I was recently interviewed about my carp fishing habits by some researchers at the University of Wollongong. It was an interesting interview and forced me to think a little harder about my relationship to carp. To put it bluntly, the relationship I have with carp is a little quixotic. On the one hand, I think they are one of the best sports-fish going around. Over the last few years chasing them on fly I have developed an immense amount of respect and admiration for them. To catch carp on fly consistently is up there in terms of any fly fishing pursuit. I don’t hesitate to call them beautiful or to admire them.
On the flip side, I kill every carp I catch. I view them as vermin, vermin that we should try our best to eradicate from our rivers.
While these two viewpoints may seem at odds, I manage to simultaneously hold both attitudes comfortable. For me, admiring carp doesn’t mean that I can’t see the damage they do or view them as an environmental pest that we would be better off without.
One thing the interview got me thinking a little harder about was how much I knew about the history of carp in Australia and how much I actually knew about the damage they cause to our ecosystems. So I decided to do a little research.
While some Australian carp populations have been established for over 100 years, it may surprise some to learn that their ubiquity in Australian rivers is only a relatively recent thing. But more on that later, I should probably start at the beginning.
The Introduction of Common Carp (Cyprinus carpio) to Australia.
The first attempts to introduce carp to Australia were made in the 1850s. The first of these was in 1858 in Tasmania. Given Tasmania’s “carp free” status until only a few years ago, this introduction was obviously unsuccessful. The first successful introduction was made in Melbourne’s botanic gardens a year later in 1859. This population thrived in this tiny area of Australia and existed there all the way up till 1962. However it didn’t spread and remained contained within the Gardens. So, like for many of Australia’s pest, the presence of carp can be traced back to the acclimatisation societies of the 1800s trying to make this vast, foreign land a little more like home. The mid 1800s were the boom time for introductions of European species. Red foxes were first introduced as early as the 1830s, the current rabbit infestation can be traced back to a release of rabbits by Thomas Austin at Barwon Park in Victorua in 1859. Carp weren’t the only fish being introduced to the country at around this time. The first attempted introductions of brown trout happened just as few years later in 1864. And for the next 100 years, brown trout would be far more successful invaders, their populations growing and expanding rapidly over the next 50 years. The differing social attitudes towards the two species are interesting in their own right, but thats for another post. Back to carp.
After the initial releases, carp introductions continued. By the 1900s a few more populations of carp had been established across the country, Prospect reservoir near Sydney being the most notable. The “Prospect strain” of carp, one of the four strains of carp in Australia bearing its name. Many more introductions had however failed, including a number of attempted introductions in Western Australia.
It wasn’t until the 1920s that carp had become established in parts of the Murray Darling Basin. Introductions into the basin continued, with the introduction of the “Yanco” strain around Yanco in the 1940s being the most successful. By the 1950s, both the “Prospect” and “Yanco” strains had become established in the basin. Still their populations were relatively contained and in check and they had not had any obvious significant ecological impacts. It wouldn’t be until the introduction of the “Boolara strain” in the 1960s and its spread throughout the basin from the 1960s onwards that all hell would break loose. For whatever reason, the “Boolara strain” had traits that the “Prospect” and “Yanco” strains lacked, traits that allowed it to thrive in Australian conditions. The “Boolara” strain had originally been a strain designed for aquaculture. Its introduction into Australia happening at a licensed fish farm (against the wishes of government) at, you guessed it, Boolara in the 1960s. In 1964, carp, that would later be shown to be “Boolara” carp, showed up at Mildura. From there “Boolara” carp interbred with the “Prospect” and “Yanco” strains, potentially passing on it favourable traits and further improving fitness. 100 years on from their introduction into Australia carp were about to take over.
What followed was a massive expansion both in range and abundance. Helped by major floods in the 1974/5 and 1993, carp quickly came to dominate the basin. Its hard to believe that carp weren’t present in either Queensland or South Australia until the 1960s. That they weren’t present in Hume Dam until the late 1970s. In a matter of a few short decades, from 1960-1990s, carp took over the basin, now representing up to 90% of fish biomass in some areas. The cat was out of the bag so to speak. Carpocalypse was upon us. Ever since we have been trying to stuff it back in.
There is one more strain of carp in Australia I have failed to mention. The “Koi” carp. “Koi” carp are a much later introduction, most introductions happening between 1976 and 2008. They are patchily distributed in the Murray Darling basin, but many east coast populations are solely made up of colourful “Koi” carp. Most of these have likely been released, either deliberately or accidentally from backyard ponds and aquaria and where conditions have been right, they have bred and prospered, establishing a number of self-sustaining populations. It seems, like “Boolara” carp, “Koi” carp have taken a liking to Australia.
What damage have they done?
In short. A lot. Although pinning down the exact nature of their impacts can be a little difficult.
At the very least, they take up a hell of a lot of space and take out a hell of a lot of resources from our rivers. Resources that could otherwise be utilised by native fish. The sheer number of carp in our rivers means they are bound to be competing with native fish and having detrimental impact.
More broadly, it gets a little more complicated. There is a school of thought that carp simply took advantage of the already degraded condition of Australian rivers, conditions already well suited to them after 100 years of neglect and environmental degradation. Rivers already far from ideal for native fish. There may be a grain of truth to this. The large declines of our native fish started well before carp were a ubiquitous part of Australian waterways. As covered here, Murray cod and other species had already suffered widespread large scale declines by the 1950s, well before carp came to dominate the basin. So in part, it may be true that the dominance of carp is partly just a symptom of already poor river health, of the years of neglect and abuse our rivers had already sustained through siltation, dams, changed water regulation, over-fishing and de-snagging to name just a few. Carp, by virtue of their feeding habits, then “moulded” these already degraded ecosystems to their advantage, increasing turbidity and reducing plant life. We don’t know what would have happened if our rivers and native fish populations were in good health at the time of carps rapid spread throughout the Murray Darling Basin. It is tempting to think that they may not have been able to spread and establish themselves quite so easily, although that may not have been the case, it may have mattered little. We will never know. As it was, carp spread into rivers that were relatively “empty” of native fish, that were already highly modified after many years of environmental changes. They quickly filled that gap. They quickly dominated. So it goes.
While its true that many of the declines in native fish numbers were initially due to non-carp factors and happened before carp even arrived and that the spread of carp may well have been helped by years of prior environmental degradation, it does not follow that carp are benign. They may and likely are doing large amounts of damage to our waterways and are likely to be having damaging impacts on native fish populations today.
The major detrimental impacts of carp are thought to be through their feeding habits. Carp feed by sucking soft sediments up into their mouths, picking out the tasty bits and ejecting the sediment back into the water. This method of feeding, known as roiling, leads to direct deterioration of water quality (due to higher sediment loads and increased nutrient levels), lower levels of light penetration leading to reduced plant growth, the smothering of plants, benthic invertebrates and eggs, the clogging of gills and can inhibit visual feeding. Added to this, by virtue of being omnivores, carp will graze on aquatic plants and uproot them during feeding, potentially exacerbating the effect they have on aquatic vegetation. This could lead to decreased invertebrate populations (especially those dependent on aquatic vegetation) and reduced stability of sediments. Juvenile carp also feed heavily on zooplankton, which may lead to increased algal growth. Carp are also likely to compete directly with native fish, both through direct predation on eggs and on small and juvenile fish and through competition for resources (there is significant dietary overlap between juvenile carp and juvenile Murray cod and Golden perch). While these impacts are likely, there has until recently been little hard evidence for much of it, mainly due to the difficulty of testing these effects in the field after the fact. Recently that has started to change, with Vilizzi (et al 2014) demonstrating many of these effects in field trials.
The above factors mean that carp are likely to be doing a large amount of damage to our waterways and to native fish populations. And while they may not have been the direct cause of many of the declines in native fish numbers, their sheer dominance now, is likely to be negatively effecting the ability of native fish populations to bounce back from their current low abundance. The fact that in some areas, carp make up 90% of the biomass, means that they must be taking out a huge amount of resources from our rivers. Logic suggests, that this MUST have an impact the survival of native fish species, especially juvenile fish which rely on zooplankton to survive and are thus going to be competing directly with juvenile carp for that resource.
It is likely that elements of both these narratives are true, that carp are both a symptom of the poor health of our rivers and a direct threat to native fish. Divorcing these two factors is difficult and may never be possible. Either way our rivers and our native fish would be far better off if habitat was rehabilitated. Our rivers and our native fish would be far better off without carp. Both strategies i.e. habitat rehabilitation and carp eradication are likely to be needed if we wish to restore our rivers and with them native fish populations.
Carp management tools and the future.
Given their invasive nature, controlling carp has been an ongoing battle. Recently a lot of progress has been made. On the ground, the development of carp exclusion traps (e.g. “jump traps”), which exploit carps tendency to jump over things, a tendency that native fish don’t possess have shown a lot of promise. Similarly, dedicated fishing effort using “Judas” fish, tagged carp that lead carp catchers to the areas where carp are hanging are being trialled with some success. Another such project run by the Upper Murrumbidgee demonstration reach is trialling a new trap that requires that carp to push their way in, exploiting carps propensity to do this far more regularly than native fish. This work is also likely to help quantify the damage carp do. It will be interesting to see how well native fish populations bounce back after extended carp trapping, something that will help us identify how much damage these fish are doing to our freshwater ecosystems. Strategies such as these have the potential to be hugely beneficial locally, however are unlikely to address carp numbers across the Murray Darling Basin.
On that level, there are a two promising options. One is Koi herpes virus. Koi herpes virus, which can produce severe gill lesions and can cause mortality rates of between 80-100% in some populations of common carp. and can cause large scale declines in carp numbers. Testing about whether the virus can jump to native fish species is still in progress, but so far the virus appears to specifically target carp, with very little risk of it jumping hosts. “Koi” herpes virus is a very promising biocontrol agent and could well be hugely beneficial at addressing carp numbers across the MDB.
Lastly, over the longer term daughterless carp technology may prove to be another highly successful control agent. Daughterless carp technology works by exploiting aromatase, an enzyme critical for the development of female embryos. Daughterless carp technology introduces a aromatase inhibiting gene into carp, which is then passed onto all offspring. The inhibition of aromatase means that all offspring will be phenotypically male, regardless of the make up of their sex chromosomes (XX or XY). So far it is thought not to have fitness effects and by introducing the gene multiple times into carp genomes, it is thought that meiosis (which would lead to only 50% chance of the each copy of the inhibiting gene being passed onto offspring) can be sidestep. If both of these are indeed true, the gene will be able to “escape” natural selection and spread widely across carp populations. Introduction of high enough densities of carp carrying this gene would then eventually lead to a vast predominance of males within populations, undermining the viability of those populations, leading to their decline. Modelling suggest that this strategy could be very successful, leading the the virtual extinction of carp populations within 30 years, however a lot more work needs to be done to determine whether this will be the case in the real world. If the strategy is successful, it would be a huge leap forward and potentially lead to similar technologies for other invasive species such as cane toads.
These technologies mean, that at least on the face of it, carp-less or at least hugely carp depleted rivers are an achievable goal within our lifetimes, which would be a remarkable achievement. It would undoubtedly be a good thing for native fish, however, to make the most of it, many of the other environmental factors that lead to the decline of native fish species and the predominance of carp within the MDB will need to be simultaneously addressed. Getting rid of carp will help, but it wont be a fix all panacea. We still need to also address historical de-snagging, siltation, river regulation and a host of other factors if native fish are really going to thrive once again.
Thanks to Eli from the University of Wollongong for doing the interview that gave me the inspiration to write this.