Umbra pygmaea
(eastern mudminnow)

U. pygmaea is a small bodied fish native to the Atlantic and Gulf slopes of the USA (Page and Burr, 1991). It was imported in the early 20^th century to Europe (France, Belgium, Germany and the Netherlands) for ornamental reasons and as food for imported North American salmonids (Verreycken et al., 2010). More recent introductions have occurred in Poland (Grabowska et al., 2010) and Denmark (Nielsen, 2012). The largest number of localities in Europe is in southeast Netherlands and northeast Belgium (Verreycken et al., 2010). In its introduced range, U. pygmaea reaches its highest abundances in lentic waters, especially moorland pools (Dederen et al., 1986), where it has little competition from other species (Verreycken et al., 2010). Aquaculture and other ponds are a major source of individuals in riverine populations (Verreycken et al., 2010). U. pygmaea demonstrates a great tolerance of poor habitat quality, such as low pH, temperature and oxygen saturation (Poll, 1949; Dederen et al., 1986). Negative impacts of U. pygmaea are generally described as low to medium (Verreycken et al., 2010; Froese and Pauly, 2014), especially in riverine systems, but it may be considered a high impact species in ditches with dense vegetation or in waters without other fish species (such as acid moorland pools) (Dederen et al., 1986).

U. pygmaea is native to the Atlantic and Gulf slopes of the USA, from southeastern New York (including Long Island) to St. Johns River drainage in Florida and west to Aucilla River drainage in Florida and Georgia (Page and Burr, 1991). It was imported in the early 20th century to Europe for ornamental reasons and as food for imported North American salmonids. It now occurs in six European countries: France, Belgium, Germany, the Netherlands, Poland and Denmark (Verreycken et al., 2010).

The risk of introduction from its native area seems very small, but unintentional introduction from infested aquaculture ponds to other ponds in its non-native range, as happened in the Marne region, France (Guidou and Keith, 2002), or escapes/drifts from ponds into riverine systems are still possible (Crombaghs et al., 2000; Verreycken et al., 2010). Intentional releases from aquaria (Grabowski et al., 2010) or use as live bait (Nielsen, 2012) cannot be excluded as potential sources of introduction (Verreycken et al., 2010).

The typical habitat of U. pygmaea is lowland waters with little to no streamflow, such as backwaters, ponds and irrigation channels (Kottelat and Freyhof, 2007). In its introduced range, the highest abundances are reached in lentic waters such as floodplain waters and especially moorland pools (Dederen et al., 1986), where U. pygmaea has little competition from other species. Aquaculture and other ponds are a major source of individuals in riverine populations (Verreycken et al., 2010).

Genetics

Haploid chromosome number is 11, diploid 22 (Froese and Pauly, 2014). Natural hybridization between U. pygmaea and U. limi in the Hudson River drainage (New York state) was documented by Schmidt and Daniels (2006).

Reproductive Biology

Age at maturity is 1.7 ± 0.3 yrs. The species is oviparous. Maximum fecundity is around 2000 ± 700 eggs. Spawning takes place in April and May at temperatures between 9 and 15°C. U. pymaea females build nests and guard them. The larvae remain in algal nests for about 6 days (Froese and Pauly, 2014; Kuehne and Olden, 2014).

Physiology and Phenology

U. pygmaea demonstrates great adaptability and tolerance of poor habitat quality, such as low pH, temperature and oxygen saturation (Poll, 1949; Dederen et al., 1986). Poll (1949) documented an incident where an U. pygmaea individual survived a whole night out of water. Extremely low pH-values (pH 3) were tolerated in an experimental setup without any mortality, and eggs and fry were observed in waters with a pH as low as 3.5 (Dederen et al., 1986). In natural environments, Dederen et al. (1986) found U. pygmaea in waters pH ranging from 3.5 to 8.1.

Longevity

Panek and Weiss (2012) found U. pygmaea up to 5 years old in the native range. In its introduced range fish up to 5.5 years were observed (Dederen et al., 1986).

Population Size and Density

Densities in lotic waters in Belgium ranged from 0.2 to 664.0 individuals per 100 m of continuous electrofishing, with the majority of the sampling sites (51.2%) containing between 1.0 and 10.0 individuals per 100 m (Verreycken et al., 2010). den Hartog and Wendelaar-Bonga (1990) reported densities of up to 20 kg/ha in moorland pools in the Netherlands. Densities may be even higher (up to 250 kg/ha) in otherwise fishless pools (Verreycken, personal observation).

Nutrition

U. pygmaea is a bottom-feeding generalist and mainly feeds on insect larvae, worms, molluscs and crustaceans. Panek and Weis (2013) found a broad diet consisting of 13-17 distinct prey classes. Although regarded as an invertivore, fish alevins (sac fry) are also eaten (Froese and Pauly, 2014). Cannibalism was documented by Panek and Weis (2013) during the summer season.

Verreycken et al. (2010) found no significant correlation between the number of U. pygmaea and the numbers of fish predators (pike Esox lucius, pikeperch Sander lucioperca and perch Perca fluviatilis more than 20 cm) in a sample site in Flemish lotic water. However, for lentic waters, Dederen et al. (1986) reported an inverse relation between the abundance of U. pygmaea and the presence of predatory fish species. In its native range, Panek (1981) similarly found an inverse relation with the number of fish species present. Grebe (Podiceps cristatus) is a possible regulator mudminnow densities (den Hartog and Wendelaar-Bonga, 1990).

Animal feed, fodder, forage

• Bait/attractant
• Fodder/animal feed

Environmental

• Biological control

General

• Laboratory use
• Pet/aquarium trade
• Research model
• Sport (hunting, shooting, fishing, racing)

Genetic diagnosis can be performed. There are 10 barcode sequences available for U. pygmaea from BOLD and GenBank (Encyclopedia of Life, 2015).

U. pymaea may be detected using conventional fish sampling methods (boat and backpack electrofishers, fyke nets, seines and minnow traps). Once captured, the species is easily identified in its native as well as non-native region because, save a few exceptions, there is no overlap in area with its close congeners U. limi (found in North America) and U. krameri (found in the Danube drainage, Europe). Analysis of meristics, morphometry and genetics can help, using the dichotomous keys such as Page and Burr (1991) or descriptions from Schmidt and Daniels (2006) and Kottelat and Freyhof (2007).

U. pygmaea looks very similar to its close congeners U. limi (found in North America) and U. krameri (found in Europe).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

Prevention

Prohibition of the sale of U. pygmaea in the aquarium trade could restrict the non-native area and prevent further spread. In addition, better control of fish stocking with fish from infested ponds could help prevent further introductions.

Eradication

Eradication in a closed system may be feasible using piscicides (such as rotenone), especially in ponds where U. pygmaea is the only fish species present. Physical removal, even after draining a pond, seems very difficult, if not impossible, given that this species can survive buried in mud. In open systems eradication would be even more problematic.

Physical/Mechanical Control

Physical control of closed water systems may be feasible by removing the majority of the fish by draining the water or by catching the fish with fykes and/or nets.

Biological Control

Grebe (Podiceps cristatus) is a possible regulator of Umbra densities (den Hartog and Wendelaar-Bonga, 1990). Pike (Esox lucius) and other fish predators may help to reduce U. pygmaea populations, as Dederen et al. (1986) reported an inverse relation between the abundance of U. pygmaea and the presence of predatory fish species.

Chemical Control

Chemical control in a closed system may be feasible using piscicides (such as rotenone), especially in ponds where U. pygmaea is the only fish species present.

Monitoring and Surveillance (incl. Remote Sensing)

Regular fish monitoring programmes (using electrofishing, fykes or seine nets) may detect U. pygmaea. In the near future, e-DNA may also be a sampling method.

It is not clear whether the situation has changed since Verreycken et al. (2010) reported the distribution of U. pygmaea’s non-native range. International cooperation would be needed to gather the current distribution in the non-native range again. In addition, more work is needed to study the impacts of U. pygmaea in lotic and especially lentic waters (such as predation on amphibian larvae and aquatic invertebrates).

Belgium: Research Institute for Nature and Forest (INBO), Kliniekstraat 25,, B-1070 Brussel, www.inbo.be

USA: US Geological Survey (USGS), 11649 Leetown Road,, Kearneysville, WV 25430, www.usgs.gov

06/03/15 Original text by:

Hugo Verreycken, Research Institute for Nature and Forest (INBO), Belgium