Abstract
Coral reefs such as the Great Barrier Reef are biodiversity hotspots which support a high biomass of coral reef fish. However, coral reefs are presently heavily degraded by anthropogenic stressors, including microplastics and oil pollution. Co-occurring stressors can synergistically or antagonistically interact to exert combined impacts on coral reef fish. Microplastics and oil have recently been found to form aggregates, via adsorption of oil onto microplastics. Microplastics may therefore act as a vector for heightened oil uptake into planktivorous fish, which are known to ingest microplastics. However, there has been minimal assessment of the combined impacts of microplastics and oil to biota, with no studies to date assessing such impacts on coral reef fish. Additionally, although microplastics and oil pollution are highly spatiotemporally variable, it is unknown how fish respond to these pollutants through time post-exposure. This research aimed to investigate the impacts of co-occurring microplastics and oil pollution on coral reef fish, examining these impacts through time. To this end, juvenile damselfish were experimentally exposed to polystyrene microplastics and crude oil WAFs individually, as well as to two ways that these pollutants may co-occur. Exposure to clean and oil-weathered microplastics reduced the in-situ antipredator-behaviour and survival over time of Pomacentrus amboinensis recruits, with the greatest reductions observed in recruits exposed to oil-weathered microplastics. To further investigate the interactive effects of microplastics and oil, an ex-situ multiple stressor study was conducted on Acanthochromis polyacanthus juveniles. Individually, neither microplastics nor oil impacted routine swimming or oxidative stress in treated fish, whereas combined exposure induced oxidative stress and recovery responses through time. Using recently proposed two-interval interaction methodology, microplastics and oil were found to synergistically induce oxidative stress in fish, and the strength of this interaction was found to change through time post-exposure. Taken together, combined exposure to microplastics and oil heightened hyper vigilance and oxidative stress in damselfish, as well as reducing their survival in-situ. Damselfish may therefore incur reduced fitness and survival when exposed to co-occurring microplastic and oil pollution, which may impact the commercially important species that damselfish act as prey for, as well as the wider coral reef ecosystem. This research suggests that the impacts of microplastic and oil pollution on coral reefs may be underestimated without considering their interactive effects, and the spatiotemporal scales of their impacts. Coral reef management can benefit from the findings of this research, as targeting the mitigation of either microplastic or oil pollution may protect fish against the synergistic impacts of both. Conservation management is moving towards understanding and addressing cumulative effects of anthropogenic stressors in the marine environment. This research demonstrates methodologies and provides information which can aid in the implementation of effective coral reef conservation efforts.