Background & Environmental Constraints Origin: China, eastern Russia, and the Korean peninsula Speciation: Channa argus is 1 of 29 snakehead species Temperament: Aggressive Maximum size: Up to 5 feet long Water parameters: Freshwater pH: 6.0 - 7.5 Temperature: 72-82˚ F (22-28˚C)
Threat to Biodiversity & Economy This top level predator fish is known to survive on land and cause major strain on local ecosystems Alter freshwater ecosystems by competing against native fish Reduce biodiversity through predation on aquatic and terrestrial species Could push endangered or threatened species to the brink of extinction May introduce fungal pathogen known as Epizootic Ulcerative Syndrome (EUS) – infects foreign freshwater fish Could damage commercial and recreational fishing industry in the Chesapeake by diminishing populations of fish that spawn in the Potomac River Millions of dollars have been spent on fish stocking, dam modifications, and other projects to limit snakehead impact
Introduction to U.S. Waters In the past, Asian markets and other grocers were allowed to import living snakeheads to the U.S. Over time, human beings introduced the species to native waters First reported in U.S. waters in 1977 in Silverwood Lake, California National Media debut in 2002 An angler caught an unfamiliar fish that measured 28 inches from a pond in Crofton, Maryland. Took the picture to Maryland Dept. of Natural Resources where it was later identified. After another fisherman caught spawning adult and baby snakeheads from the same pond, the national media disseminated the first stories of the snakehead. To prevent the fish from migrating to the Potomac River, Maryland wildlife officials dumped the pesticide, Rotenone. Killed all fish, including 6 adult and over 1000 snakehead young. An unsuccessful effort as Channa argus appeared in the Potomac River 2 years later.
Northern Snakehead possesses competitive advantages that may allow it to out-compete native fish species Primitive lungs Sacs above each gill that can fill with air and draw oxygen from the stored air. Allows it to survive waters that are low in oxygen. Allows it to survive out of water for 2 – 3 days. Allows it to live under a sheet of ice Aggressive Both parents guard the eggs and the fry Known to attack fishing lures. Highly prolific Spawning females can release up to 15,000 eggs at a time Can mate up to 5 times per year. Equates to 75,000 offspring per year. Indiscriminate feeders Feed on native fish, amphibians, crustaceans, birds, small reptiles and small mammals. Snakehead video
Purpose: In 2004, the VA Dept. of Game and Inland Fisheries initiated a study to monitor the snakehead’s expansion in the Potomac from 2004–2006.
Methods: Surveyed on a monthly basis: Areas of known snakehead concentrations. Bodies of water contiguous but farther away to assess range. Relative abundance of snakeheads was based on the number of fish retrieved per hour of electrofishing, and catches per unit effort of angling. Explanation of electrofishing Sampled fish were separated into 8 groups, ages 0-7 (approximated based on size of sagittal otolith at necropsy). Accumulates in rings as it grows Used in age or growth studies Counts of daily rings on sagittal otoliths of newborns (age-0) were used to estimate when fish hatched. Data collection included total length (mm), weight (g), stomach content evaluation, gonad condition, and length at age. Stomach content evaluation entailed only removal and identification. Gonadosomatic indices (GSI) of ovary weight to body weight was used to quantify spawning period.
Results: Table 1. Total number of northern snakeheads captured from the Potomac River system, Virginia, during 2004-2006 by use of several gear types (HL = hook and line; EP = electrofishing; BP = Backpack; other = dip net, trap net, seine, standing, and bow angling). Year HL Boat EF BP EF Barge EF Other Total 2004 13 4 0 0 3 20 2005 11 59 127 71 3 271 2006 24 171 11 1 7 214 Total 48 234 138 72 13 505 Table 3. Length-at-age data (mm) for 167 northern snakeheads sampled in the Potomac River system, Virginia, during 2004-2006 (all years combined). Age N TL mean SD Min. Max. 0 9 133 23 88 166 1 18 190 20 155 227 2 24 287 85 163 480 3 53 357 85 218 588 4 39 431 100 308 669 5 17 545 111 362 674 6 4 578 52 531 634 7 3 560 9 531 569 Table 2. Mean total length (TL) and weight (W) of northern snakeheads sampled in the Potomac River system, Virginia, during 2004-2006. TL (mm) ____ W (g)___________ Year N Mean SD Min. Max. N Mean Min. Max. 2004 20 406 105 88 634 782 553 7 2635 2005 270 273 120 113 655 493 621 13 3145 2006 215 475 146 164 765 1381 1009 36 4598
Table 4. Frequency of occurrence (percentage of stomachs containing the taxon) of identifiable taxa consumed by 219 northern snakeheads collected in the Potomac River system, Virginia, during 2004-2006. Common name Scientific name Frequency (%) Banded killifish Fundulus diaphanus27 White perch Morone americana 5 Pumpkinseed Lepomis gibbosus5 Bluegill L.macrochirus 5 Goldfish Carassius auratus 2 Gizzard shad Dorosoma petenense 1 American eel Anguilla rostrata 1 Yellow perch Perca flavescens 1 Largemouth Bass Microplevus salmoides1 Sponail shiner Notropis hudsonius 1 Eastern silvery minnow Hybognathus regius < 1 Mummichog F. heteroclitus < 1 Channel catfish Ictalurus punctatus < 1 Green sunfish L. cyanellus < 1 Tessellated darter Etheostoma olmstedi < 1 Frog Rana spp. < 1 Crayfish Cambaridae < 1
Conclusions: Dramatic increase in catch rates suggests an overall increase in snakehead population, although an increase in electrofishing capture efficiency could have added some degree of bias. Maximum size, including total length and weight, increased every year, which suggests that the population matured. 17 species were identified in stomach contents. Female Gonadosomatic Indices rose in early April, peaked in June, and diminished through September. Counts of daily rings on otoliths of newborns indicated hatching from mid-June to early September.
Implications: Despite the apparent increase in snakehead population, the area of colonization did not appear to increase over the original 23-km main stem of the Potomac. Yearly increase in maximum size is consistent w/ theory that the population was recently established. Stomach contents consistent w/ theory that snakehead is piscivorous and opportunistic in feeding (ie. Frog, crayfish). Female GSIs, estimated hatch dates, and the discovery of a nest in early September, indicate a spawning period of at least 5 months in the Potomac.
Works Cited: Virginia Invasive Species Council. 2005. Virginia Invasive Species Management Plan. Virginia Department of Conservation and Recreation Division of Natural Heritage. Richmond. 84 p. Invasive Species. 2004. Marek Topoleski, Mary Fabrizio, Ron Lauda. Pg 77-122. Aquatic Community. Northern Snakehead. 2004. Retrieved on Oct. 26, 2009.. Odenkirk, John, and Steve Owens. Expansion of a Northern Snakehead Population in the Potomac River System. American Fisheries Society 136:1633-1639, 2007. The Environmental Literacy Council. Snakeehead Fish.. 2008. Retrived on Oct. 25, 2009..