Abstract
Globally, marine environments are subjected to anthropogenic stressors, such as pollutants and climate change. One particular driver of anthropogenic stressors is the sourcing, transport, and use of crude oil. Crude oil itself enters the environment through a range of pathways, including leakage from source sites, effluent and other marine shipping waste, land-based run off, and accidental spills from ships or oil rigs. Crude oil contains toxic components such as polyaromatic hydrocarbons (PAHs) that have a complex chemical structure which allows them to persist in the environment and makes them particularly harmful to marine organisms. It is well understood that crude oil exposure can impair a range of important processes in marine organisms, such as development and growth, immune responses, metabolic capacity, and behaviour. Much of the existing research has focused on medium to high crude oil concentrations, such as those that marine environments might experience in the aftermath of an oil spill. Furthermore, much of this research has investigated the impacts of larger pelagic fish species, often from temperate and polar regions. However, many marine ecosystems are frequently exposed to low-level crude oil contamination, such as that caused by high marine shipping use.
Tropical marine environments, such as coral reef systems, are experiencing increasing marine traffic, thus the incidence of exposure to shipping related waste such as crude oil is also increasing. Despite this, there is a significant gap in the field of crude oil research that focuses on the effects of exposure on coral reef fish physiology and behaviour. As corals rely on a complex symbiosis with other organisms to provide essential ecosystem functions, external stressors that alter these interactions can threaten the stability of the environment. Thus, understanding how acute, low level crude oil exposure impairs coral reef fish physiology and behaviour will provide insight into how predator-prey interactions may be altered as a result. Moreover, stressors rarely occur in isolation and coral reefs are experiencing an increased frequency and intensity of marine heatwaves. These environments and their constituents are already being pushed to their physiological limits which can increase their vulnerability to other stressors. Specifically, crude oil toxicity is exacerbated by increased temperature, as PAH solubility increases which in turn makes it more bioavailable.
The overarching objective of this thesis was to determine the effect of acute, low concentration crude oil exposure on the metabolic physiology and escape behaviour of coral reef fishes. We also aimed to understand how crude oil exposure and temperature may interact to these traits in coral reef fishes.
To investigate this, Chapter 2 aimed to determine the effect of crude oil exposure and temperature on the metabolic performance of juvenile Pomacentrus chrysurus. Fish were exposed to a low concentration of crude oil prepared as a WAF (water accommodated fraction) for 24 h before being exposed to a period of elevated temperature to simulate a short heatwave (5 days at 31.5˚C, +4˚C from ambient). Fish underwent an exhaustive chase protocol prior to their oxygen uptake being measured as a proxy for metabolic rate using intermittent-flow respirometry. We captured the oxygen consumption of fish immediately after exhaustive exercise (MO2MAX) and continued to measure their oxygen uptake for 22 h to see how recovery was affected by crude oil and temperature. After respirometry, fish were dissected to determine hepatosomatic index (HSI) to determine where there was any effect of treatment on body condition associated with energy stores. We found that crude oil alone reduced the time to recovery, while temperature caused an increase in metabolic rates, including MO2MAX, MO2MIN, and aerobic scope. Fish exposed to crude oil and temperature had a significantly lower HSI than control fish, further suggesting that one or both stressors was causing a reallocation of energy sources in response crude oil exposure or warming. Despite the lowered HSI, P. chrysurus appeared to have some metabolic tolerance as a result of crude oil exposure and elevated temperature.
To understand how crude oil exposure might impair the ability of coral reef fishes to detect predators, Chapter 3 examined how Dascyllus trimaculatus responded to olfactory and visual predator cues, using changes in oxygen consumption as a response proxy. Fish were treated with crude oil as described in Chapter 2 and then underwent intermittent-flow respirometry to measure their oxygen consumption prior to any cues being added, and then under the presence of olfactory predator cues, followed by visual predator cues. By calculating the change in their metabolic rate from on period to the other, we were able to determine how fish were responding to the threat of a predator but examining their oxygen consumption. We found that crude oil had no effect on the response of D. trimaculatus compared to control fish experiencing olfactory or visual predator cues. However, fish exposed to crude oil that were not exposed to any fish cues had significantly higher oxygen consumption than control fish. These findings suggest that fish may be expending additional energy while resting when they do not perceive a predator threat but are still capable of altering their metabolic rate to in response to predators when they do perceive a threat.
Previous research has shown that crude oil exposure can cause cardiotoxicity and increase metabolic costs that could affect the execution of essential tasks with high energy requirements such as escape responses. Chapter 4 aimed to determine the effect of crude oil exposure and temperature on the metabolic activity and escape behaviour of Pomacentrus amboinensis recruits. Fish were exposed to a low concentration of crude oil prepared as a WAF (same concentration as detailed above) for 24 h before being exposed to a period of elevated temperature to simulate a short heatwave (5 days at 32.5˚C, +4˚C from ambient). After exposure, fish were stimulated to execute a C-start escape response which was analysed kinematically for locomotory and non-locomotory variables to assess how crude oil and temperature may affect the success of the escape. Fish exposed to crude oil exhibited longer response latencies which would jeopardise escape success while temperature caused reduced escape latencies, likely through elevated muscle activation. White muscle tissue was analysed for lactate dehydrogenase (LDH) and citrate synthase (CS) enzyme activity to assess whether crude oil and temperature exposure were altering anaerobic or aerobic metabolism. LDH activity was found to be significantly higher in fish as a result of a synergistic effect of crude oil and temperature, suggesting these were significantly affecting anaerobic metabolisms. CS was significantly reduced by crude oil alone, suggesting a cost to aerobic metabolism in fish recovering from crude oil exposure. The findings from this chapter provide evidence that crude oil and warming can interact to significantly alter anaerobic metabolisms of coral reef fishes, which has potential consequences for their escape performance.
Often, when considering how a stressor may impact the anti-predator behaviour of fishes, the predator itself is treated as a constant variable and one that has not also been influenced by the stressor. Due to the typically ubiquitous and non-selective nature of crude oil exposure, there is a chance that if either predator or prey has been exposed to crude oil, it is likely that the other has also been exposed. Therefore, Chapter 5 explored the interaction between oil-exposed Pseudochromis fuscus predators and Pomacentrus amboinensis prey, using high-speed videography to kinematically assess the components of the predator attack and prey escape. Predator and prey were exposed to crude oil in the same manner as described above and were then paired in four treatment combinations – control predator - control prey, control predator - oil prey, oil predator - control prey, oil predator - oil prey – and their interaction within an arena was filmed for 10 min (or until the prey was consumed). This fully crossed design was chosen to evaluate exactly how crude oil exposure influenced predator-prey dynamics. We found that oil exposure in prey significantly reduced reaction distance and ALT in prey fish, appearing to give them an advantage over predators. Oil exposed in both the predator and prey individually influenced capture success and strike rate, where either control or oil-exposed predators were less successful and reduced their strike rate with oil-exposed or control prey respectively. These findings suggest that predators are disadvantaged under crude oil exposure, whether they themselves are exposed, or just their prey. This tips the scales in favour of prey success under crude oil pollution, which could have severe repercussions for individual predator success and alter the role each play in coral reefs.
Understanding how stressors affect coral reef fishes, both in isolation and simultaneously, provides researchers with insight as to the magnitude with which coral reef systems may be affected under the increasing threat of anthropogenesis. The findings of this thesis demonstrated that both crude oil, elevated temperature, and both interactively can impact the metabolic physiology and escape behaviour of coral reef fishes. Specifically, this research has shown that responses to multiple stressors are often more complex than the responses to each stressor in isolation and to assume so could result in misinformed predictions of the resilience or tolerance of certain species or systems. Crucially, the findings of this thesis are the first to demonstrate that even acute, low concentration exposure to crude oil can alter coral reef fish physiology and behaviour in the context of predator-prey dynamics, provided insight as to how predator-prey dynamics of coral reef fishes may be altered under the increasing threat of anthropogenic stressors.