A Study of the Effects of Black and Yellow Grubs on Lepomis cyanellus and L. humilis in a Natural Environment

Abstract

Questions concerning the effects of parasitism on fish have stimulated considerable investigation; whether parasites damage their hosts to any appreciable extent seems to depend on a variety of conditions. Works dealing with this problem have been varied in nature and are too numerous for a complete review in this paper. Certain investigations (Van Oosten 1944; Elliot and Russert 1949) have indicated that parasites are of minor importance in fixing the standards of growth in an entire population. There are records, however, of populations having been damaged by heavy parasitism (Fasten 1922; Essex and Hunter 1926; Hubbs 1927; Reighard 1929; Cross 1938; Venard 1940; Fantham and Porter 1947). In certain instances, it has been shown that the reproductive'-capacity, at least, is impaired (Reighard 1929; Hunninen 1935; Hunter 1942). Experimentally, it has been demonstrated that tissue parasites especially may be a cause of stunting or loss of weight and sometimes death where infections are heavy. Noteworthy among works of this type are those of Krull (1934) and Hunter and Hunter (1938). The work reported here is based on a study made during the fall of 1949, on a mixed population of Lepomis cyanellus and L. humilis taken from the Little River where it crosses the south campus of the University of Oklahoma in Norman. This population of fish was chosen for study because it represented a condition of overpopulation and stunting, as well as heavy parasitism. The fish were concentrated in small holes along the stream and were heavily parasitized by the strigeid metacercariae, commonly called black and, less heavily, by yellow grubs of Clinostomum marginatum. This offered an opportunity to study relations between the general well-being of the fish, or K factor, and parasitism under natural conditions. The fish were poisoned with rotenone. Upon collection, they were classified and weighed; standard length was determined and scale samples were taken for age determinations. Specimens were preserved in alcohol for future study of growth. A total of 341 fish were selected for study. Of these 203 were Lepomis cyanellus and 138 were L. humilis. They were separated into age groups as revealed by scale readings, and the K factor determined for each specimen. Determinations and counting of the grubs in the muscles were made by examining longitudinal slices of the muscle tissues over a light. All other organs were dissected for cysts. The fish were divided into five-millimeter groups within each age group, the age being expressed as the year in which they lived when collected. Growth was determined as the percentage of total growth in the last year,