Early life history of a landlocked amphidromous fish: migration, critical traits and ontogeny

Life-history plays a key role in determining species distributions and population structuring. Many studies evaluate the role of adult characteristics when predicting species distribution and population structuring, but very few evaluate the role of early life-history which is often the most vulnerable, and therefore limiting, phase of life-history. This thesis examines the role of early life-history in distribution and population structuring leading to divergence, and potentially speciation, by evaluating short and long term population structuring mechanisms, and the role of early life-history alteration in a facultatively amphidromous fish. Landlocked populations of Galaxias brevipinnis were found to be genetically divergent from diadromous populations, despite no obvious physical barriers to population connectivity. On shorter timescales, catchment level meta-populations were formed within lakes and along coasts. Larvae were found to be highly rheotactic through lab trials, and the majority of larvae were found to be retained within river plumes through larval trawling and otolith microchemistry, suggesting a simple behavioural mechanism responsible for both the long term isolation leading to divergence and short term meta-population structuring patterns seen here. Egg and larval size was larger in diadromous populations than in landlocked populations suggesting a potential pathway for the formation of landlocked populations and adaptation to local conditions. Despite size differences, responses to behavioural cues were similar in both diadromous and landlocked populations likely due to retaining a bipartite fluvial-pelagic life-history pattern. A review of amphidromous fish shows landlocking and life-history plasticity is common both within species and families, suggesting patterns seen here may be applicable across many amphidromous fish taxa. Further, the consistent use of pelagic rearing environments highlights amphidromy as an obligate benthic-pelagic migration where the various life-history patterns are a response to landscape centered on rapidly moving larvae to a pelagic environment. This thesis shows landlocked populations of an amphidromous fish diverging from their diadromous counterparts, due to a simple behavioural response, resulting in adaptation of early life-history in a novel environment, allowing fecundity optimization and providing the potential for context dependent isolation, adaptation, divergence and speciation