Background

A NSW Department of Primary Industries (NSW Fisheries) project ‘Assessment of wild carp populations required for successful implementation and monitoring of daughterless carp gene technology’ was funded by our predecessor, the Pest Animal Control CRC.  It proved effective in demonstrating that carp reproductive activity is not widespread throughout NSW, with specific areas being identified as the origin of huge numbers of carp larvae.

Larval sampling in NSW over the several breeding seasons  identified the Gwydir wetlands, Namoi wetlands, the lower Boomi River, Barmah-Millewa Forest, Hume Dam and Lake Wyangala as ‘hot-spots’ of carp recruitment in their respective catchments.  However, given that the sampling regime was restricted by climatic variables, with a very small window of opportunity for sampling, a portion of NSW river systems were not sampled under that project.

Continuation of the sampling program was considered important to enable complete coverage of all river systems in NSW.  The replication of sampling in a selection of river systems was also needed to strengthen the scientific validity of the results.   The Invasive Animals CRC decided to continue the work.

Sampling of drifting larval carp is one of two means to definitively pin-point the location of hot-spots of carp reproduction (the other is otolith microchemistry). Although more cost effective than otolith microchemistry, sampling of larval carp is strictly only a demonstration of carp spawning activity, and does not necessarily translate into recruitment of those larvae into the adult population.  It has been hypothesised that drifting larval carp may be a physiologically weak component of the cohort, and that stronger larvae remain within the spawning habitats.  Under this hypothesis, the density of drifting larvae would be independent of the abundance of recruits in the system, which will be more closely associated with the density of non-drifting larvae found within spawning habitats.

In order to validate the results obtained by larval drift sampling, this project is testing the hypothesis that dispersing carp larvae are weaker than carp larvae which remain resident within spawning areas - by using a suite of larval condition indices measured from samples of drifting and resident carp larvae. 

The identification of carp recruitment ‘hot-spots’ project aims to assist an integrated pest management strategy for carp by guiding:

• appropriate spatial parameterisation of the CARPSIM 2.0 model
• targeted control of adult carp migrating towards spawning areas
• targeted control of spawning aggregations
• exclusion of spawning adults from spawning areas
• control of dispersing juveniles from spawning areas
• control of sink populations through limiting immigration from source populations.
• identification of priority control sites.

The current NSW DPI (NSW Fisheries) project ‘Assessment of wild carp populations required for successful implementation and monitoring of daughterless carp gene technology’ funded by the PAC CRC has proven effective in demonstrating that carp reproductive activity is not widespread throughout NSW, with specific areas being identified as the origin of huge numbers of carp larvae. Larval sampling in NSW over the past 2 breeding seasons has identified the Gwydir wetlands, Namoi wetlands, the lower Boomi River, Barmah-Millewa Forest, Hume Dam and Lake Wyangala as ‘hot-spots’ of carp recruitment in their respective catchments. However given that the sampling regime is restricted by climatic variables, with a very small window of opportunity for sampling, a portion of NSW river systems were not sampled under the current PAC CRC project (particularly as the past 2 years have been significantly drought affected). Continuation of the sampling program will enable complete coverage of all river systems in NSW and the replication of sampling in a selection of river systems will strengthen the scientific validity to the results, which are at present only based on single replicate samples. Secondly, the Daughterless Carp Reference Group (CRG –the scientific body advising the PAC CRC) recommended at its meeting of 13 January 2004 that the field sampling and data collection be extended throughout the entire Murray-Darling Basin.

Carp larvae can’t swim, they only drift with river flows.  Sampling of drifting larval carp is therefore one of two means to definitively pin-point the location of hot-spots of carp reproduction (the other is otolith microchemistry). Although more cost effective than otolith microchemistry, sampling of larval carp undertaken under this proposal is strictly only a demonstration of carp spawning activity, and does not necessarily translate into recruitment of those larvae into the adult population.  It has been hypothesised that drifting larval carp may be a physiologically weak component of the cohort, and that stronger larvae remain within the spawning habitats (Smith, pers. comm.). Under this hypothesis, the density of drifting larvae will be independent of the abundance of recruits in the system, which will be more closely associated with the density of non-drifting larvae found within spawning habitats. In order to validate the results obtained by larval drift sampling, this project will test the hypothesis that dispersing carp larvae are weaker than carp larvae which remain resident within spawning areas by using a suite of larval condition indices measured from samples of drifting and resident carp larvae. 

The identification of carp recruitment ‘hot-spots’ will assist an IPM strategy by guiding:

• Appropriate spatial parameterisation of the CARPSIM model (developed by  the Victorian Department of Primary Industries with CRC support)
• Targeted control of adult carp migrating towards spawning areas
• Targeted control of spawning aggregations
• Exclusion of spawning adults from spawning areas
• Control of dispersing juveniles from spawning areas
• Control of sink populations through limiting immigration from source populations.
• Identification of priority control sites.

Progress 

Our researchers aimed to sample at 26 locations in Queensland, 33 in Victoria and 10 in South Australia and the Culgoa, Paroo, Castlereagh, Macquarie, Bogan and Lachlan catchments in NSW. 

Over the 2005/06 and 2006/07 sampling seasons, there were a total of 7 catchments sampled. Catchments sampled in the 2005/06 breeding season were Murrumbidgee (06/10/05) (> 10^th percentile), mid Murray (8/11/05) (> 10^th percentile), Macquarie (14/11/05) (> 10^th percentile), Kiewa 14/11/05 (percentile unknown) and Gwydir (24/02/06) (> 2^nd percentile).

During the 2006/07 breeding season only two catchments were sampled (due to flooding events); the Border Rivers (> 6^th percentile) in NSW and the Paroo (>6^th percentile) both in Queensland and NSW. 

A high number of native fish have been sampled over the last two years, which suggests that the sampling strategy was functioning effectively. Consequently, the absence of carp larvae in the data is likely to be a true reflection of poor carp reproduction over the past two years.  During 2005/06, a total of 279 native fish were sampled from five catchments, and in 2006/07 there were 1396 native fish sampled from two catchments.  Of the 1396 fish, 99% of the total catch was collected in the Paroo River, whilst in the Border Rivers catchment there were low numbers of native fish (only seven) caught representing 1% of the total fish caught for during 2006/07.

The low number of carp caught in the Border Rivers in 06/07 and Paroo coupled with previous sampling of the Border Rivers catchment during the PAC CRC larval drift component, which yielded no larval carp, is a strong indication that there has been no carp recruitment in the Border Rivers system and the Paroo over the past two years.   

From the larval carp sampled in the 2005/06 season there were a significant number (93%) caught from one site on the Murray River downstream of the Barmah-Milewa wetland at Echuca.  From a previous carp project this particular area of the Murray was described as a ‘Hot spot’ for carp recruitment and the high percentage of larval carp caught once again supports this possibility.

Existing daily ageing data has been used to estimate the expected spawning date for those larvae sampled.  This has produced some unexpected results suggesting that spawning is not flow triggered and occurs under base flow conditions.  Therefore, the limitation on only sampling the 2^nd flow peak in the season is unnecessary and will be ignored in the 07/08 season.  However, given that the larval drift sampling concept is only effectively implemented under high flow conditions, the team will continue to only target sampling towards high flow events.

Using GIS software and spot-5 imagery, maps have been created where sites have been plotted (red circle) from carp larval samples that have been collected from the last two years of sampling and during the previous PAC CRC project.

The maps show sites where carp have been sampled.  There is an indication that these sites are occurring downstream of potential carp recruitment ‘hot spots’. 

All the data collected has been collated, checked and entered into the NSW DPI Freshwater database.  

During the 07/08 breeding season, nine sampling trips were undertaken, with three rivers being sampled for the first time since the project began.  In comparison to previous sampling years there was an increase in sampling and a larger data set was able to be collected.  This was due to higher rainfall in a number of catchments.

All project milestones have now been met and the final report is due for submission.  Each state agency has been resourced with the equipment necessary to apply carp larval sampling during any suitable sampling opportunity that may arise in catchments that have not been sampled during the life of the current project.  NSW DPI is committed to applying the larval sampling strategy to fill in the remaining gaps within NSW catchments.

Summary 

In two years of research, no carp larvae have been found drifting in the Murray River upstream of Barmah-Millewa Forest, only downstream – this indicates that carp do not spawn upstream of the Barmah-Millewa Forest.  As such, the Barmah-Millewa Forest is considered a ‘hot spot’ for carp breeding.  Other identified ‘hot spots’ in the Murray-Darling Basin include the Gwydir Wetlands, Namoi Wetlands, lower Warrego River and the lower Boomi River.