Continued impacts of the Black Summer bushfires on native fish and platypus

Australian bushfires are renowned for their unimaginable heat and destruction. The 2020 Black Summer bushfires were a reminder that it is not only the lives and urban ecosystems of human beings that are affected. The media cycle produced countless statistics and images of stranded, burnt wildlife, especially those found in our forests like koalas, kangaroos and more.

However, a little considered aspect of bushfires is how they affect animals that live in and around waterways, including native fish and other aquatic animals like frogs and platypus. Bushfires cause direct mortality and also degrade the habitats for aquatic animals.

A recent study looked at the impact of the Black Summer bushfires on native fish and platypus populations in the Buffalo River in North-east Victoria, where three-quarters of its upper catchment was burned between late 2019 – early 2020. In particular, the study looked at the impact of the first substantial rains in early autumn after the fires. Post-fire rain events are known to cause significant damage to water quality, habitat structures and resource availability when large amounts of ash, charcoal and eroded soil are washed into waterways with greater run-off due to less vegetation and canopy cover around the riverbanks.

Thick layers of bushfire ash and sediment wash into the Upper Murrumbidgee river, smothering macroinvertebrate habitat.

Thick layers of bushfire ash and sediment wash into the Badja river, smothering macroinvertebrate habitat. Photo credit: Antia Brademann.

This section of the Upper Murrumbidgee has endured a thick flow of ash which has turned the water black. Photo credit: Antia Brademann.

This section of the Badja River has endured a thick flow of ash which has turned the water black. Photo credit: Antia Brademann.

The study compared the different responses of platypus and native fish to the bushfires, particularly focusing on long-term impacts. Interestingly, the study did not find significant declines in fish population numbers directly due to the fire. However, there were major impacts on fish numbers from rainfall and run-off after the fire – around 90% of two-spined Blackfish and Macquarie perch were lost from the river after the post-fire flooding. In contrast, invasive species had less severe responses to the influx of sediment after the fires, with Redfin perch numbers actually increasing in Buffalo River.

Comparatively, the study found that platypus survived the bushfires and post-fire effects better than native fish species. Platypus breeding actually increased post-fire, with higher numbers of juvenile platypus recorded in the first post-fire breeding season. Because platypus can breathe through lungs instead of gills, they are more resilient to bushfire than native fish. This is especially important when waterways have extremely low dissolved oxygen concentrations due to the large amounts of sediment and ash run-off following post-bushfire rainfall. Platypus also have the ability to seek refuge in their burrows, which protects them from the direct impacts of flame and smoke.

An Australian gum tree sprouts new growth after being damaged by the 2019–20 Black Summer bushfires. Photo credit: Christian Bass.

These important findings help inform future management of native fish populations after major bushfires, as well as platypus populations. One clear implication is that the management of aquatic environments after bushfires needs to take into account species-specific responses to different impacts. For example, native fish are most likely to require additional support in the weeks and months following heavy post-fire rainfall, instead of immediately during or after the actual fires.