Impact of Biological Variability on the Dispersal of Coral Reef Fish Larvae: A Study of Early Life Behaviors and Group Dispersal

Abstract

Demersal coral reef fish all share a complex life cycle, with a demersal phase as juvenile and adults, and a pelagic larval phase. This larval phase is thought to disperse offspring, connecting distant fish populations and influencing meta-population dynamics. Accurately estimating the connectivity between fish populations is important for the conservation, protection, and managements of coral reef fish; however, this estimation remains a difficult and challenging task. During dispersal, fish larvae are transported by the currents but actively modify their trajectories to improve their odds for survival/settlement. Studies conducted on pre-settlement stage larvae have shown that they use their swimming and sensory abilities to detect environmental cues and orient toward their settlement grounds. Although the behaviors of coral reef fish larvae have been the topic of many studies, there are still a lot of open questions concerning their early life stage and the development of their behavioral abilities. This dissertation attempts to fill this gap by documenting the beginning of the life of reef fish larvae and quantifying the impact of biophysical interactions after hatching and during development. This dissertation is divided in four parts: a description of coral reef fish hatching strategies, a quantification of the potential for cohesive dispersal, a study of the impact of biological uncertainties on estimates of connectivity, and a study of the impact of orientation behaviors on estimates of connectivity. In the first part of this work, following in situ observations, we describe the hatching strategies of demersal coral reef fish. These observations lead us to investigate whether larvae disperse in cohesive groups. With numerical models developed to quantify biophysical interactions, we demonstrate that with certain swimming strategies, fish larvae could counteract micro-scale turbulence and maintain group cohesion. Then, we implemented larval behaviors in a biophysical model to account for larval active swimming during dispersal. The simulated larval behaviors rely on empirical parameters to model biological traits and behaviors --such as mortality, swimming abilities and orientation-- and modulate larval settlement and dispersal forecasts. These parameters are, however, uncertain because they are poorly constrained by observations and vary naturally between individuals. We construct and use a polynomial chaos surrogate to quantify the impacts of these uncertain biological parameters on estimates of larval connectivity. We then highlight the uncertainties associated with the mechanisms used by fish larvae to orient and quantify their impact on the dispersal of a model fish species. Finally, we discuss the early life behaviors and their impact on connectivity estimates, and we propose different methods that can be used to quantify and reduce uncertainties.</p