) compared with common carp (mean ± SD: 1 ± 0.54 m; 0.08 ± 0.09 m·s-1) respectively. The presence of juvenile Murray cod only amongst submerged wood is an indication that these habitats are important and should be preserved. Conversely, juvenile common carp were equally present among all habitats sampled, suggesting that habitat selection is less specific, possibly contributing to their widespread success.