Introduction and dispersal of non-native bullseye snakehead Channa marulius ( Hamilton , 1822 ) in the canal system of southeastern Florida , USA

An established population of bullseye snakehead (Channa marulius), a large predatory fish from southeastern Asia, was identified for the first time in North America from waters in southeastern Florida, USA, in the year 2000. Since then, it has dispersed throughout the extensive canal system in the area from West Palm Beach south to Miramar. Collection data were compiled to determine the extent of the distribution. The range encompasses three separate areas totaling approximately 830 km2. Over an 18-year period, the range increased an average of approximately 46 km2 per year. There is concern that this nonnative species may threaten the fauna in unique protected natural areas of southern Florida, such as Everglades National Park.


Introduction
The state of Florida in the United States has a long history of non-native fish introductions (Courtenay and Robins 1973;Courtenay et al. 1974;Schofield and Loftus 2015).Introductions may occur in many ways including intentionally through deliberate releases or unintentionally through incidental releases and escapes (Benson 2000).To date, over 160 nonnative fish species have documented occurrences in fresh waters of Florida (USGS 2018).Of those 160, 34 species have reproducing populations in the State (Shafland et al. 2008).Florida's native fishes evolved from North American temperate stocks, but the nonnative fish fauna is comprised primarily of tropical and subtropical species from Asia, Africa, and the Americas that are well-suited to southern Florida's mild climate (Loftus and Kushlan 1987).
Nearly two decades ago, a member of the Channidae family of snakehead fishes, Channa marulius (Hamilton, 1822), was collected for the first time in the United States (USA) from waters in Florida.Snakeheads are a group of air-breathing freshwater fishes comprising two genera.The genus Channa Scopoli, 1777 is recognized with 42 valid species that are native to Asia in a wide range of habitats spanning from tropical to boreal, and the genus Parachanna Teugels and Daget, 1984 has three valid species from tropical Africa (Fricke et al. 2018).The common name "snakehead" refers to the snake-like appearance, having a long cylindrical body shape and eyes that are forward on the head.
Four species of snakehead have been documented in open waters of the USA, three of which have established reproducing populations.Channa argus (Cantor, 1822) (northern snakehead) is established in the Mid-Atlantic states and was initially discovered in 2004 in the Potomac River drainage of Virginia (Odenkirk and Owens 2005) as well as in the lower White River system of Arkansas several years later (Holt and Farwick 2009).Channa maculata (Lacepède, 1801) (blotched snakehead) has been established in Hawaii for more than a century and was believed to have been imported for human consumption (Devick 1991).Channa micropeltes (Cuvier, 1831) (giant snakehead) has been collected in six states in the conterminous USA as single specimens only and no reproducing populations have been reported.Lastly, Channa marulius (bullseye snakehead; Figure 1) at first thought to be a species complex (Britz et al. 2017), has been established in southeastern Florida since sometime prior to 2000 (Courtenay and Williams 2004).
Channa marulius, the largest of the Channidae family, is native from Pakistan and India to Southeast Asia and China (Talwar and Jhingran 1991) where it is known to reach 120 cm in length (Bardach et al. 1972).Coloration is grey to green on the dorsal side and white on the ventral side (Figure 1).Lateral sides are rusty orange to brown with scattered small white spots and a row of five to six large dark spots (Talwar and Jhingran 1991).All fins are without spines and dorsal and anal fins are long, running much of the length of the fish (Wee 1982).It is distinguishable from all but one other snakehead species C. marulioides (Bleeker, 1851) by a distinctive ocellus near the base of the caudal fin that fades with age (Talwar and Jhingran 1991).The identity of the Florida population was eventually confirmed through DNA testing as C. marulius (Serrao et al. 2014).It represents the only known established population of C. marulius in North America.
Although C. marulius has been present in Florida's southeastern canal system for at least 18 years, there has been no single account providing information on its distribution.In this paper, we compile observations and collections of C. marulius to produce a timeline of introduction and dispersal in southeastern Florida while also providing life history information that may help explain its success.

Methods
Channa marulius observation reports and collection information were accessed from the U.S. Geological Survey's Nonindigenous Aquatic Species Database (USGS 2018), a national repository for spatiallyreferenced data.Records in this database are compiled from scientific literature, published reports, and museum specimens.In addition, unpublished data are voluntarily contributed from a variety of sources including different levels of government (federal, state, county, municipalities), universities, non-government agencies, citizen scientists, and anglers.Collection data from active monitoring of C. marulius each year after their discovery by the Florida Fish and Wildlife Conservation Commission (FWC) were also included.
From these data, a geospatially-referenced data layer of C. marulius occurrence location points was constructed.ArcGIS (ESRI, Redlands, CA) convex hull tool, which creates a polygon using the outermost points, was used to calculate cumulative areas of snakehead range annually from 2000 to 2018.Because the current distribution of C. marulius in south Florida is not contiguous, ranges of three separate populations in the towns of Wellington, Miramar, and Tamarac were calculated.The Wellington, Miramar, and Tamarac distribution ranges were then added together to produce a total cumulative area range.Four collection locations were not included in the convex hull calculations as they were single fish, distant from the general population (Figure 2).These four fish were removed, and we do not believe there were additional fish in these locations.We used linear regression to examine the relationship between year and cumulative geographic range (km 2 ).The analysis was performed using NCSS 11 Statistical Software (NCSS, LLC, Kaysville, UT).

Results
A total of 224 C. marulius locations in Florida were compiled from 2000 through June 2018.The first documented occurrence of C. marulius in the USA was October 5, 2000 in a residential pond in Tamarac, Broward County, Florida (Figure 2; Table S1).The To the south, the species extended its distribution when collections were made in Miramar just north of the Miami-Dade County line.The known distribution range encompassed an area of 830 km 2 by June 2018.Over an 18-year period, there was an average increase in the distribution range of approximately 46 km 2 per year.The range expansion across years closely fit a linear function (r 2 = 0.96; Figure 3).
In general, C. marulius inhabits canals that comprise the interconnected human-made water management system constructed to provide freshwater and flood protection for southeastern Florida (Miami-Dade, Broward, and Palm Beach counties).The large main canals in this area generally flow in an easterly or southerly direction and empty into the Atlantic Ocean.Those canals are generally box-cut with vertical shorelines cut into the lime rock substrate and average approximately 30 m wide and greater than 3 m deep.They can also have aquatic vegetation on a 1-2 m wide littoral shelf.Main canals connect with an extensive system of increasingly smaller, shallower and more vegetated canals that drain urban surface waters during heavy rain events.Many canals are connected to human-made "lakes" that range from less than one to several hundred acres, a few of which are deep (e.g., 18 m).All main canals and some lateral canals have water control structures that can act as barriers to fish movement.From survey observations, C. marulius was often associated closely with shoreline structure including floating aquatic vegetation, overhanging woody vegetation, and boat docks.They also tended to be more abundant in shallow, heavily-vegetated secondary canals than in the main canals.The species is present in a number of isolated ponds and small lakes in southeastern Florida, suggesting it is being moved by people.

Discussion
The Channidae family was recognized early on as a potential problem by the State of Florida.In March of 1970, the State recommended that the genera Channa and Ophicephalus (currently a junior synonym of Channa) "be included on our list of restricted fishes" (pers.comm.from V.E.Ogilvie to J.W. Woods 1970).In 1979, the Channidae family was placed on the State of Florida's Prohibited Non-native Aquatic Species list (F.A.C.Rule 68A-23.008)that prohibited the possession of live fish with permitted exceptions for research and exhibition (FWC 2010).Despite being on the State's prohibited list, C. marulius was later collected in Florida and established a reproducing population.
Like so many other introductions of fishes, it is not known exactly how, when, and where C. marulius first entered the United States.However, why it was introduced is likely explained.Several snakehead species are valued as food fishes and to a lesser degree aquarium pets and have been introduced widely across Asia through trade and aquaculture (Courtenay and Williams 2004).Channa marulius, however, is not often found in the aquarium trade because of its aggressiveness towards other fish (Schmidt 2001).In Thailand, it is a popular gamefish among tourists which may have led to illegal stocking in Florida although there is no direct evidence (Courtenay and Williams 2004).In India and Pakistan where it is native, C. marulius is cultured as a food fish for human consumption (Bardach et al. 1972) and commercially harvested from the wild (Dua and Kumar 2006).Live fish fetch the highest price, but they are also sold freshly dead or dried (Wee 1982).Snakehead is viewed as an economically important species within its native range (Parameswaran and Murugesan 1976) so much so that its importance in local human diets has led to the decline of some wild populations in India (Habib et al. 2011;Ali et al. 2013).As part of their cultural heritage, tribes in West Bengal, India have chosen to protect C. marulius where industrial development has had a negative impact on biodiversity (Deb and Malhotra 2001).
Little has been documented about the reproduction of C. marulius in Florida.In its native range, sexual maturity for some individuals begins at about 360 mm total length (TL) in males and females alike; but it is not until males reach 560 mm TL and females 660 mm TL are they all sexually mature (Devaraj 1973).Naturally, C. marulius lay their floating eggs in areas with vegetation; however, they will spawn in fish farm ponds without aquatic vegetation (Parameswaran and Murugesan 1976), which could help explain their success in artificial habitats of southeastern Florida.
In general, the fecundity of snakeheads is considered relatively low compared to other fishes (Wee 1982) and depending on the maturity stage (i.e.size), the number of eggs for C. marulius ranged from approximately 2,200-18,400 (Devaraj 1973).Snakeheads exhibit biparental nest guarding behavior, thereby increasing reproductive success in all habitats (Parameswaran and Murugesan 1976;Wee 1982;Schmidt 2001).Like all other species in the Channidae family, adult C. marulius are carnivorous (Talwar and Jhingran 1991), feeding primarily on fishes and crustaceans.Very young snakeheads feed on protozoa and algae as larvae; then as they grow into fry, the diet shifts to zooplankton, and eventually to include insects, benthic crustaceans, mollusks, and larvae and fry of other fish (Wee 1982;Bhuiyan et al. 2006).Its ability to adapt to a wide variety of available food is also likely to allow the snakehead to easily adapt to novel environments.
The native habitat of C. marulius is described as lakes and rivers, with deep pools and clear water (Talwar and Jhingran 1991).In addition, the species can be found in canals, lakes, and swamps with little or no flow in its native range (Rainboth 1996), which is similar to the habitats C. marulius occupies in Florida.Courtenay and Williams (2004) stated the native range spanned from latitudes 7ºN to 34ºN, which partially includes a temperate climate.Using an ecological niche model to predict suitable habitat that included variables such as frost frequency and air temperatures, Herborg et al. (2007) showed a North American distribution for C. marulius limited to most of Mexico and the southern United States up to approximately 35ºN latitude.However, preliminary testing of the lower temperature limit is approximately 10 °C (KBG, unpublished data), which may restrict the species to southern Florida.
The habitat of extreme southern Florida, including Everglades National Park (ENP) and Big Cypress National Preserve, consisting of wetlands, swamps, and coastal river headwaters is home to 92 native freshwater fishes (Loftus and Kushlan 1987).It is possible that C. marulius could spread beyond the canal system into unique natural ecosystems such as those in ENP, a designated UNESCO World Heritage Site, which may potentially provide suitable habitat and a diverse array of prey.Within novel environments of the canal system of southern Florida, Gandy and Rehage (2017) showed that fish assemblages favor non-native species over natives.This canal system likely provides a conduit for non-native fishes to disperse into protected areas (Loftus 1988); 17 non-native fish species have already been documented in ENP (Kline et al. 2014).Kline et al. (2014) stated that legislation is in place to minimize the spread of non-native species into ENP, but at the same time, plans to restore the Everglades region will use the canal system to deliver water to those natural areas.Unfortunately, canal habitat may protect non-native fish from drought (Loftus and Kushlan 1987) and unseasonably cold winter temperatures in some waters (Schofield et al. 2010), thereby serving as a potential continuous source of non-native fish to natural areas.We believe Channa marulius has not yet reached its potential distribution and will likely continue to disperse in southeastern Florida where suitable habitats exist.

Figure 2 .
Figure 2. Cumulative collection and observation data for bullseye snakehead (Channa marulius) in southeastern Florida, USA.Data are from the USGS Nonindigenous Aquatic Species database available at http://nas.er.usgs.gov.Collection locations with black squares represent historical collections of single fish.These were not used in the range calculations.

Figure 3 .
Figure 3. Linear regression of geographic range by year for bullseye snakehead (Channa marulius) in southern Florida.Each dot represents the cumulative range encompassed by the species by year.