Sander vitreus
(walleye)

Walleye, Sander vitreus, is a freshwater fish native to North America. The walleye has been intentionally stocked as a food fish and for sport fishing within North America for over a century. Natural migration, as well as walleye being flushed downstream during years of high water, has allowed this species to colonize many downstream sections of rivers in the western USA (McMillan, 1984).

The introduction of walleye has been shown to negatively affect other fish species through competition, predation, or by altering species composition. Juvenile walleye coexist with the young of a number of other fish species. McMahon (1992) suggested that competition would occur between walleye, perch and trout fry in the Canyon Ferry Reservoir where no thermal stratification would separate them. Through competition, the numbers and health of brown trout Salmo trutta were found to decrease after introduced walleyes consumed a large portion of the crayfish population, the brown trout’s main food source (McMahon and Bennett, 1996). When walleye were initially introduced into Salmon Falls Creek Reservoir, Idaho, yellow perch Perca flavescens comprised 80% of the sport fish. However, 12 years later, walleye made up 80% and perch only 1% of the fish in the reservoir (McMahon and Bennett, 1996). Walleye have been shown to prey on smolts of Pacific salmon and consume an estimated two million smolts annually in the Columbia River, thereby accounting for about one third of total predation loss of Pacific salmon smolts (McMahon and Bennett, 1996). A study in Seminoe Reservoir, Wyoming, found walleye stocking resulted in a sharp decline in native minnows Hybognathus spp., darters Etheostoma spp., suckers Catostomus spp., rainbow trout Oncorhynchus mykiss and crayfish Orconectes obscurus.

Walleye occurs in all US states except Hawaii and Alaska (Hartman, 2009).

Throughout the history of this species’ introductions, walleye has been introduced into lakes and reservoirs predominantly as a sport fisheries target as well as a food fish; however, colonization of new waters beyond the point of release is a major concern, and regardless of many US state agencies initiating detailed environmental reviews to evaluate the risks and benefits of proposed introductions, illegal introductions of walleye continue (Vashro, 1990; 1995). Natural migration, as well as walleye being flushed downstream during years of high water, has allowed this species to colonize many downstream sections of rivers in the western USA (McMillan, 1984).

Walleye generally prefer large (>100 ha), cool water bodies with a gravel and sandy substrate and with high turbidity. This species occurs in lakes, pools, backwaters and runs of medium to large rivers (Etnier and Starnes, 1993; Frimodt, 1995). They prefer areas with extensive littoral areas and large areas of rocky substrate. Ideally, 25-45% of surface area should provide cover in the way of boulders, logs, brush and vegetation (Scott and Crossman, 1973; McMahon et al., 1984).

Genetics

Walleye has a diploid (2n) chromosome number of 48 and a haploid/gametic (n) of 24 (Danzmann, 1979). The genetics of this species has been intensively studied (April et al., 2011). It is also known to hybridize in nature with sauger (Sander canadense) (Scott and Crossman, 1973; Billington and Koigi, 2003).

Reproductive Biology

Walleye are broadcast spawners, usually preferring shallow shorelines and riffle areas with good water circulation for spawning on gravel shoals (Scott and Crossman, 1973; McMahon et al., 1984). The spawning act usually involves 1-2 female and 2-6 male fish. The spawning group rushes near to the surface in shallow water, where eggs and sperm are released (Ellis and Giles, 1965; Scott and Crossman, 1973). Females release 200 to 300 eggs at a time, as often as every five minutes. Eggs eventually settle to the bottom and fall into crevices in the substrate. Most spawning activity takes place at night, although spawning does occur during daylight hours. Many walleye populations are known to migrate to rivers or tributary streams characterized by fast flowing water and gravel-coarse rock substrates. Walleye also spawn on lake shoals. In some inland lakes, shoal spawning may actually occur under the ice (Dimond et al., 1996).

Associations

Walleyes may be infected with a wide range of diseases and parasites. Protozoan parasites that infect walleye include Ichthyoptirius multillis, Myxosporidia, copepod parasites including the many species of fish lice, and Ergasilus centrachidarum (Hartman, 2009). Three genera of nematode, Contracaecum sp. Eustrongylides sp. and Rhaphidascaris sp., and one species of acanthocephalan, Neoechinorhynchus tenellum, also parasitize walleyes (Dechtiar, 1972). Four genera of cestodes have also been identified as walleye parasites: Bothriocephalus sp., Proteocephalus sp., Triaenophorus sp. and Diphyllobothrium sp. (Poole and Dick, 1985).

The monogenean flukes Cleidodiscus aculeatus and Urocleidus aculeatus are external parasites that infect walleye. Walleye are susceptible to several bacterial diseases such as Columnaris disease. Lymphocistis is a common viral disease among walleye (Hartman, 2009).

Environmental requirements

Walleye have a relatively wide range of environmental tolerances, with an upper temperature range of 29-34°C and a preferred range of 20-24°C in summer (DFO, 2011). This species is able to tolerate low oxygen (to 2 mg·L-1) but prefers levels greater than 5 mg·L-1. Suitable lakes are usually >400 ha, with large littoral zones.

Environmental

• Biological control

General

• Sport (hunting, shooting, fishing, racing)

Human food and beverage

• Meat/fat/offal/blood/bone (whole, cut, fresh, frozen, canned, cured, processed or smoked)

Sauger (Sander canadensis) is similar to walleye in appearance but has a more limited North American distribution. The sauger is smaller and more slender than the walleye. Its dorsal fin, unlike the walleye's, is marked by rows of dark spots and lacks the dark blotch at the rear base. The sauger also lacks the white lower tail tip.

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.

Rapid Response

Rapid response is established at the national level.

Public Awareness

There is not much awareness of the species’ invasiveness and walleye are still stocked.

Eradication

Montana Fish, Wildlife and Parks are currently proposing a 6-year effort to investigate suppressing illegally introduced walleye in the Noxon Reservoir by using a variety of sampling gear and techniques; however, no proven methods have yet been found.

Control

As established populations are difficult and costly to control, further introductions or stocking should be avoided.

Physical/Mechanical Control

Electrofishing and seine netting.

Movement Control

There are some regional regulations. Movement control could be improved by increased public awareness.

Chemical Control

The only effective method of fish eradication is the application of rotenone, a piscicide that is also toxic to non target species.

Monitoring and Surveillance (incl. Remote Sensing) 

Both radio and acoustic telemetry can be used.

01/04/13 Original text by:

Michael Godard, consultant, UK