Did the mice stray too close to the riverbank? Did they drown during a flooding event?
The scientists can't be sure and say either is possible.
"The catfish may be altering their diets according to what's available," explained Erin Kelly, lead researcher on a new study of the fish, in a statement. "This mouse species has been reported to construct deep burrow systems in the sand of riverbanks. If a burrow of mice is flooded and collapses into the river, the catfish are likely to be taking advantage."
"Both species are nocturnal, and it is also possible that the catfish are actively hunting mice on the riverbank," added Kelly.
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It's not entirely unheard of for fish to eat mammals. Fish in Alaska have been documented regularly consuming shrews, and catfish can even be found on Youtube snaring pigeons that get too close to the water. Monkfish, for their part, have been found to eat puffins.
But researchers in this case say it's the first they have seen of high-volume consumption of mammals by Australian catfish, and they hope they can learn more about how the land- and water-based food webs might be changing, especially in a warmer Australia. The better they understand the changing habitats, the better they can manage and conserve them.
"This is of increasing importance as Australia's climate warms," said Kelly. "Changing weather patterns and the increasing challenge to meet growing water demands is likely to disrupt the flow of many of Australia's rivers and threaten the unique biodiversity of these ecosystems.
"Understanding whether species like the lesser salmon catfish can adapt to their changing environment, and how the successful ones do it, will help us to conserve these creatures," said Kelly.
Kelly and her co-authors' findings have been published in the Journal of Arid Environments.
Hat tip: NewScientist