Pheromones are chemicals produced naturally by fish to trigger a social response in other fish of the same species, such as a spawning aggregation. Scientists at the University of Minnesota have been working on identifying carp pheromones and developing products containing these chemicals, which can be used to manipulate carp behaviour and potentially assist in controlling carp populations1. At least five sex pheromones have been characterised for carp: one is prostaglandin (a type of fatty acid), which is released by carp when they are ovulating and spawning. Prostaglandin has been proposed as a lure to attract and trap sexually mature and receptive male carp.
The Invasive Animals Cooperative Research Centre (IA CRC) funded research that found the male carp sex pheromone to be sex-specific and powerful. The research also found that ovulating female carp release a prostaglandin-based attractant for males and that non-ovulating females can be primed with a synthetic dose of prostaglandin to release the attractant. This synthetic prostaglandin can be implanted in a slow-release capsule in the fish, causing the female carp to produce their attractant pheromone for a longer period than usual (up to two weeks).
To test how applicable attractant pheromone technology is in the field, trials of pheromone attractants were conducted in Tasmania (Lake Crescent and Lake Sorell) and New South Wales (Lake Cargelligo and the Lachlan River).
Tasmania was selected because of the expertise and infrastructure already invested in carp management in that state, especially the Integrated Carp Management (ICM) program that has been operating since 1995 in Lakes Sorell and Crescent (see PestSmart Case Study: Carp removal in Tasmania). The NSW sites were selected because of the abundance of carp in that catchment and the existing resources dedicated to carp control.
Partners and Management
The organisations involved in the pheromone research and trials were the IA CRC, Inland Fisheries Service (IFS) Tasmania, NSW Department of Primary Industries and the University of Minnesota.
Carp with artificially-inserted prostaglandin implants (called ‘odour donors‘) were used to attract and trap feral (wild) carp in a series of experiments at Lake Sorell between October 2010 and January 20111.
All odour donor females received two prostaglandin implants (slow release osmotic pumps), except in one experiment, where only one of the fish received two implants and the other two received a single implant. At the same time, male odour donor fish were also implanted with the slow release prostaglandin. A number of carp were fitted with radio transmitters that accurately relayed their location.
New South Wales
Field trials of pheromone attractant traps were conducted in Lake Cargelligo (September/October 2010 and September 2011) and the Lachlan River at Hillston (October/November 2010). The purpose of the trials was to compare pheromone traps in a lake and in a flowing water environment. A measured dosage of the pheromone (based on fish weight) was surgically implanted into female and male carp. Implanted fish were then placed into individual holding nets and randomly assigned to a trap each day. Five identical sets of three traps were set for each trial, over a 10 day period. In each of the three traps were i) a sexually mature female carp implanted with pheromone, ii) a sexually mature male carp implanted with pheromone and iii) no fish.
No feral or radio-tagged carp were caught in traps set close to the prostaglandin primed females. In contrast, two adult carp (a radio implanted male and a feral female) were caught in a trap set close to the male odour donor fish. Although the number of feral carp trapped was low and occurred at only one site, the results may still be significant given the low number of adult carp in the lake (as a result of the ongoing control program). The latter results agree with previous captures of feral carp in these lakes following the use of either the slow release prostaglandin derivative, or females primed with prostaglandin taken from their pituitary gland.
The movement of two radio-tagged fish was also monitored. No specific behaviour in response to the placement of donor carp could be inferred, although one of the radio-tagged males was caught close to the male odour donor carp. Before the experiment had finished the radio-tagged male carp had to be removed due to the high risk of spawning during the summer of 2010– 11.
The surgery to implant the osmotic pumps is invasive and the dosage rates of prostaglandin needed are difficult to gauge. The best results have occurred when there has been ovulation or increased spermiation. This can be effectively achieved using an analogue of gonadotropin-releasing hormone (GnRH), also known as luteinizing-hormone-releasing hormone (LHRH), or pituitary extract injected into the donor fish. Injection must be critically timed to coincide with favourable environmental conditions like rising water levels and warmer water temperatures. This simple process is far less invasive to administer and will form an integral part of ongoing integrated carp management in Tasmania.
New South Wales
The Lake Cargelligo trial, while successfully trapping over 6000 juvenile carp, only captured two adult carp — both of which were caught in control traps that contained no pheromone implanted fish. The Lachlan River trial did not capture any carp, with only a single native fish (Golden perch) caught.
A third trial was conducted in spring 2011. Water temperatures within Lake Cargelligo were monitored with the aim of coinciding the timing of the trial with a natural carp spawning event. Once water temperatures were consistently 16°C, the trial commenced. Five trapping stations were again established: two at sites with high numbers of radio-tagged males, and the remaining three at the inflow channels of each of the lakes within the system (Sheet of Water, Curlew Water and Lake Cargelligo) to utilise any possible flow to disperse the pheromone. A total of 393 carp were captured during this trial, however only seven were sexually mature.
In contrast to the first trial, all adult fish caught were male. Radio tagged carp were tracked daily and three were tracked adjacent to one of the trapping stations (but not caught) on-route to a large spawning aggregation. As in the previous trials, no significant differences were found between the three treatments in catch rates, sex ratios or the size of carp caught. So the use of the pheromone implanted lure carp did not enhance trapping catches.
What was learnt
At present there are several issues associated with the use of this type of pheromone in the natural environment:
- High doses must be used because fully active (natural) mixtures are required to out-compete other stimuli in the environment.
- The pheromone must be added in a way that produces concentration pathways that the fish can follow.
- Even spawning male carp are trap-shy: to effectively capture any fish that are attracted, larger trapping schemes that carp cannot detect are needed.
- The timing of application must be precise, and pheromones must be added at places where fully mature and sexually active males are present (ie pheromone lures are only effective during narrow windows of time).
Results from the Tasmanian trials suggest that pheromones released by females stimulate males to search in a general area, and the attracted carp are most likely to be caught using large traps. ‘Judas’ fish (which are radio-tagged and reveal the location of carp aggregations) may also be used to find spawning fish. Implanted fish may be toxic and should be properly disposed of after use.
The results of the NSW trials suggest that pheromone traps in their current form are not cost-effective carp removal tools in the Murray–Darling Basin.
At present the pheromone cue will only work as an attractant immediately before, during, or just after spawning. Implants should be used during or just after spawning because that is when males may be searching in earnest for any remaining ovulated females. Un-natural and therefore stressful settings for implanted fish may not work, and the pheromone is probably best suited as a short-to-medium range attractant rather than a long range one.
Pheromone attractants will form part of the standard methodology for trapping in Tasmanian lakes, but only at optimal times, such as when the water is warming and water levels are rising. The use of radio-tagged fish will be critical to identify the best sites, by identifying where carp aggregations are occurring.
The use of pheromones to guide fish movement is still a relatively new concept, both in Australia and overseas, and requires further research. In addition to their use as an attractant, other potential applications include disrupting fish movement and migration patterns, adversely affecting fish reproductive success, repelling individuals, augmenting fish trapping and assessing population sizes and distribution.
|Author||Centre for Invasive Species Solutions|
|Publisher||Centre for Invasive Species Solutions|
|ISBN/ISSN||PestSmart code: CPCS4|
|Control method||Integrated Pest Management|
|Region||Australia - national|
|Documents||PestSmart Case Study: Carp pheromone attractant trials [490 kb PDF]|