ASFB Home > The effects of spatial and temporal factors on the abundance of seven key finfish species along south-western Australia.

Herbivory by fish and sea urchins at Mewstone, south-western Australia

Mat Vanderklift, Gary Kendrick, Euan Harvey and Samantha Lee

University Of Western Australia, CRAWLEY, AUSTRALIA

THEME: ASFB

Herbivores can exert a profound influence on the macroalgae growing on temperate subtidal reefs. Our objective was to study the effects of sea urchins and herbivorous fish on the macroalgae of a subtidal reef at Mewstone, near Perth, Western Australia. We studied the effects on both recruiting populations and adult stands of macroalgae by deploying cages (excluding both fish and sea urchins), roofs (excluding fish only) and fences (excluding sea urchins only) for 13 months. Results provided equivocal evidence to suggest that fish influenced the biomass and species composition of recruiting macroalgae, but not the assemblage of adult macroalgae. There was no evidence to suggest that sea urchins influenced either recruiting or adult macroalgae. Herbivorous fish exerted the greatest influence during winter and early spring; by late summer no influences were observed. Growth and biomass is greatest for many local algae during the summer period and this might explain why herbivores exerted no detectable influence during this time. The absence of any influence by sea urchins probably relate to the preferred mode of feeding of the dominant sea urchin, Heliocidaris erythrogramma, which feeds primarily on drift algae in the area studied. There are alternative explanations for the patterns observed that invoke caging artefacts, rather than herbivory, as the reasons for our results. Two possible changes introduced by the cages (other than herbivore exclusion) were sedimentation rate and light availability. These results are discussed in the context of the role of herbivory in structuring macroalgal assemblages in temperate Western Australia and to previous caging and fencing experiments.