HABITAT STRUCTURE, CONSPECIFIC PRESENCE AND SPATIAL VARIATION IN THE RECRUITMENT OF A TEMPERATE REEF FISH

Pronounced spatial variation in recruitment occurs in many marine inve rtebrate and fish populations and is thought to be critical to the dem ography of these species. In this study I examined the importance of h abitat structure and the presence of conspecific residents to spatial variation in larval settlement and recruitment in a temperate fish Tau togolabrus adspersus. I define settlement as the movement of individua ls from the water column to the benthic habitat, while I refer to recr uitment as numbers of individuals surviving some arbitrary period of t ime after settlement. Experiments in which standard habitats were stoc ked with conspecifics showed that resident conspecifics were not an im portant factor contributing to small-scale variability in recruitment. Further correlative analyses demonstrated that large-scale variation in recruitment could not be explained by variability in older age clas ses. By contrast, manipulations of macroalgal structure within a kelp bed demonstrated that recruitment was significantly higher in habitats with a dense understory of foliose and filamentous algae than in habi tats with only crustose algae. Understory algae varied in their patter n of dispersion among sites, and the dispersion of fish matched that o f the plants. In order to determine the effects of differences in patt erns of algal dispersion on the demography of associated T. adspersus populations. I used experimental habitat units to manipulate patterns of dispersion. Settlement was significantly greater to randomly placed versus clumped habitats, however, no differences in recruitment betwe en random and clumped habitats were detected. Because recruitment is a function of the numbers of settlers minus the subsequent loss of sett lers, rates of mortality or migration must have been higher in the ran domly placed habitats. These results are counter to the current paradi gm for reef fishes which suggests that larval settlement is the crucia l demographic process producing variability in population abundance. I n this experiment patterns of settlement were modified by varying the patch structure of the habitat.