The effect of motorboat noise on early life stages of coral reef fishes

Underwater anthropogenic noise is now recognized as an environmental pollutant. Vessel noise is the most common source of underwater anthropogenic noise. With vessel noise increasingly being propagating through the world's waters, it has become of urgent concern to understand the effects of boat noise on aquatic organisms and reduce negative impacts. I focused on fish, which have received less research attention than mammals in terms of anthropogenic noise impacts. In particular, I examined early life stages, which are important to fish population persistence, but are understudied with respect to the impacts of anthropogenic noise. In order to gain insight into the animal's perception of anthropogenic noise and their associated mechanistic responses, I examine indicators of physiological stress during exposure to boat noise in my study species. The aims of my studies were to increase empirical evidence of the effects of boat noise on early life stages of coral reef fishes so that noise mitigation programs can be developed and implemented more effectively. Chapter 2 investigated whether fish embryos were affected by boat noise and whether the effects differed with engine type (e.g., two vs. four stroke engines). Embryos of the damselfish Amblyglyphidodon curacao were used in a novel recording setup to examine changes in heart rate, which can indicate stress in response to real boat noise. Findings showed an increase in embryo heart rate when boats were passing. The magnitude of increase was greater upon exposure to boats with two-stroke engines compared to the response to four-stroke outboard engines. These are the first field data demonstrating that fish embryos can be affected by boat noise. I also analyzed the differences between two-stroke and four-stroke sound spectra and concluded that a new metric – total sound energy produced above the sound detection threshold and within the sound detection range of the animal (relSEL) – may be useful in indicating differences between biological effects of different noises. Chapter 3 examined the energetic cost of boat noise induced stress in settlement stage coral reef fishes, using respirometry to measure oxygen uptake rates and approximate metabolic costs associated with the secondary stress response initiated by boat noise. The energetic cost of boat noise induced stress is relevant because a high energetic cost would be associated with depleted energy available for other fitness promoting processes and activities. I examined oxygen uptake rates in settlement-stage Pomacentrus amboinensis, and employed a new design for intermittent flow respirometry that does not require pumps and therefore allows quiet acoustic conditions during respirometry trials. Findings showed no effect of boat noise playback on oxygen uptake rates. These differing results in stress response indicators between the two species and life stages highlights the complexity of responses of fish to anthropogenic noise. Both of these studies provide a significant advancement in technology with which to monitor anthropogenic impacts during early life stages, which I hope will be used in future studies on this topic. Overall, these findings contribute to our understanding of how boat noise can affect fish and increases our understanding of how noise pollution can be managed and/or mitigated