Habitat degradation has species-specific effects on the stress response of coral reef fishes

Abstract

Coral reefs are one of the world's most threatened ecosystems, with transition from coral- to algal-dominated habitats becoming increasingly common. While habitat degradation in other ecosystems can lead to heightened stress among individuals, thereby increasing energetic demands and reducing fitness, the effects of habitat degradation on stress and metabolic performance of coral reef fish is unknown. Therefore, we investigated whether cortisol levels (an indicator of stress), metabolic performance, behaviour, and growth, were influenced by habitat quality in three fish species with varying degrees of reliance on live coral. We reared newly-settled fishes (Pomacentrus amboinensis, P. moluccensis and Dascyllus aruanus) for three weeks in one of three habitat treatments: live coral, dead coral covered in turf algae and cyanobacteria, or a combination of the two. After three weeks we quantified each individuals metabolic performance, cortisol concentration, morphology, and habitat use. Pomacentrus moluccensis had higher cortisol concentrations in the dead coral treatment, compared to the live coral, whereas there was no difference in cortisol levels among habitat treatments for P. amboinensis or D. aruanus. Overall, metabolic performance (aerobic scope, standard and maximum metabolic rate) showed no consistent response to habitat treatments. Regarding morphology, we only found D. aruanus to be impacted, with individuals from the live coral treatment having a significantly higher mass than individuals from the dead coral treatment. Behavioural space-use analysis revealed that D. aruanus and P. moluccensis spent more time away from their habitats in the dead coral treatments, whereas the space-use of P. amboinensis was unaffected by habitat degradation. Understanding how coral loss affects stress levels and fitness of reef fishes is critical to predict future reef fish communities. This study highlights that vulnerability of reef fish to coral reef degradation is species-specific, and related to their reliance on live coral. These changes may provide a mechanistic explanation to observed population declines following coral loss and indicate that remaining individuals have reduced fitness potentially compromising the subsistence of future populations.