MEPS 321:41-53 (2006)  -  doi:10.3354/meps321041

ABSTRACT: In their early larval stages, red drum migrate through coastal inlets and settle into shallow seagrass meadows within estuaries. This study describes environmental rhythms in red drum nursery habitats and evaluates their role in larval growth and survival to determine nursery habitat quality. Well-defined diel cycles were observed in temperature (amplitude 3 to 7°C) and dissolved oxygen (DO) (range 2.9 to 17.5 mgO₂ l^–1), while sporadic cold fronts lowered temperatures by 6 to 10°C in 24 to 72 h. Groups of settlement and post-settlement larvae (3.9 to 17.3 mm standard length) were exposed in the laboratory to cycles of temperature and DO, and to combined temperature and DO diel cycles and then compared to fish grown in constant conditions (control). Relative to controls, growth was significantly reduced only in DO cycles with prolonged daily exposure to hypoxia (>8.2 h d^–1). Survival was similar in all treatments. In response to water cooling during simulated cold fronts, fish previously exposed to temperature cycles grew faster and had a higher food intake than control fish. Fish exposed to DO cycles maintained a greater food intake than control fish but grew at a similar rate. These results indicate that (1) diel cycles impart a physiological advantage to red drum larvae, (2) field measurements of environmental characteristics at a frequency of once a day may be inadequate for assessing nursery values for red drum, and (3) increased exposure to transient hypoxia in oscillating environmental conditions can affect growth and reduce nursery value and recruitment strength.

KEY WORDS: Seagrass · Diel rhythms · Nursery quality · Hypoxia · Fish larvae · Growth · Red drum · Sciaenops ocellatus