Striped Bass Habitat Selection Rules in Reservoirs without Suitable Summer Habitat Offer Insight into Consequences for Growth

—The traditional view of habitat requirements for inland striped bass Morone saxatilis suggests that these fish need dissolved oxygen (DO) levels above 2–3 mg/L and temperatures below 258C to thrive. However, striped bass are found in reservoirs where hypolimnetic hypoxia forces them into warm temperatures (27–308C) for much of the summer, and contrary to expectations, these populations do not consistently experience poor growth or mortality. We used telemetry of adult striped bass in Badin Lake, North Carolina, to characterize habitat selection by striped bass in systems with unsuitable summer habitat. As summer stratification developed, striped bass selected preferred temperatures of 20–238C as long as the DO was at least 2 mg/L. Once hypoxia forced striped bass into warmer water, the fish concentrated at the top of the oxycline (defined as the depth just above the largest decline in DO occurring over a 1-m change in depth), which was 1–28C warmer but had greater DO levels (4–8 mg/L) than the coolest water, with DO of 2 mg/L. Striped bass remained at the top of the oxycline into the fall, even after deeper water with preferred temperatures and a DO level of 2 mg/L became available. We suggest that these patterns, supported by observations in the literature, represent summer habitat selection rules for striped bass in reservoirs where all oxygenated habitat exceeds temperatures traditionally considered suitable for striped bass. We also show that the depth distribution of Badin Lake striped bass in response to physical habitat constraints causes them to overlap spatially with warmwater prey inhabiting shallow, warmwater depths both in the summer and early fall. Badin Lake striped bass continue to feed and grow over the summer, providing evidence that the availability of adequate prey resources can offset the costs of poor summer habitat. Warm, productive reservoirs without permanent thermal refuges may therefore provide better habitat for maintaining quality growth and condition than those systems where occupation of cooler temperatures segregates striped bass from their prey. The thermal preferences of striped bass Morone saxatilis in reservoirs and the consequences of those preferences for growth and mortality have been topics of great interest to fisheries scientists and managers for nearly three decades. As interest in recreational fisheries for striped bass in inland systems grew (Axon and Whitehurst 1985), so did questions regarding what constituted acceptable habitat for adult striped bass, particularly in southern reservoirs where the cool, oxygenated habitat preferred by this species was often not available through the summer months. These questions were lent additional urgency as patterns of summer mortality and poor growth of adult striped bass were observed in some systems (Matthews 1985). Coutant (1985) hypothesized that these problems were the result of physiological constraints dictating that inland striped bass must select dissolved oxygen (DO) levels above 2–3 mg/L and temperatures below 258C. During summer stratification, fish may become squeezed between epilimnetic water with temperatures that are too warm and hypolimnetic water with DO levels that are too low to provide suitable habitat for adult fish. In severe cases, suitable habitat for adult striped bass may be entirely lacking during some portion of the summer. Since the development of Coutant’s (1985) temperature–oxygen squeeze hypothesis, numerous striped bass telemetry studies have been conducted in reservoirs that fall into one of two categories: (1) systems with some suitable habitat (temperatures below 258C and DO above 3 mg/L) available throughout the summer (Cheek et al. 1985; Coutant 1985; Van Den Avyle and Evans 1990; Wilkerson and Fisher 1997; Schaffler et al. 2002; Young and Isely 2002) and (2) systems without any suitable habitat for at least a portion of the summer (Matthews et al. 1985; Farquhar and Gutreuter 1989; Zale et al. 1990; Van Horn et al. 1998; Jackson and Hightower 2001). * Corresponding author: [email protected] 1 Present address: Department of Biology, Chemistry, and Environmental Science, Christopher Newport University, 1 University Place, Newport News, Virginia 23606-2998, USA. 2 Present address: Kansas Department of Wildlife and Parks, 2131 180 Road, Glen Elder, Kansas 67446, USA. Received July 31, 2009; accepted May 13, 2010 Published online September 13, 201