Hybridisation dynamics of Murray cod (Maccullochella peelii) and trout cod (M. macquariensis) in Cataract Reservoir

Context The successful mating of individuals from two genetically differentiated lineages, hybridisation, becomes a concern for conservation when it leads to the loss of distinctive gene pools. Aims Murray cod (Maccullochella peelii) and trout cod (M. macquariensis), two species of conservation concern, were translocated in 1915 into Cataract Reservoir, near Sydney, New South Wales. We assess the extent of hybridisation between these two species in Cataract Reservoir. Methods We obtained nuclear and mitochondrial genotyping data from fish sampled in 2003–06 and 2013. Fish sampled in 2019 and 2025 were genotyped with Diversity Arrays Technology using a double-digest restriction site-associated DNA sequencing (RADseq) approach (DArTseq). Key results We found that first-generation hybrids have trout cod mothers. The current composition of cod in Cataract Reservoir on the basis of DArTseq genotyping of 562 fish is 67% genetically pure Murray cod, 7% genetically pure trout cod, 22% F1 hybrids and <4% other hybrid classes of fish. Conclusions Over 110 years, hybridisation has not led to the loss of genetically intact Murray cod and trout cod in Cataract Reservoir, but the proportion of trout cod appears to be in decline. Implications The risk of species collapse from hybridisation appears low; however, strategies to encourage reproductive isolation between Murray cod and trout cod should be identified.