Background

The Invasive Animals CRC  is very interested in developing sex pheromones for use to capture and remove common carp (Cyprinus carpio). The remarkable specificity of pheromones and the low concentrations at which they have activity has distinguished them as tools of great promise for use as attractants in carp trapping schemes in spite of the difficulties associated with identifying them.

Although little is known about carp pheromones, a great deal is known about goldfish (Carassius auratus) pheromones, a very close relative.  Numerous studies suggest carp employ the same or very similar pheromonal compounds in different mixtures (Sorensen and Stacey 2004, see below).  This situation allows the investigators to employ goldfish as laboratory model to understand the identity and function of carp pheromones which are extremely difficult and awkward to study in the laboratory, thus saving both time and money.

All pheromones identified to date in insects, fish, and mammals have been shown to be mixtures of relatively common metabolites whose precise mixture varies between closely-related species.  (Sorensen and Stacey 1999; Sorensen and Stacey 2004; Stacey and Sorensen 2002, 2005).  This makes sense because there are far too many species (there are 25,000+ species of fish) for each to have its own unique pheromone.   Compelling evidence suggests that carp and goldfish pheromones in particular also follow this trend.   Briefly, five hormonal compounds with pheromonal (e.g. behavioral or endocrinological) activity have been now been identified in the goldfish, all of which have also now been shown to be detected with extreme sensitivity and specificity by the carp olfactory system as well (Irvine and Sorensen 1993).  Further, when examined, all of these compounds have also been found to be produced by carp (Stacey and Sorensen 2002).  Two of the goldfish pheromones have also been shown to have at least some biological activity in carp.  Specifically, studies have shown male carp to respond with increased sperm production to the identified goldfish pheromonal component 17,20b-dihydroxy-4-pregnen-3-one when introduced into tanks with a background of carp odor (Stacey et al. 1994). 

Other work has also shown that prostaglandin F2a induces female sexual behavior in carp, exactly as it does in the goldfish (Sorensen, unpublished), and that background/odor context (i.e. metabolite mixture) strongly influences the strength of pheromonal responsiveness (Sorensen et al. 2000). 

Finally, goldfish and carp are known to hybridize, at least to limited extent (Taylor and Mahon 1977) while olfaction is known to mediate reproductive activity. 

Progress

We have discovered that sexually-mature male carp (Cyprinus carpio) release a sexual attractant for sexually-receptive conspecific females.  This cue can be isolated and purified.   Work commenced in late January 2007 with the arrival of Dr. Hangkyo Lim from the University of Kansas where he earned a Ph.D. in insect chemical ecology.  Dr. Hangkyo is now being assisted by a half-time technician, Mr. Mario Travaline, who started in early April 2007.  Mr. Travaline has a B.S. degree in biology and had previously been working with carp. 

As planned, our initial studies focused on the goldfish (Carassius auratus), a close relative of the carp which employs many of the same olfactory cues and is much easier to study because of its calmer disposition and smaller size. 

A set of mazes, each with three inter-connected compartments, overhead infrared lighting and video recording systems were constructed.  For each experimental trial, a group of sexually-receptive females (injected with  prostaglandin) was placed into a maze and acclimatised.  Behaviours were then noted while either a control odour, male goldfish odour, a food odour, or juvenile odour was added to one side using pumps.  These tests have clearly demonstrated that sexually-receptive females are strongly attracted to male odour (p<0.001). 

Remarkably, we have found that the potency of male odour is equivalent to that of food odour.  Future isolation and purification is feasible.  The project has also confirmed that females injected or implanted with prostaglandin released a potent male attractant.

Our second set of experiments asked whether the common carp, like the goldfish, also uses a male-derived pheromonal attractant.  These experiments are still underway.    We have already discovered that sexually-receptive (prostaglandin-injected) female carp will spawn with spermiated male carp in the lab and that they too are strongly attracted to conspecific male odour; i.e., carp also employ a male-derived sex pheromone.  This pheromone can be extracted and purified and tests show that it is highly stimulatory.

The project has progressed to testing the behavioural potency of the extract.  We need to know whether the product produces the whole of the chemical cue needed to affect fish behaviour - is the extract producing similar results to whole male odour?   If so, large scale isolation is the next step and tests will progress to large aquarium tanks (3 m in diameter), and then to the field. 

Initial field experiments will monitor attraction of fish in Lake Susan, a site in Minnesota in which the team have placed two dozen radio-tagged adult carp and whose residents strongly support our research program.

The Sorensen laboratory is also presently funded by the State of Minnesota to elucidate and develop a juvenile-derived attractant for carp.