During the summer months, we receive numerous phone calls from concerned anglers regarding visible sores or lesions on sport fish.  These concerns are prevalent not only at Sam Rayburn and Toledo Bend reservoirs, but also at many other East Texas reservoirs and private ponds.  In general, all anglers express concerns regarding overall health of the fish population, and some assume that these infections are very abnormal.  However, fish parasites are extremely common in most southeastern U. S. waters and include bacteria, protozoa, fungi, flukes, tapeworms, roundworms, and copepods.  In fact, essentially all adult fish are carriers of numerous species of parasites but are otherwise healthy.  Parasite abundance is usually low and nearly harmless to individual fish.


Parasites are animals that live on or in other animals.  Most parasitic infections of fish occur from spring through fall, but heaviest infestations are typically observed during summer months.  There are several reasons for this seasonal pattern.  Fish parasite abundances are usually at their peak when water temperatures are warm, increasing the potential of fish infection.  Sport fish are also more susceptible to sickness or infection during this time.  Spring spawning activities consume considerable amounts of energy and are stressful to fish, reducing their resistance to disease.  In addition, spawning behaviors of many sport fish involve physical contact during courtship and egg deposition.  Resulting scrapes and wounds greatly increase infection rates.  However, these infections are rarely severe.  Typically, less than 10% of fish will exhibit external signs of significant infection, even when infection rates are at their highest.  Most infected fish quickly recover from these infestations.

Most of the external ailments are likely caused by bacterial infections.  Bacteria are one-celled, microscopic organisms that live in nearly all environments.  The two most common types affecting fish are Aeromonas and Pseudomonas.  Infections are usually initiated by open wounds or slime removal resulting from spawning activities or angler handling.  Common sites for infection include the mouth and jaws, sides of body, and the tail fin.  Epistylis, a microscopic protozoan, can also cause external red sores on fish but is not actually a parasite.  It only uses the host fish as a substrate for attachment and feeds on bacteria and other small particles in the water.  However, the specialized attachment structure secretes an enzyme that dissolves fish mucus and scales, leaving visible lesions.

Fungi are as widespread as bacteria in all water systems and can cause disease.  Similar to bacterial infections, fungal disease is typically associated with some form of external injury.  The most common fungus is Saprolegnia, or water mold.  Infection results in fuzzy grey patches that resemble cotton balls.

The most common, externally visible fish parasites in our area are Argulus, or fish lice.  Fish lice are copepods (small crustaceans) that are nearly transparent but have a light yellow color.  They live on the surface of fish and feed on blood, mucus, and skin cells.  Largemouth bass appear to be preferred hosts of fish lice.  Lice are most often observed on the white, underside of bass directly beneath the gills.  Similar to other fish parasites, rarely are lice abundant enough to cause significant harm to fish.

If you have questions about the fisheries of area lakes, stop by the Inland Fisheries office at the Jasper State Fish Hatchery or contact us by phone (409-384-9572) or email ( This e-mail address is being protected from spambots. You need JavaScript enabled to view it ).  Good luck and good fishing!


Todd Driscoll is a district fisheries management biologist for Texas Parks and Wildlife Department where he has worked for 10 years.  He received a B.S. in Fisheries Biology from Kansas State University and a M.S. in Fisheries Management from Mississippi State University.  His primary responsibilities include fisheries management of Sam Rayburn and Toledo Bend reservoirs in Southeast Texas.  Todd is an avid bass angler and participates in approximately 25-30 local and regional bass tournaments per year.  He also represents Lowrance Electronics as a Technical Pro Staffer, working tournament support and service at BASS, FLW, and owner’s tournaments across the country.