Suspended sediment and its effects on the behavior and physiology of coral reef fishes

Urbanization and increased agricultural activities are causing long-term changes to many of the key physical characteristics in coastal aquatic environments through increased sediment and nutrient loading, elevated turbidity and reduced light penetration. Inshore coral reef habitats and their associated species are vulnerable to sediment loading, which has been responsible for habitat degradation and a reduction in biodiversity of corals. Many coral reef fish species depend on live coral for habitat at some stage of their life history and declines in the abundance and diversity of coral reef fishes have been linked to an indirect effect of habitat loss. Direct effects of suspended sediment on coral reef fishes may compound the indirect effects of habitat loss, leading to further changes in population dynamics. Importantly, early life history stages of coral reef fishes may be particularly vulnerable as the small scale ecological processes during early growth and development happen on short time scales. A quantitative evaluation of the direct interaction between sediment and coral reef fishes is crucial to increase our understanding of how changing water quality directly affects coral reef fishes. The overall aim of this project was to provide a comprehensive assessment of the direct effects of suspended sediment on early life history stages of coral reef fishes. This research was conducted experimentally in aquaria, using a suite of planktivorous coral reef damselfish species and one predator species, all commonly found associated with live coral, including in areas that are exposed to riverine flood plumes with sediment. All suspended sediment concentrations used have been consistently recorded on nearshore reefs.\ud \ud The first critical stage of development post-hatching is larval development, during which time foraging efficiency and rapid growth are key factors in survivorship. Correlative studies suggest that suspended sediment may affect coral reef fishes, however, whether it could alter recruitment patterns in areas prone to high suspended by impacting larval development is unknown. Chapter two investigated the effects of suspended sediment on larval development of Amphiprion percula. The fish in the no-sediment control experienced a median pelagic larval duration of 11 days, compared to 12 days for the sediment treatments. Fish in suspended sediment showed increased variation in time to metamorphosis, with 76% of the fish in the control settling by day 11, compared to only 40-46% in the sediment treatments, with some individuals extending their larval phase by 100%. Length and weight of fish in the low sediment treatment were significantly larger compared to the control, medium and high sediment treatments at settlement, suggesting an effect of sediment on foraging ability. The altered developmental patterns caused by suspended sediment on coral reef fish larvae may be one of the key mechanisms behind distribution patterns seen across tropical continental shelves, from inner to outer reefs.\ud \ud Another potential driver of fish distributions from inner to outer reefs could be an impairment in the ability to successfully choose optimal habitat, which would reduce successful recruitment and survivorship. Chapter three tested the hypothesis that sediment-enriched water impairs the ability of Pomacentrus amboinensis and Pomacentrus moluccensis to find suitable settlement sites. In a clear water environment (no suspended sediment), pre-settlement individuals exhibited a strong preference for live coral over partially dead and dead coral, choosing live coral 70 and 80% of the time, respectively. However, when exposed to suspended sediment, no habitat choice was observed, with each habitat type being selected in equal frequency. An examination of chemosensory discrimination in sediment-enriched water showed that sediment also disrupts the ability of P. moluccensis to respond to chemical cues from different substrata. A reduction in settlement success in conjunction with altered developmental patterns could reduce cohort strength during turbid conditions.\ud \ud Chapter four expanded on chapter three by investigating the sediment thresholds for the behavioral changes that were observed in P. moluccensis. This chapter also examined postsettlement migration and home range movement in different concentrations of suspended sediment. The results elucidated that a clear threshold for habitat selection and home range movement existed, beyond which the fish were settling to, and remaining on sub-optimal habitat. A review of the literature highlighted that P. moluccensis could experience sub-optimal conditions between 8 and 53% of the time on inshore areas of the Great Barrier Reef. This chapter emphasizes that the sediment threshold for behavioral changes is already reached relatively frequently, and that the restricted movement due to a loss of visual cues will compound the increasing habitat loss occurring on coral reefs. Additionally, the results of the literature review underscore the importance of studying coral reefs vulnerable to sediment loading, as little information currently exists about these reefs.\ud \ud Chapter five examined the influence of suspended sediment on the fitness-associated traits of individuals persisting in turbid environments. Here, I tested the hypothesis that suspended sediment can negatively affect foraging efficiency, nutritional state and survival in Acanthochromis polyacanthus, a damselfish commonly occurring on inshore, turbid reefs. Fish took longer to find food and consumed less of the food provided with increasing sediment. The decline in food acquisition was associated with a significant reduction in juvenile growth and body condition. Fish held in the medium and high sediment treatments had half the density of hepatoctye vacuoles (a proxy for condition) than the fish held in the control and low sediment treatments. Suspended sediment also caused a significant increase in mortality of the juvenile fish in the high sediment treatment. Mortality reached almost 50% in the high sediment treatment, in contrast to the low and medium sediment treatments, which had less than 10%, and the control, which had no mortality. A reduction in growth and condition will have major implications for survivorship and reproductive success, which could change population demographics on reefs increasingly subjected to suspended sediment.\ud \ud Although suspended sediment reduces visual cues necessary for vital processes in the development of planktivorous fishes, it may protect them from predation as predators often require a larger visual field to spot their prey. Chapter six examined whether suspended sediment could change predation patterns between Chromis atripectoralis and a ubiquitous predator, Pseudochromis fuscus, whose diet largely consists of damselfish. Increased turbidity led to a nonlinear response in predation patterns. The prey had about 50% survivorship in the control and low sediment levels. However, in the medium sediment level, survivorship declined to less than 30%, contradicting the hypothesis that the prey would be protected in higher turbidity. Interestingly, in the high sediment level, survivorship increased to resemble predation rates in the control. This non-linear predation pattern exposes different tolerance levels to suspended sediment, whereby the prey was more sensitive to suspended sediment initially, allowing the predator to capitalize on their reduced visual acuity. This study demonstrates that coral reef fishes from different families and functional groups are susceptible to increased turbidity. Changing predation patterns due to suspended sediment could lead to an imbalance in trophodynamics on coral reefs.\ud \ud This research provides important evidence that coral reef fishes are directly affected by suspended sediment and that clear trigger values exist for behavioral changes. This work underscores the need to reduce suspended sediment inputs into coral reef environments due to its potential effect on larval development, settlement success and habitat use, foraging, and survival of coral reef fishes. Changes in these fundamental processes that regulate fish assemblages may have long-term effects on the persistence of populations, particularly as habitat loss on coral reefs increases