ROLE OF MACROALGAL STRUCTURE IN THE DISTRIBUTION AND ABUNDANCE OF A TEMPERATE REEF FISH

Habitat structure can dramatically influence patterns of distribution and abundance of reef fishes. On temperate reefs, much of this structure consists of macroalgae, especially large kelps. I explored the role of such algae, and in particular the structural features of the giant kelp Macrocystis pyrifera, in the distribution and abundance of the kelp perch Brachyistius frenatus. Several measures of the abundance of M. pyrifera were positively related to the density of kelp perch. An ontogenetic shift in habitat use occurred within reefs in that young resided initially within the floating surface kelp canopy, older juveniles descended into the water column and became more loosely associated with bundles of fronds, and adults were distributed more uniformly among several defined depth strata. The density of juveniles varied widely both spatially and temporally, which was linked to variation in the percentage cover of the kelp canopy. Not surprisingly, the density of juvenile kelp perch among reefs was related to the density of adults, since young are born directly into the reef habitat. However, the densities of 1 yr old adults on reefs did not reflect the densities of that cohort as juvenile recruits the previous year, possibly due to temporal variation in the abundance of M. pyrifera. The hypothesis that macroalgal structure may better predict recruitment strength for algal-associated reef fishes that are capable of only limited dispersal than for those that are first subjected to a planktonic dispersive phase was not supported here. However, to make such comparisons between species, recruitment should be measured concurrently under the same environmental conditions and fluctuations in macroalgal abundance, and the particular relationships between recruiting species and the structural features of macroalgae must be considered.