Tennessee Wildlife Resources Agency

TWRA's Region 4 - East Tennessee

Reservoir Fisheries Management Program

"Gill Maggot" Infestation in Tennessee

Updated on February 15, 2012

Have you been seeing parasites in the mouths of striped bass or largemouth bass lately? Are they attached to the tongue, gills, and/or the roof of the mouth and look sort of like maggots? What you are seeing is most likely a parasitic copepod in the genus Achtheres.

There is no cause for alarm, however, since these unsightly critters have not been linked to any fish mortality in Tennessee. They are also not harmful to humans since they are destroyed by cooking and are not found in the flesh of fish.

Achtheres is one of several genera of external parasitic copepods that attach to the oral cavity of numerous North American fresh and saltwater fishes. During severe infestations, it appears as if the fishes' mouths are full of maggots; hence the common term, “gill maggots”. Several species of parasitic copepods can inflict great harm and even kill fish, but Achtheres is not considered to be one of them.

Generally, the adult female copepods attach to various places in the oral cavity of fish and can remain there for extended periods. They feed off the soft tissues present and produce eggs, which are carried in egg sacs until they hatch as nauplii. The nauplius molts quickly into the first copepodid stage, which is free swimming and infective to fish. The copepod attaches to the fish gill by means of the frontal filament and becomes chalimus I. There are a total of four chalimus stages still attached by the filament. Adult males are freely moving on the fish gills, while the adult females re-attach permanently by a mushroom shape bulla. Fertilized females deposit eggs into their egg sacs to start the process over again.

The first known record of a severe outbreak of Achtheres in Tennessee was from samples taken from largemouth bass from Tellico Reservoir in 1981. While working as a creel clerk, TWRA’s Rick Bivens identified the parasites taken from severely infested largemouth bass. The infestation did not seem to hurt the population and went away quickly.

The critter surfaced again in 2000. This time it was not in largemouth, but in striped bass from Watts Bar and Tim’s Ford Reservoirs. Samples of these parasites were sent to Auburn University and Tennessee Technological University for identification. There was no cause for alarm since they were the “good” copepod, Achtheres, and the striped bass appeared to be in good condition.

Recent outbreaks have occurred in striped bass from Old Hickory, Melton Hill, Tellico, Cherokee, and Norris Reservoirs. Parasites from Old Hickory were sent off in 2002 and were identified as Achtheres. Samples taken from Norris in January of 2004 were also identified as the “good” copepod.

Achtheres should not cause significant harm to healthy fish. One would suspect, however, that heavy infestations could lead to secondary bacterial and fungal infections in fish already compromised by poor water quality or poor condition. These secondary infections could lead to a limited amount of fish mortality. For this reason, they are being monitored by TWRA biologists.

Anglers are encouraged to report sightings of this parasite in any reservoir not mentioned in this article or in species other than striped bass or largemouth bass, to their local TWRA fisheries biologist.

Below is an abbreviated “time-line” of when Achtheres was first reported in area reservoirs. This parasite still persists to some degree in many of the reservoirs listed:

1981 Tellico Reservoir, TN - largemouth bass
2000 Watts Barr Reservoir, TN - striped bass
2000 Tim’s Ford Reservoir, TN - striped bass
2001 Melton Hill Reservoir, TN - striped bass
2001 Watauga Reservoir, TN - 1 smallmouth bass
2002 Old Hickory Reservoir, TN - striped bass
2002 Norris Reservoir, TN - striped bass
2002 Smith Mountain Lake, VA - striped bass
2003 Kerr Reservoir, VA - striped bass
2003 Leesville Resrvoir, VA - striped bass
2004 Lake Norman, NC - striped bass
2004 Gaston Reservoir, NC - striped bass
2004 Tellico Reservoir, TN - 1 striped bass
2004 Smith Mountain Lake, VA - largemouth
2004 Cherokee Reservoir, TN - black bass
2005 Cherokee Reservoir, TN - white bass (angler report)
2006 Ouachita Lake, AR - striped bass
2006 Fort Patrick Henry Reservoir, TN - striped bass (angler report)
2006 Hiwassee River, TN - striped bass (angler report)
2007 Cherokee Reservoir, TN - striped and hybrid striped bass
2007 Holston River - Cherokee Res. tailwater, TN - striped bass
2007 Santee Cooper, SC - striped bass (angler report)
2008 Lake Murry, SC - striped bass (angler report)
2010 Norris Reservoir, TN - largemouth (many, but low density)
2012 Boone Reservoir, TN, - striped bass (angler report)

Drawing/picture? of Achtheres with loaded egg sacs from: http://www.glsc.usgs.gov/greatlakescopepods/


Achtheres in Striped Bass
Photo by Jim Negus, TWRA, Norris Reservoir Striped Bass, December 2003


Achtheres in striped bass from Smith Mountain Lake Virginia, Tom Shahady and Joe Ashwell


Histological preparation of Achtheres attached to striped bass. Photo care of Karl Hayden Auburn University


Copepods found in Cherokee Reservoir largemouth bass in March, 2004 by Jim Negus


Parasitic copepod from Cherokee Reservoir, TN largemouth – Jim Negus


Parasitic copepod from Cherokee Reservoir, TN largemouth – Jim Negus