On yesterday's ABC 666 spot, Dr. Ben Smith of the South Australian Research and Development Institute reported that carp have begun to move in the Murray River, maybe signalling the end of the Australian winter. Ben is monitoring carp behaviour on Banrock Station in South Australia. Last year, the Station's wetlands were drained and tens of thousands of European Carp were left stranded - very few carp left the wetlands with the flow of water. Australian native fish, however, took the opposite decision, leaving with the draining water.
This phenonmenon probably results from different evolutionary responses of native fish compared to carp. Native fish in a place as dry as Australia probably equate lowering water levels with coming drought and so swim with the flow. Carp might be associating the water flow with wetlands flooding, which is more in line with their native Asia ("European" carp are actually Asian originally). Hence, the carp swam against the water flow, into the draining wetland.
Pictured behind Ben in the picture is a carp separation cage. These cages rely on carp behaviour of jumping over a barrier, whereas Australian natives don't generally jump. This behaviour is being exploited to separate carp from the rest of the fish population. Because the Murray-Darling Basin Commission have built fishways from the Hume Dame to the sea, these separation cages are a brilliant development in practical carp control. The one behind Ben is actually also fitted with a push-trap device to exploit pushing behaviour as well.
Over the past few months, this trap on the inlet to the Banrock wetlands would not contain carp because they are not active at this time of year. Ben said in the interview that on the drive from Adelaide he noticed Almond trees coming into bloom - a sign of the turning season. On getting to Banrock he was presented with this haul of carp.
This is another example of the "Achilles' Heel" approach to pest management - exploiting some physical or behavioural feature of the pest to isolate it in some way from the native population. It's also an example of one of the many, many great things happening on our river system - the lack of water obviously dominates the headlines and we forget that goods things are happening as well.