Freshwater ecosystems are a major player; they support about 9.5% of all species. However these ecosystems are being increasingly disrupted by human activity. This disruption can cause mass fish death. Mass deaths of fish, or fish kills, are naturally occurring events. But ecosystem disruption and human impacts are increasing the severity and frequency of fish kills. Fish kills can be crippling for the ecosystem; they dramatically lower water quality by increasing the concentrations of ammonia and nitrates, which then drive phytoplankton and cyanobacteria blooms. Bacterial blooms decrease dissolved oxygen concentrations which further increases risk of fish death. Taken together these effects fish kills can lead to devastating biodiversity losses and ecosystem breakdown.

Carp are a major pest in these ecosystems. In response to these pests in the Murray-Darling basin the government plans to introduce a virus to lower numbers. However introduction of a carp virus could result in millions of carp carcasses rotting in river systems. This could have huge impacts on people living and relying on these rivers for water. 

This is where the scavenging turtles come in. 

The new study attempted to see just how important these turtles are to freshwater systems. To measure the impact of the scavengers they used five groups of four male turtles in artificial wetlands. Dead fish were introduced into these wetlands, as well as into wetlands with no turtles. The carcasses were kept in the wetlands until they had been completely eaten or had completely decomposed.

When turtles were present, the water quality showed dramatic improvement. Carp carcasses were removed three times faster than without the scavengers present. Water quality was quantified by ammonia levels and dissolved oxygen levels. These parameters returned to initial conditions much faster with the turtles present.

These results prove that turtles are very important in maintaining healthy and normal levels in this ecosystem. However these turtles are vulnerable and are often found in very low numbers. Foxes predate on their nests and they are often killed on road crossings. The Murray-Darling basin has also seen significant alteration in the last century, with reduction in turtle-friendly vegetation and habitat loss. 

It is concluded that the turtle’s scavenging efforts are critical in removing fish carcasses rapidly before they decompose and taint the water. They are also critical in rapidly bringing the water quality back to normal after fish kills. If left to fester the water quality will be greatly reduced.

The ill effects from fish kills won’t just be felt in the environment as the Murray-Darling basin supports 40% of Australia’s agricultural production and it is the main water source for more than 2.8 million people. If the water quality lowers dramatically this will have knock on effects for the human population and agricultural efforts.

Therefore having a healthy turtle population is critical in these ecosystems, especially when to deal with carp pests the government authorizes occasional fish kills. Turtle conservation and population recovery should hopefully be brought to the fore to help protect both the ecosystem and human populations which rely on it. The control of invasive European foxes, ensuring hatchlings survival, and reducing turtle mortality rates on roads is a good start! 

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Citation:

Santori, C., Spencer, R., Thompson, M.B. et al. Scavenging by threatened turtles regulates freshwater ecosystem health during fish kills. Sci Rep 10, 14383 (2020). https://doi.org/10.1038/s41598-020-71544-3

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