Ontogenetic shifts in microhabitat preference of the temperate reef fish Forsterygion lapillum: implications for population limitation

Quantitative surveys of habitat associations of New Zealand's common triplefin fish Forsterygion lapillum suggest that young settlers disproportionately occur within the fronds of the large brown algae Cystophora retroflexa and C. torulosa (hereafter collectively referred to as Cystophora). This apparent preference is specific to Cystophora (i.e. fish do not appear to respond to structurally similar macroalgal species) and is not exhibited by older juveniles or adults of the species, which suggests that age-specific microhabitat requirements may act as a bottleneck to recruitment. To determine whether the overuse of Cystophora by newly settled E lapillum could be attributed to a behavioural preference for this specific habitat, we conducted a lab-based choice experiment. Using a purpose-built 'choice chamber' that facilitated assessments of behavioural responses of focal individuals provided with a choice of 4 common habitat types, we determined that newly settled F lapillum selected chambers containing Cystophora more frequently than predicted by random assortment. To test the ecological consequences of this behavioural preference, we conducted a field experiment (replicated at 2 sites) that manipulated Cystophora cover and examined subsequent patterns of recruitment by F. lapillum. Experimental additions of Cystophora cover enhanced local recruitment of F lapillum, and the effects of Cystophora additions were most pronounced at the site where Cystophora was naturally rare. Overall, these results suggest that age-specific behavioural preferences may mediate ontogenetic shifts in microhabitat use and limit recruitment in local populations.