Oral Presentation Australian Society for Fish Biology Conference 2025

Hybridisation dynamics of Murray Cod and Trout Cod in Cataract Reservoir (124869)

Matthew A Campbell 1 , Katherine Harrisson 2 , Meaghan Duncan 3 , Dean M Gilligan 4 , Luke Pearce 5 , Peter J Unmack 6
  1. The University of Sydney, Brownlow Hill, NSW
  2. Latrobe University, Melbourne, VIC
  3. NSW DPIRD, Narrandera, NSW
  4. Bush Heritage Australia, Docklands, VIC
  5. NSW DPIRD, Albury, NSW
  6. Unmack AquaEvolutionary, Canberra, ACT

Background

Hybridisation is a phenomenon defined as the successful mating of individuals from two genetically differentiated lineages. Hybridisation can serve an important role in the exchange of adaptive genetic material, the reinforcement of species boundaries, and the creation of new taxa. This phenomenon occurs naturally but may become of conservation concern when anthropogenic actions promote hybridisation leading to the loss of distinctive gene pools or homogenisation of progenitor lineages. Populations of Murray Cod (Maccullochella peeli) and Trout Cod (M. macquariensis) were established by translocation in 1907 in Cataract Reservoir, near Sydney. Since that time, these fishes have hybridised resulting in an abundant hybrid cod population in Cataract Reservoir. In contrast, hybrid cods are rarely detected in the native range of the species.

Methods

In 2025, all size classes of fish were sampled from Cataract Reservoir. In addition, we obtained 87 tissues from a previous study that sampled cods in 2019. Two earlier studies also provided mitochondrial data matched to nuclear data (microsatellites and paired-end RADseq) from 225 fish. A total of 564 fish were genotyped with DArTseq from 2019 and 2025 and we characterise the relative proportions of Murray Cod, Trout Cod and hybrid cod in contemporary Cataract Reservoir samples. We also examine the ancestry proportions and hybridisation classes of hybrid cods from DArTseq data (F1, F2, F1 x Murray Cod, etc.).

Results

The data indicate that genetically intact Murray Cod and Trout Cod are found in Cataract Reservoir. The mitochondrial data demonstrate that all putative first generation hybrid cod have Trout Cod mtDNA, thus they originate from male Murray Cod x female Trout Cod crosses. Ongoing hybridisation is evident, such as F1 x Murray Cod backcrosses.

Conclusions

Our findings provide a perspective on the persistence of pure Trout Cod in Cataract Reservoir, an important source of fish for ongoing efforts to re-establish this fish in its former native range. The Cataract Reservoir system, while artificial, may be a model system for understanding and managing Murray Cod and Trout Cod hybridisation, especially in impoundments.