In north-eastern New Zealand, adults of the blennioid fish Forsterygion varium Bloch et Schneider exhibited extreme differences in density among habitats of low, intermediate and high physical complexity. The age composition also differed among habitats, with complex sites supporting a greater proportion of older fish (> 1 + yr). The relative importance of recruitment and postrecruitment processes in determining these patterns was evaluated by monitoring the abundance, survival, growth and movement of two separate cohorts of juveniles in habitats of different complexity. The demographic data were based on records of individually recognised fish monitored from time of recruitment in permanent quadrats. Despite high temporal and spatial variability in the intensity and distribution of recruitment between years, adult F. varium were highly dependent on early postrecruitment mortality. Greater rates of early juvenile mortality were experienced within habitats of low complexity. The greatest impact of mortality in structuring spatial patterns occurred within the first week after settlement. Seasonal peaks in movement may have partially contributed to changes in patterns of distribution between habitats. The effects of habitat on growth were less clear, although there was a trend for growth to be better within habitats of high topographic complexity. The effect of topographic complexity on survivorship of juveniles was examined using a manipulative experiment which increased the complexity of simple habitats and reduced the complexity of heterogeneous habitats. Recruitment occurred roughly evenly across all treatments. Low complexity treatments suffered 100% mortality compared to only 13% in high complexity treatments. The demography of natural populations and the subsequent experiment suggest that differences in the abundance of adult F. varium between habitats can be attributed essentially to the early failure of juveniles to survive in large numbers in low complexity habitats.
