Hypophthalmichthys molitrix and Hypophthalmichthys nobilis, two of the many types of Asian carp, have been a nuisance to waterways in the United States since their establishment (Irons et al., 2007). These species of Asian carp have displayed numerous signs of being ideal invaders, as their populations have exploded and continue to spread (Williamson & Garvey, 2005). This high abundance of H. molitrix and H. nobilis has caused shortages of resources for native fish due to diet overlaps, as well as, a fear of a trophic cascade effect by devastating populations of plankton, caused by these invaders’ extraordinary appetite (Domaizon & Devaux, 1999; Lu et al., 2002; Radke & Kahl, 2002; Irons et al., 2007; Sampson et al., 2009). In light of these threats, action is being taken to prevent the spread of these fish into the Great Lakes, by means of sound and electric barriers (Lovell et al., 2006). Further analysis will examine how H. molitrix and H. nobilis are so successful in their invasive range, the severity of damage they inflict on the environment, and how further spread of these fish is being prevented.

Hypophthalmichthys molitrix, or the Silver carp, possess many of the traits that make organisms effective invaders. Initially, H. molitrix was introduced to the United States as a form of biocontrol to algae overgrowth in small lakes (Radke & Kahl, 2002). This proved moderately effective until dispersal allowed this carp to move into large river and lake systems, where both algae and plankton became a food source for H. molitrix. By superior competition, H. molitrix causes native filter feeding fish to undergo reduced overall fitness and decrease the survival of native fish larvae and nauplii, due to overconsumption of plankton, their main food source (Domaizon & Devaux, 1999; Irons et al., 2007; Sampson et al., 2009). The average size of a one-year old Silver carp from an established population in the Mississippi river was three times as long as a one-year old in its native range (Williamson & Garvey, 2005). This indicates that Hypophthalmichthys molitrix uses resources more efficiently when in highly productive water systems, perhaps by consuming both phytoplankton and zooplankton, in its invasive range, such as the Mississippi and Illinois Rivers (Sampson et al., 2009). Correspondingly, the heightened rate of growth, the reproductive ability of the silver carp, increases as they reach sexual maturity at younger ages in their invasive range (Irons et al., 2007; Kaefer et al., 2007). In addition to reaching sexual maturity earlier, Silver carp, in its invasive range, was found to have increased fertility as a direct consequence of these fish growing to larger sizes (Williamson & Garvey, 2005). The elevated growth and fertility rates of the Silver carp can likely be attributed to its planktiverous feeding habits and introduction into highly productive waters (DeGrandchamp et al., 2008).

Silver and Bighead carps are filter feeders with a diet of primarily zooplankton and phytoplankton (Williamson & Garvey, 2005; Irons et al., 2007). Competition for resources has worried researchers as substantial diet overlap exists between the introduced H. molitrix and H. nobilis and at least two native filter feeding fish species, Dorosoma cepedianum and Ictiobus cyprinellus (Irons et al., 2007). Both native species have been negatively affected by the introduction of both the Silver and Bighead carp (Irons et al., 2007). Not only was there a significant decrease in populations of D. cepedianum and I. cyprinellus after the invasion of the Silver and Bighead carp, but also a significant decline in the average body condition, or overall fitness, of the native species (Irons et al., 2007). With decreased overall fitness caused by the competitive pressures placed on D. cepedianum and I. cyprinellu,s by the introduced carp species, decreased relative mass, fertility, and overall population often result, as well as, allowing for higher susceptibility to disease (Irons et al., 2007). The native D. cepedianum and I. cyprinellus cannot compete with the introduced Asian carp species in terms of resource attainment, especially because of H. molitrix and H. nobilis’ plastic diets (Burke et al., 1986; Kolar et al., 2005).

H. molitrix and H. nobilis have shown a remarkable ability to adapt to new environments depending on the resources available. H. nobilis has been shown to prefer zooplankton, however, when zooplankton abundances are low H. nobilis shows no difficulty in changing its diet to more available phytoplankton or detritus, an adaptation that most native fish, such as D. cepedianum, are not capable of (Kolar et al., 2005). This evidence suggests that if in high enough concentrations, H. nobilis and H. molitrix could exhaust the supply of zooplankton, the most common prey items of I. cyprinellus and D. cepedianum. Then they could easily transition from predatory behavior to herbivorous consumption of phytoplankton, while native fish will be forced to adapt or perish (Sampson et al., 2009).

The consumption of zooplankton is a similarity Hypophthalmichthys molitrix shares, not only with native filter feeders, but also fish larvae, nauplii, and small crustaceans, called cyclopoids (Lu et al., 2002). The diet overlap between the Silver carp and these crustaceans, as well as superior competitive ability, causes concern for a trophic cascade effect, or disturbance in the food web, causing ecosystem collapse due to elimination of the food source of an organism, in the invasive range of this fish (Lu et al., 2002). Evidence for this occurrence has been well documented in small lakes with introduced H. molitrix populations, as it has been determined that a higher biomass of Silver carp causes a decrease in zooplankton biomass, thereby decreasing cyclopoid density as well (Lu et al., 2002). A French reservoir in which H. molitrix was introduced was found to be the cause of up to ten different declining populations of native microscopic organisms as its biomass increased (Domaizon & Devaux, 1999). The vast consumption ability of these carp not only affects microscopic communities, but also populations of native fish.

In the Illinois River and backwater lake systems, H. molitrix and H. nobilis have been shown to have significant overlaps in diet with native fish (Sampson et al., 2009). This overlap of diet has been shown to cause an overall decrease in the amount of cladocerans, a type of zooplankton key to the structure of the plankton community and common diet item, to both Asian carp and the native I. cyprinellus and D. cepedianum, available to native fish (Sampson et al., 2009). Not only does competition between numerous species of fish for this type and related zooplankton negatively affect native fish I. cyprinellus and D. cepedianum by reducing fitness and causing population decreases in the La Grange reach of the Illinois River, but also negatively affects the quality of water, as this zooplankton is necessary for control of algae (Burke et al., 1986; Radke & Kahl, 2002). With the elimination of zooplankton and the plastic diets of H. molitrix and H. nobilis, these species are virtually unaffected and proceed to consume detritus and algae, which other native fish are less efficient at (Burke et al., 1986).

On a microscopic level, with zooplankton being a primary food choice of most Asian carp, a trophic cascade effect is a potential concern regarding H. molitrix and H. nobilis’ destructive and excessive consumption capabilities. Even in highly productive waters, a high biomass of Asian carp causes a decrease in the overall biomass of crustacean zooplankton (Lu et al., 2002). This decrease in overall zooplankton biomass is due to both the ability of H. molitrix and H. nobilis to outcompete larger zooplankton for herbivorous activity on phytoplankton and direct predation on zooplankton (Lu et al., 2002). Not only do high concentrations of these species of Asian carp have negative effects on the small scale plankton community, but also negatively affect the environment on a macroscopic scale by outcompeting native fish for food resources such as zooplankton (Lu et al., 2002; Sampson et al., 2009). Stress on the populations of zooplankton by multiple predators, such as both Asian carp species and native fish, not only further decreases the ability of native fish to obtain nutrition, but interestingly, has little detrimental effects on H. molitrix and H. nobilis (Kolar et al., 2005).

The threat of H. molitrix and H. nobilis entering the Great Lakes system is a primary concern, as their presence is predicted to cause potential trophic cascade and ecosystem collapse (Sampson et al., 2009). Methods of physical control, such as barriers and deterrents, block H. molitrix and H. nobilis passage into the Great Lakes as they are strategically placed in the Des Plaines River and other key entryways into Lake Michigan (Jerde et al., 2013). Another form of experimental control involves manipulation of the water level near dams during spawning seasons of H. molitrix and H. nobilis (Lohmeyer & Garvey, 2009). During the spawning season, Asian carp larvae float freely and survive best in strong river currents (Lohmeyer & Garvey, 2009). By using dams to lower the water level and prevent river flow, juvenile Asian carp will have reduced survival rates and lower dispersive potential (Lohmeyer & Garvey, 2009). With the risk of preventing native fish species from freely entering the Great Lakes, more selective barriers, such as the Bio-acoustic Fish Fence, has shown promise (Lovell et al., 2006). This method of mechanical control takes advantage of H. molitrix and H. nobilis’ specialized auditory perception, meaning that these species are more sensitive to sound fields at different frequencies (Lovell et al., 2006). A Bio-acoustic Fish Fence would emit a sound field at certain frequencies that would deter the approach of both the

Bighead and Silver carp, while allowing other native fish species to pass by undisturbed (Lovell et al., 2006).

H. molitrix and H. nobilis’ superior ability to compete for resources and adaptability to fluctuations in the food supply makes these species of Asian carp an unmatched adversary for native fish, such as I. cyprinellus and D. cepedianum, causing not only population decreases, but reductions in fitness (Irons et al., 2007). The disruption caused to the structure of plankton communities, reduction of native larval spawn survival, and contribution to potential ecosystem failure provides all of the pieces to consider these Asian carp harmful invaders (Burke et al., 1986; Lu et al., 2002). Implications of control to prevent the ever present increasing invasive range of H. molitrix and H. nobilis are moderately effective; however, most current barriers are impassible to native fish. With promising advancements in the understanding of these Asian carp underway, more selective barriers, such as the Bio-acoustic Fish Fence, show promise to prevention of further dispersal of the invaders, while causing minimal disturbance to native fish (Lovell et al., 2006). The focal point of controlling H. molitrix and H. nobilis seems to be primarily on prevention of further spread, specifically to prevent establishment of these carp in the Great Lakes. In addition to the present physical blockades in rivers attached to the Great Lakes, further emphasis should be placed on legal status of transport of live H. molitrix and H. nobilis, as well as, harvesting these fish to reduce their population and lower propagule pressure, or amount of attempted introductions into the Great Lakes.

References

Burke, J. S., D. R. Bayne, and H. Rea. 1986. Impact of silver and bighead carps on plankton communities of channel catfish ponds. Aquaculture 55:59-68.

DeGrandchamp, K. L., J. E. Garvey, and R. E. Colombo. 2008. Movement and habitat selection by invasive Asian carps in a large river. Transactions of the American Fisheries Society 137:45-56.

Domaizon, I., and J. Devaux. 1999. Experimental study of the impacts of silver carp on plankton communities of eutrophic Villerest reservoir (France). Aquatic Ecology 33:193-204.

Irons, K., G. Sass, M. McClelland, and J. Stafford. 2007. Reduced condition factor of two native fish species coincident with invasion of non‐native Asian carps in the Illinois River, USA Is this evidence for competition and reduced fitness? Journal of Fish Biology 71:258-273.

Jerde, C. L., W. L. Chadderton, A. R. Mahon, M. A. Renshaw, J. Corush, M. L. Budny, S. Mysorekar, and D. M. Lodge. 2013. Detection of Asian carp DNA as part of a Great Lakes basin-wide surveillance program. Canadian Journal of Fisheries and Aquatic Sciences 70:522-526.

Kaefer, Í. L., R. A. Boelter, and S. Z. Cechin. 2007. Reproductive biology of the invasive bullfrog Lithobates catesbeianus in southern Brazil. Pages 435-444 in Annales Zoologici Fennici. JSTOR.

Kolar, C. S., D. C. Chapman, W. R. Courtenay Jr, C. M. Housel, J. D. Williams, and D. P. Jennings. 2005. Asian carps of the genus Hypophthalmichthys (Pisces, Cyprinidae)―a biological synopsis and environmental risk assessment.

Lohmeyer, A. M., and J. E. Garvey. 2009. Placing the North American invasion of Asian carp in a spatially explicit context. Biological Invasions 11:905-916.

Lovell, J., M. Findlay, J. Nedwell, and M. Pegg. 2006. The hearing abilities of the silver carp (Hypopthalmichthys molitrix) and bighead carp (Aristichthys nobilis). Comparative Biochemistry and Physiology Part A: Molecular & Integrative Physiology 143:286-291.

Lu, M., P. Xie, H. Tang, Z. Shao, and L. Xie. 2002. Experimental study of trophic cascade effect of silver carp (Hypophthalmichthys molitrixon) in a subtropical lake, Lake Donghu: on plankton community and underlying mechanisms of changes of crustacean community. Hydrobiologia 487:19-31.

Radke, R. J., and U. Kahl. 2002. Effects of a filter‐feeding fish [silver carp, Hypophthalmichthys molitrix (Val.)] on phyto‐and zooplankton in a mesotrophic reservoir: results from an enclosure experiment. Freshwater Biology 47:2337-2344.

Sampson, S. J., J. H. Chick, and M. A. Pegg. 2009. Diet overlap among two Asian carp and three native fishes in backwater lakes on the Illinois and Mississippi rivers. Biological Invasions 11:483-496.

Williamson, C. J., and J. E. Garvey. 2005. Growth, fecundity, and diets of newly established silver carp in the middle Mississippi River. Transactions of the American Fisheries Society 134:1423-1430.

Related Links:

• Analysis of the invasion of the asian carp_edited.pdf (PDF)