Environmental conditions and timing for the spawning of Murray cod (Maccullochella peelii peelii) and the endangered trout cod (Maccullochella macquariensis) in southeastern Australian rivers

The environmental conditions and timing of spawning of Murray cod and trout cod were investigated over three successive years in the regulated Murray River and in the nearby, unregulated Ovens River. Larvae were collected in drift samples from early November. Murray cod larvae were present for up to ten weeks, but trout cod larvae were present for only about two weeks. Cod larvae were collected in both rivers in each year sampled, despite a range of flow conditions. Spawning periods, estimated by back-calculating larval ages and egg incubation times, were in part, concurrent for the two species, beginning in October when water temperatures had exceeded 15 degrees C, allowing the occasional hybridization that has been noted between these two species. Trout cod larvae (10.0-18.2 mm) were significantly larger than Murray cod larvae (9.5-14.8 mm) in both years and the larvae of both species were significantly larger in 1995/6 than in 1994/5 in the Murray River. There was no relationship between larval size and water temperature, but later spawning times at the upper Murray River site coincided with lower water temperatures. Larval abundance varied significantly between sites, samples and years, with peak larval abundances occurring in November. Murray cod larval abundance was best explained by the explanatory variables of year, day length and change in flow over the previous 7 d. Environmental conditions for the spawning of Murray cod and trout cod are similar, and both species exhibit a similar larval dispersal strategy by emergence into the drift. Spawning occurred regularly under a range of flow conditions and it is likely that recruitment of these species in these rivers is driven by the subsequent survival of larvae and juveniles. Copyright (c) 2005 John Wiley & Sons, Ltd.