Background
In temperate lowland rivers, carp are allied with two important
habitats: a shallow well-vegetated spawning habitat, preferably in
off steam wetlands, and a deep over wintering habitat in the main
river channel. The shallow wetland habitat enables spawning and the
replenishment of populations via recruitment. The deep habitat is
thought to provide refuge from flow and maintain stable
temperatures in comparison with shallow surface waters. Movement
between these two habitats is annually predictable, and results in
localised accumulations of carp (spawning and over wintering
aggregations). Carp aggregations are vulnerable to targeted removal
programs and allow focussed, effective carp management efforts.
Existing CSC technology has proven effective in trapping
migrating carp by exploiting their jumping behaviour. However,
field and laboratory observations have shown that carp prefer to
push their way under barriers rather than jump over them.
Modifications to existing CSCs to incorporate a gated, spring
tensioned or weighted pushing mechanism therefore had considerable
potential to further increase trapping effectiveness by exploiting
this behaviour.
Aims
This project is identifying methods of exploiting carp spawning
aggregations at wetland inlets by evaluating:
- the migratory behaviour of carp through wetland inlets
- the application of existing carp separation cage (CSC)
technology for trapping and removing carp at wetland inlets
- modifications to the existing CSC design, including the
incorporation of a “pushing” trap component and trash
racks to collect floating debris
- physical (current-flow) and chemical (food odours, other
chemical stimuli) attractants to optimise the uptake of carp into
traps.
Activities
Field sampling was undertaken during late summer and early
autumn 2007 to indirectly determine the pushing power of carp. The
results of these measurements were used for the
development/manufacture of wetland CSCs incorporating a pushing
component (in a “cat-flap” configuration). Two of these
traps are currently being tested on Banrock Station, with promising
results. Project researchers also took the opportunity for further
study when the Banrock Station wetland was drained in mid-February
2007. Lateral movements of carp and native fish were evaluated,
confirming that native fish were quick to leave a drying wetland,
while carp stayed and even swam further into the water body,
against the out-flowing water. Water samples were taken from
the wetland, the outlet creek and the river, and are undergoing
chemical analysis. It is suspected that current flow, amino acid
content, water temperature and/or sound may play a role in carp
behaviour.
Recent progress
The carp separation cages (funded by MDBC as part of its Native
Fish Strategy) and modified by SARDI to work with carp in wetlands,
have removed around seven tonnes of breeding carp from the Murray
River in just 10 weeks. This was in only one wetland.
Trials of the cages form part of our project 4.F.12 working on
Achilles' heel exploitation of unique carp behaviours.
This work, when linked to our identification of breeding
hotspots, and work on sensory attractants, offers real hope that
carp can be successfully targeted for removal in large
numbers. SARDI senior research officer Dr Leigh Thwaites
hopes the traps will eventually be used to help control carp
throughout the Murray-Darling Basin.
The push trap designs are available on the SARDI website, along
with further information about carp separation cages. [
Read the MDBC media release here].
For further information contact Dr Ben Smith: smith.ben2@saugov.sa.gov.au
or Dr Leigh Thwaites: thwaites.leigh@saugov.sa.gov.au.
