The population history and demography of the long-spined sea urchin (Centrostephanus rodgersii) in Aotearoa New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Biological Sciences at Massey University, Albany, New Zealand
Climate-driven changes in range and abundance can go undetected, particularly in regions like north-east New Zealand (NENZ) where there have not been routine surveys for many species. The Long-spined sea urchin (Centrostephanus rodgersii) has extended its range and increased its abundance in Tasmania over the past 40 years, a change that has dramatically impacted the ecosystem and local fisheries. Centrostephanus rodgersii is also found in NENZ but we lack systematic survey information to understand whether a similar range extension could have occurred here. Given the similarity of the Tasmanian and NENZ ecosystems, C. rodgersii poses a potential threat to New Zealand’s (NZ) marine biodiversity and fisheries. The sizes of individuals within populations were analysed and population genomics was used to study the population history, population structure and recruitment dynamics of C. rodgersii across its NZ range. Although population size structure revealed no overall signatures of a poleward range extension, the northern part of the NENZ C. rodgersii range did have patterns indicative of a poleward range extension. Furthermore, the size structure of populations further south and east suggested that these populations had more regular recruitment than northern populations. Population genomic analysis revealed that Rangitāhua (the Kermadec Islands) populations and populations of NENZ are genetically differentiated, but there is some ongoing migration from Rangitāhua to NENZ. Within NENZ there was no evidence of population genetic structure, however, population graphs revealed that some groups of populations were more similar in genetic composition, and presumably shared higher geneflow. Two demographic groups (younger than 15 years and older than 15 years) were created to examine differences in genetic composition among age classes found within the same populations. This comparison recovered different patterns of connectivity and within-population variance between the two demographic groups that was not present when the groups were combined. My results indicate that the population demography and structure of C. rodgersii in NZ is changing and therefore we need to routinely monitor this urchin across its NZ range to prevent damage to our ecosystems and fisheries.