fish-hook water flea

In addition to sexual reproduction, Cercopagis most commonly reproduces parthenogenically (asexually) during the summer, which allows it to quickly establish new populations with a relatively small seed population without the need for a large number of the smaller males along with females. Eggs produced in the early part of the season are delicate and very susceptible to damage, with low recruitment rates. Later in the season, as surface water temperatures decline, Cercopagis females switch to sexual reproduction, producing over-wintering or resting eggs (the species is also known to produce resting eggs anytime during the year when environmental conditions become inhospitable). Such resting eggs can successfully overwinter in an inactive state and replenish the population after hatching in the spring. Resting eggs are also resistant to desiccation, freeze-drying and ingestion by predators (such as other fish). They can be easily transported to other drainage basins by various vectors, particularly if they are still in the female's body (the barbed caudal spine allows attachment to ropes, fishing lines, waterfowl feathers, aquatic gear, vegetation and mud). Resting eggs can hatch regardless of whether the carrier female is alive or dead.

Summary of species impacts derived from literature review. Click on an icon to find out more...

Environmental	Socioeconomic	Beneficial

Realized:
Cercopagis pengoi, a relatively small species, is a consumer of other small zooplankton nearly as large as itself, including small cladocerans (e.g., Bosmina longirostris, early instars of Daphnia spp.), as well as nauplii and early copepodite stages of copepods (Pichlová-Ptácniková and Vanderploeg, 2009). As such, it competes with other planktivores of the Great Lakes, including alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax) (Bushnoe et al. 2003). Its long spine makes it less palatable to small planktivorous fish. For these reasons, C. pengoi could have a serious effect on the food supply of planktivores.

Yearling alewife directly compete with C. pengoi because they are planktivorous but cannot consume C. pengoi due to its caudal appendage. Once an alewife reaches its first year, it is large enough to handle this tail spine (Bushnoe et al. 2003). The fishhook water flea is known to make up a portion of the adult alewife diet in Lakes Ontario, Erie, and Michigan, but this contribution does not appear significant relative to Bythotrephes longimanus, another nonindigenous spined cladoceran, when these species co-occur (Pothoven et al. 2007, Stewart et al. 2009, Storch et al. 2007). The alewife preference for Bythotrephes is related to its conspicuousness and larger size; C. pengoi is not only small but is hard to see because of its transparency (Pothoven et al. 2007, Vanderploeg et al. 2002). The fishhook water flea’s establishment in Lake Ontario in 1998 corresponded with the lowest alewife populations in twenty years (Makarewicz et al. 2001). Surveys in the following year indicated that C. pengoi was found to account for as much as 73% of crustacean zooplanktonic biomass in the lake (Ojaveer et al. 2001).

Unlike B. longimanus, C. pengoi is too small to impact populations of the native predatory cladoceran, Leptodora kindtii, via predation. However, it is likely to be a competitor of L. kindtii because of similar food preferences, similar life histories, and similar habitat preferences; both are found in the epilimnion (Cavaletto et al. 2010, Pichlová and Vijverberg 2001, Pichlová-Ptácniková and Vanderploeg 2009).  Because of its large feeding appendages, it is possible that C. pengoi is a more effective predator of zooplankton of a broader range in size and escape abilities than is L. kindtii (Pichlová-Ptácniková and Vanderploeg 2009).

Many studies have been conducted on the food web effects of C. pengoi in Lake Ontario. A 2002 study showed that the depth at which C. pengoi exists is depleted of small organisms (<0.15 mg) in Lake Ontario (Benoit et al. 2002). It was unclear as to whether this is due to predator evasion or C. pengoi consumption, but in either case, the smaller organisms are forced into deeper, cooler strata, causing growth rate changes (Benoit et al. 2002). Further study in Lake Ontario indicated that in the years following C. pengoi invasion, the density of small zooplankton began to drop in the late summer and fall seasons (when C. pengoi is most abundant) (Warner et al. 2006). Importantly, Laxson et al. (2003) found that increasing C. pengoi abundance was correlated with declines in populations of native zooplankton Daphnia retrocurva, Bosmina longirostris, and Diacyclops thomasi in Lake Ontario between 1999 and 2001. Daphnia retrocurva and B. longirostris are important prey items of C. pengoi, and appeared to be limited by predation rather than food availability or any decrease in fecundity (Laxson et al. 2003). Evidence thus suggests that C. pengoi may have played a role in the decline of zooplankton abundance in Lake Ontario. It does not appear, however, that zooplankton species richness has been altered as the result of C. pengoi invasion (Stewart et al. 2010).

Based on findings in the Baltic Sea and Gulf of Finland, it has been predicted that increased predation pressure on zooplankton caused by increases in C. pengoi abundance could lead to an increase in phytoplankton abundance and an eventual shift in the energy fluxes and eutrophication rates in an ecosystem (Litvinchuk and Telesh 2006). Additional data collected by Laxson et al. (2003) supports the plausibility of this prediction. In Lake Ontario, the authors documented an additional correlation between an increase in chlorophyll a concentration and the increase of C. pengoi and decrease of herbivorous zooplankton (Laxson et al. 2003). This suggests that C. pengoi likely had a significant top-down (albeit variable) effect on zooplankton communities in Lake Ontario, although these predatory effects appear to have declined steadily since the species’ establishment (Laxson et al. 2003).

Potential:
Initial research in southwestern Lake Michigan suggested that C. pengoi could have an effect on the food web due to predation of rotifers, whose abundance dropped significantly following C. pengoi establishment (Witt et al. 2005). However, this implication is taken with caution, as overall zooplankton abundance had been in steady decline previous to this study (Witt et al. 2005). In Lake Ontario, it was also thought that the addition of a zooplanktivorous invertebrate could alter the food web and increase toxin biomagnification levels in top predators. Conversely, studies indicate that this is probably not the case, largely because alewife does not feed heavily on C. pengoi (Thompson et al. 2005).

Cercopagis pengoi has a high socioeconomic impact in the Great Lakes.

Realized:

Costs in damages and control associated with C. pengoi in the U.S. are currently estimated at about five million US dollars annually (Pimentel et al. 2005).

Cercopagis pengoi fouls fishing lines, which acts both as a nuisance and as a possible mechanism of its dispersal and expansion. In a study by Jacobs and MacIsaac (2007), fouling was found to be most intense with longer lines and larger trolling distances; accumulation of C. pengoi on a single fishing line towed 1 km in Lake Ontario was as high as 1,024 individuals and 106 diapausing eggs. Lines specially designed to reduce water flea fouling experienced diminished C. pengoi accumulation (Jacobs and MacIsaac, 2007).

There is little or no evidence to support that Cercopagis pengoi has significant beneficial effects in the Great Lakes.

Potential:
It is possible that C. pengoi could benefit planktivorous fish by preying on smaller zooplankton, which are difficult for fish to catch, and storing this energy in a larger body mass, which is easier for fish to prey upon. However, this potential benefit is likely insignificant (Vanderploeg et al. 2002).

In Wisconsin, the fishhook waterflea is a prohibited invasive species (Wis. Admin. Code § NR 40.04), meaning that it is unlawful to transport, possess, transfer, or introduce the species within or into the state without a permit as defined under Wis. Admin. Code § NR 40.06.

Note: Check federal, state/provincial, and local regulations for the most up-to-date information.

Control
Cercopagis pengoi is most likely to be spread on aquatic equipment, especially fishing lines. Consequently, public education is a significant method of control which can greatly reduce incidences of species transfer by unaware or incautious anglers (Jacobs and MacIsaac 2007).

Biological
Pothoven et al. (2007) found that adult large alewives (Alosa pseudoharengus) (>100 mm) consume Cercopagis pengoi in Lake Michigan, but not significantly enough to control the species, concluding that the alewife prefers Bythotrephes longimanus due to its larger size and conspicuousness. In contrast, a study of C. pengoi as a prey item in Lake Ontario found that at least 70% of alewives larger than 70 mm contained C. pengoi spines (Bushnoe et al. 2003). The same study also found spines in rainbow smelt (Osmerus mordax) stomachs (Bushnoe et al. 2003). Rainbow smelt historically consume cladocerans in the Great Lakes, but prefer larger prey and may select B. longimanus over C. pengoi where both occur (Pothoven et al. 2009). Gorokhova et al. (2004) found that in the northern Baltic proper, herring (Clupea harengus L.) and sprat (Sprattus sprattus L.) are the dominant predators of C. pengoi, and a possible source of biological control through fisheries management, though it is possible that fully mature resting eggs may survive passage through fish digestive systems as has been observed with B. longimanus eggs in yellow perch (Perca flavescens). B. longimanus is also known to consume C. pengoi, but not as a main prey item (Cavaletto et al. 2010).

Physical
Cleaning all aquatic/fishing equipment, including downrigger lines and monofilament on reels, is important in areas where this species is present. Responsible maintenance and cleaning methods are recommended to prevent spread between water bodies, including cleaning all aquatic equipment with high pressure (>250 psi) or hot (>50°C) water after each use (Ontario’s Invading Species Awareness Program). Bythotrephes longimanus has been documented spreading by transfer of diapausing eggs on fishing gear, which are more resilient than adult waterfleas (Jacobs and MacIsaac 2007). Because of its similar life history to B. longimanus, it is likely that Cercopagis pengoi can also be spread by introduction of diapausing eggs to previously uninvaded waters as well as by transfer of fully developed adult specimens (Jacobs and MacIsaac 2007). Fishing lines designed specifically to prevent the spread of waterfleas, such as the Flea Flicker brand, have been proven effective in significantly reducing fouling on lines, indicating their importance as a management tool (Jacobs and MacIsaac 2007).

Electron beam irradiation has been used to control microorganisms in aquatic pathways, including Cercopagis pengoi (GLMRIS 2012). Electron beam irradiation is a non-selective control method which exposes water to low doses of radiation using gamma-sterilizers or electron accelerators, breaking down DNA in living organisms while leaving behind no by-products (GLMRIS 2012). Ultraviolet (UV) light can also effectively control microorganisms, including C. pengoi, in water treatment facilities and narrow channels, where UV filters can be used to emit UV light into passing water, penetrating cell walls and rearranging DNA of microorganisms (GLMRIS 2012).

Chemical
There are no known chemical control methods for this species.

Note: Check state/provincial and local regulations for the most up-to-date information regarding permits for control methods. Follow all label instructions.

According to the EPA's GARP model, C. pengoi, a free-swimming macroinvertebrate, would likely find suitable habitat throughout the Great Lakes region, except for the deeper waters of Lake Superior. However, population densities of the fishhook water flea increase with distance from shore (IUCN 2010), suggesting that this species may be able to occupy the entire region, including the deeper waters of Lake Superior, given sufficient time (USEPA 2008).