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

摘要

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 25 degrees C to thrive. However, striped bass are found in reservoirs where hypolimnetic hypoxia forces them into warm temperatures (27-30 degrees C) 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 20-23 degrees C 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 1-2 degrees C 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.