Reproductive sources and sinks within a sea urchin, Evechinus chloroticus, population of a New Zealand fjord

Free to read on publisher website We examined population structure of New Zealand sea urchins Evechinus chloroticus across the Doubtful-Thompson Sound complex, a fjord with a large influx of freshwater, a low salinity surface layer and a mean estuarine circulation. Measurements of abundance, growth, gonad development and larval settlement were collected at 9 sites. We observed large differences in each of these vital population parameters among sites. The highest growth rates and gamete production occurred at the entrances of the fjord. We used 3 alternative growth models to test whether the spatial pattern in growth rates was robust. Variability in growth among sites likely reflected differences in nutritional history, as indicated by the Aristotle's lantern index. Size distributions taken 2 yr apart at each of the 9 sites showed that the adult mode was relatively stable in time. As a consequence much of the variance in mean size among sites could be explained by differences in growth rather than in mortality or recruitment. The highest abundance and aggregation of sea urchins occurred at mid-fjord and entrance sites, which coincided with the highest rates of larval settlement and suggested limited larval supply elsewhere. The observed spatial patterns in vital rates were used to evaluate the likely influences of spatial heterogeneity in larval production on larval dispersal patterns within the fjord, based on particle movements within a validated hydrodynamic model. We then tested the sensitivity of these model results to magnitude of wind and rainfall events and duration of larval development. Results from our field observations and modeling studies suggest that the sea urchin population in Doubtful-Thompson Sound is likely composed of distinct areas of reproductive source and sink