1830 Environmental Toxicology and Chemistry, Vol. 19, No. 7, pp. 1830–1836, 2000  2000 SETAC Printed in the USA 0730-7268/00 $9.00 + .00 SEASONAL VARIATION IN RADIOCESIUM LEVELS OF LARGEMOUTH BASS (MICROPTERUS SALMOIDES): IMPLICATIONS FOR HUMANS AND SENSITIVE WILDLIFE SPECIES JOHN D. PELES,*² T OM PHILIPPI,‡ MICHAEL H. SMITH,‡ I. LEHR BRISBIN JR.,‡ and J. WHITFIELD GIBBONS‡ ²Pennsylvania State University, McKeesport, Pennsylvania 15132, USA ‡Savannah River Ecology Laboratory, Aiken, South Carolina 29802, USA ( Received 27 July 1999; Accepted 1 December 1999) Abstract—To examine seasonal variation in levels of radiocesium ( 137 Cs) within largemouth bass (Micropterus salmoides; N = 589), fish were collected monthly over a one-year period from an abandoned reactor cooling reservoir. Month of collection, sex, age, and body mass (log transformed) were all significant factors influencing 137 Cs concentrations. Levels of 137 Cs reached a peak in late winter/early spring (February/March), and minimum values occurred in the fall (October). An asymmetric sawtooth model with a four-month period of increase and an eight-month period of decrease fit the data for monthly 137 Cs values significantly better than symmetric sinusoidal and sawtooth models. The mean concentration of 137 Cs for bass collected during all months was 7.09 Bq/g wet mass. All individuals examined, regardless of month, sex, age, or body mass, had 137 Cs levels (2.95–12.60 Bq/g) that were much higher than the maximum level (0.60 Bq/g wet mass) generally considered safe for human consumption. Radiocesium is relatively long-lived within this reservoir and will continue to remain an important issue in risk assessments for both humans and wildlife species. Keywords—Bioaccumulation Largemouth bass Radiocesium Seasonal variation INTRODUCTION Contaminants released into the environment invariably move to and become concentrated within aquatic ecosystems. Thus, the consumption of fish is a potentially important mech- anism by which environmental contaminants may enter food chains leading to humans or wildlife species. Because of the widespread occurrence of radiocesium ( 137 Cs) due to global fallout and accidental releases at nuclear facilities, knowledge regarding the bioaccumulation of this contaminant in fish has applications for biomonitoring, environmental remediaton, and risk assessment studies [1]. The process of 137 Cs bioaccumu- lation in fish has been the subject of numerous investigations during the past three decades [2]. Recently, these types of investigations have received increased attention following the Chernobyl nuclear accident [3–5]. One relevant aspect of 137 Cs bioaccumulation in fish, dem- onstrated by Kolehmainen [6], was that levels of this contam- inant varied seasonally in bluegill (Lepomis marcrochirus) from a lake in the southeastern United States. Seasonal changes in 137 Cs levels have also been observed for several fish species from a lake in southern Sweden [7]. Understanding such sea- sonal changes in 137 Cs levels of fish may be relevant to risk assessments for wildlife that utilize fish on a seasonal basis for activities such as the feeding of offspring. However, despite the numerous investigations regarding 137 Cs bioaccumulation in fish, seasonal differences in levels of this contaminant have seldom been reported or evaluated. Concentrations of 137 Cs in the water column are known to vary seasonally in a system of former nuclear reactor cooling reservoirs located on the Savannah River Site (SRS), near Aiken, South Carolina [8]. These reservoirs represent an ex- cellent opportunity to address questions concerning the sea- * To whom correspondence may be addressed (jdp12@psu.edu). sonal pattern of 137 Cs bioaccumulation in fish. Moreover, in- formation concerning seasonal patterns of 137 Cs in fish from these SRS reservoirs has immediate application to the making of management decisions. For example, fish in these reservoirs are used seasonally by sensitive wildlife species, such as the threatened bald eagle (Haliaeetus leucocephalus), to feed nes- tlings. In addition, since production activities have ceased at the SRS, there is now interest in the possibility of opening the site’s reservoirs to public recreational fishing. Previous investigations in SRS reservoir systems have dem- onstrated seasonal differences in 137 Cs levels in a fish species functioning at an intermediate trophic level (mosquitofish, Gambusia holbrooki; [9]) as well as in a species of migratory waterfowl (American coots, Fulica americana; [10]). Based on these observations, we hypothesized that 137 Cs levels would also vary seasonally in other species within this ecosystem. The purpose of this investigation was to examine seasonal changes in 137 Cs levels of largemouth bass (Micropterus sal- moides) collected over a 12-month period. The relatively high trophic position at which largemouth bass feed, along with its popularity as a sport fish and its importance as a food item for wildlife (e.g., bald eagles), makes this species particularly important to study at the SRS site and elsewhere throughout its range. MATERIALS AND METHODS Study site This investigation was conducted between November 1973 and October 1974 in Pond B on the SRS. This 87-ha reservoir was constructed in 1961 to receive thermal discharges from an operating production reactor [11]. Releases of radionuclides into Pond B, resulting from failure of reactor fuel elements, occurred from September 1961 through June 1964. This reactor