Early life-history drivers of connectivity in a temperate marine fish metapopulation

© 2016 Dr. Emily Kate FobertMetapopulations are connected by the movement of individuals between local populations. For marine organisms that are benthic or demersal as adults, this movement occurs entirely in the earliest life stage, as pelagic eggs or hatched larvae, dispersing through the plankton for days to months before settling to a new population. This dispersal phase, and the carry-over effects from dispersal into subsequent life-stages, has fundamental implications for population dynamics and metapopulation persistence, yet for most marine organisms very little is known about what drives dispersal, settlement, and recruitment within individual species. This thesis investigates early life-history drivers of population connectivity in a temperate marine fish metapopulation, by picking apart, and then putting back together, the complex and interacting processes acting on each stage of the early life-history – dispersal, settlement, and survival – of the southern hulafish (Trachinops caudimaculatus). In the dispersal phase, it is well recognized that the environment encountered by dispersing larvae can contribute to differences in the quality of dispersers (disperser phenotype), however, very little is known about how the disperser phenotype directly influences the dispersal process. Findings from CHAPTER TWO show how phenotypic variability influences dispersal outcomes through the influence of disperser phenotype on relative vertical distribution throughout the water column. Using otolith elemental signatures from the dispersal period of recruit T. caudimaculatus collected from 13 populations around Port Phillip Bay (the study system), CHAPTER THREE provides evidence that dispersal pathways are not independent, and explores the implications of shared dispersal histories for patterns of recruitment and population connectivity. At the end of the dispersal period, larvae must locate suitable habitat for settlement, and making the right choice at this transitional stage can be critical for successful recruitment into the population. Through a series of manipulative laboratory and field experiments designed to untangle key extrinsic (habitat-associated) drivers of settlement and survival, findings from CHAPTER FOUR demonstrate a link between sensory information, habitat selection at settlement, and survival in the post-settlement environment, CHAPTER FIVE investigates the implications of these habitat selection decisions at settlement on post-settlement performance, and illustrates how habitat quality and social context can influence shoaling behaviour. The combined results from these studies provide evidence that larval phenotype is a critical factor influencing the likelihood of individual success at every stage of its early life, through direct effects on fitness, and carry-over effects that determine fitness and survival at each subsequent stage. At the metapopulation level, phenotypic variation can indirectly but significantly alter patterns of recruitment and connectivity of local populations. Since the stages across early life history of marine organisms are ecologically linked, this research thesis spanning the early life stages of one temperate reef fish species makes considerable strides towards our understanding of both individual performance in this vulnerable early stage, and how this variation can scale up to influence the dynamics of marine metapopulations