Hollow logs for cod habitat
Since European settlement, populations of the endangered Mary River cod (Maccullochella mariensis) have plummeted as a result of land clearing and long-term habitat degradation, overfishing, extreme weather, invasive fish species and recreational fishing during the cod breeding season from August to November.
This iconic freshwater fish, a top order predator native to the Mary River system, has specific habitat requirements that include complex instream woody structures – hollow logs and undercut banks, and deep pools surrounded by intact riparian vegetation which shade the water and keep it cooler. This is the habitat cod need to breed and feed.
The record floods in 2022 severely impacted on riparian and aquatic species habitat in waterways throughout the Mary River catchment. The MRCCC was subsequently successful in receiving flood recovery funding from the Commonwealth and State Governments for a suite of actions aimed at reinstating habitat for a range of aquatic species including the Mary River cod.
Teaming up with the Jinibara Peoples Aboriginal Corporation, Griffith University and the Burnett Mary Regional Group and gaining expert advice from the Noosa Hinterland Hatchery and NSW Fisheries, a plan was developed which would test the cod’s preferences for various structures for refuge and spawning. Developing a resilience intervention that ensured post-flooding recovery, a number of ideas were floated, from concrete pipes to timber ply triangular nesting boxes.
Funding doesn’t last forever so another aim was to give landholders, like-minded individuals and community organisations the tools to replicate the project work; particularly following a flood event. In reality, these
structures can be added in-stream anytime to give the cod more hollows to spawn within.
The final cod hollow design met all the criteria; light enough to transport and a nifty person with a chainsaw can whip one up. Arborists split 1300mm long by 7-800mm diameter logs into two pieces and carved each of the split pieces of log into the required internal dimensions, these halves were then bolted back together, thus creating an artificial hollow log. These logs were then moved onto the banks of various creeks on properties where landholders were supportive throughout the catchment.
The installation took place in late July 2024 with water temperatures sitting from 9.5-11.5 degrees. A fortnight later the weekly monitoring commenced at all 24 of the cod hollows and 24+ naturally occurring structures. Several weeks later, in one of the hollows in the upper Mary, a large male cod was filmed fanning a substantial number of eggs. Since then, a number of cod and many other species have been documented using the cod hollows. A brilliant and undeniable success, which was reported on by ABC Rural Journalist, Jennifer Nichols.
A video of the ABC report can be found here