Natural and Man-induced Variability in a Silverjaw Minnow (Ericymba buccata) Population

Abstract

During 8 years of fish sampling in an agricultural watershed, density of the silverjaw minnow Ericymba buccata fluctuated widely with a population outbreak in 1976 followed by a rapid decline. Accompanying these fluctuations were changes in population structure that were linked to differential mortality and variation in recruitment. Factors responsible for the demise of this species in the watershed included: severe droughts during 1976 and 1978; consecutive harsh winters in 1976-1977, 1977-1978 and 1978-1979; fish kills; and modified habitat conditions brought about by stream alterations. INTRODUCTION Fish populations in freshwater streams vary in space and time due to a variety of factors in and man-altered environments. Because of the complexity of factors responsible for this variation, it is difficult to interpret the results of programs designed to minimize negative effects of man's activities on biological integrity of water resources, particularly when only 1 or 2 years of data are collected. This report examines population variation of silverjaw minnow (Ericymba buccata) over 8 years in an agricultural watershed in northeastern Indiana. The study area was the target of intensive application of soil conservation practices designed to reduce soil erosion and thereby improve water quality (Morrison, 1977). Thus it was possible to evaluate the population dynamics of Ericymba in light of natural environmental variability and human-induced perturbations in the watershed. Ericymba buccata is a common inhabitant of small headwater streams in the Midwest, where it lives and feeds in schools on or near the bottom (Trautman, 1957; Hoyt, 1970; Pflieger, 1975; Wallace, 1976; Smith, 1978). It is most abundant in areas with sandy substrates and occurs in low densities over silt (Wallace 1972). In Indiana, spawning takes place from late April through July (Wallace 1973a). METHODS The study was conducted in conjunction with an interdisciplinary demonstration project (Black Creek Project) carried out on 48.5 km2 watershed in Allen Co., Indiana. The primary objective of the project was to develop and implement plans for controlling soil erosion and to evaluate effectiveness of traditional conservation methods in improving water resources. This included analysis of the shortand long-term effects of project activities on the fish fauna. Twenty-five fish sampling stations were established in the Black Creek watershed (Fig. 1), but most of the sampling effort was focused upon the main channel (particularly stations 6, 18, 17, 28, 29, 15 and 12). Samples generally covered 100 m at each station and were taken using 3.1 or 6.3 mm mesh minnow seines with block nets at the upper and lower ends of the station. Seines were 5-8 m long. Sampling frequency was not uniform throughout the study period but is believed to accurately reflect general population trends among adult (> 1 year) Ericymba. Initial I Department of Ecology, Ethology, and Evolution, 606 E. Healey, University of Illinois, Champaign 61820. 2 Division of Surveillance, Ohio Environmental Protection Agency, 361 E. Broad, Columbus, Ohio 43215. 3 Museum of Natural History, University of Kansas, Lawrence 66045. 4 Address reprint requests to J.R. Karr.