Knothead Carp

By the late 1800s, the carp is the most important commercial fish in the Illinois. Anything that affects the growth and development of this species is of concern. The large increases in the pollution load in 1916-1918 lead to dramatic changes in the plant and animal life of the middle Illinois River. Other changes that occur at this time, noticeable to fishermen, are alterations in the carp's growth and appearance: first, a retardation of the growth rate throughout its life cycle, and second, marked anatomical abnormalities, particularly to the bones. This malformation to the bones is believed to be rickets, the first known record of this among fishes. Rickets in higher vertebrates is due to a lack of vitamin D, and there is evidence that this is also the case for the carp. The depletion of green aquatic vegetation may be a contributing factor to the bone malformations (Thompson 1928).

The most common name given to these malformed carp is “knothead” but “lunkhead,” “popgill,” “clam-jaw,” and “lump-jaw” are also used. These names all refer to the malformation of the head and opercles (gill covers, Figure 29).

Figure 29: Head of carp (from left): a. Normal, b. Moderate Knothead, and c. Extreme Knothead (Thompson 1928)

Two anatomical changes that occur in the knothead carp but not in normal carp are a reduction in fat and softer flesh; these changes make it difficult to dress them for market without tearing the flesh (Thompson 1928). Another change is to the respiratory system.

Fishermen report that the knothead carp begin to appear in their catch in 1920 and that the percentage of knotheads in their catch increases rapidly, reaching 50% by 1922. The range of the knothead carp is from Utica to Copperas Creek (north of Havana, Table 14). Effective 1912, no fish survive above Utica because of the severe pollution. The knothead carp is found in portions of the river where there is a significant loss of aquatic vegetation due to pollution. Conditions are sufficient to allow fish to survive, but the degree of pollution causes malformations.