Effects of nursery-environment condition on habitat use, growth, survival and endocrine physiology during larval settlement in the red drum (Sciaenops ocellatus)

Abstract

Settlement in red drum (Sciaenops ocellatus) involves vulnerable early-life stages that rely upon fast growth and reduced mortality within seagrass nursery areas. Environmental parameters fluctuated widely in seagrass as a result of daily cycles or weather systems. Major diel variability was found in temperature (amplitude: 3-7 ºC) and dissolved oxygen (DO) (range: 2.9-17.5 mg O2 l-1; hypoxia-hyperoxia). Hypoxia events were regularly observed, especially at the core of seagrass beds where they lasted for longer periods compared to deep-edge areas. Cold fronts were associated with important and fast changes in temperature and salinity. This study describes patterns of use of the nursery during settlement, as well as growth and mortality estimates during subsequent recruitment. The ratio between instantaneous growth in weight and mortality coefficients (G’:Z) was used as a criterion to address the value of seagrass areas for young fish. In the laboratory environmentally realistic temperature and DO fluctuations did not affect growth or survival of larvae. I detected an early activation of thyroid and interrenal glands during the yolk-sac phase and a second activation of the thyroid gland during transformation into juveniles which was coincident with settlement. Settlement-size larvae exposed to handling expressed stress-related cortisol changes. However, no such increase was induced by dawn-hypoxia or normal diel temperature cycles. This study detected a pulsed supply of settlers from early September to late October coupled with high mortality rates, suggesting that population size largely depends on supply and larval mortality immediately after settlement. Red drum larvae settle at the edge and core seagrass, but accumulate at the core seagrass. Mortality was substantial and variable, determining the value of G’:Z. No seasonal trends in G’:Z were observed and probably most cohorts contributed to recruitment. Cages stocked at edge and core areas with hatchery-reared larvae failed to demonstrate habitat differences. Caged and wild fish grew at similar rates suggesting that cultured red drum can be used to estimate growth rates of wild counterparts during settlement. I suggest that edge seagrass areas are very important in determining successful settlement in this species since they provide the first and crucial contact with the nursery habitat.