Feeding selectivity of the herbivorous fish Scartichthys viridis: effects on macroalgal community structure in a temperate rocky intertidal coastal zone

The ecological importance of fish herbivory was examined in a temperate roc ky intertidal zone along the central Chilean coast. In this system, the ble nny Scartichthys viridis is the most abundant and only herbivorous fish. We describe its diet, determine its field food selectivity (gut contents vs m acroalgal field availability comparison), and report on its laboratory diet ary preferences in a multiple-choice feeding experiment. Finally, we evalua te the relative effects of fish herbivory and grazing invertebrates on macr oalgal abundance, distribution and diversity patterns in the field, using r eplicated exclusion cages to manipulate grazing intensity. S. viridis was f ound to be a selective grazer; its diet was dominated by 2 macroalgae: the green Ulva and the red Gelidium, representing 32.6 and 41.4% of its total g ut content biomass, respectively. Ulva was consumed much above its field av ailability while Gelidium was selected only during fall-winter. In the labo ratory, S. viridis selected the green macroalgae Ulva rigida and Codium dim orphum and avoided Gelidium chilense. We suggest that Gelidium may be inclu ded in its diet due to the limited availability of Ulva in the field. Exper imental exclusion of this fish from rocky surfaces resulted in increased ab undance of green foliose macroalgae (U. rigida and Enteromorpha linza), and , to a much lesser extent, of the red foliose macroalgae G. chilense, Porph yra columbina, and Mazzaella laminarioides, and of the brown macroalgae Col pomenia phaeodactyla and Petalonia fascia, as well as an extension of the d istribution of P. columbina from the high intertidal to the mid intertidal zone, where it normally does not occur. Exclusion of grazing invertebrates resulted in a decrease in foliose macroalgal species richness, but had no e ffect on biomass. To our knowledge, this is the first experimental demonstr ation of the effect of an herbivorous fish on macroalgal community structur e in a temperate rocky intertidal area. The selective feeding displayed by S, viridis, its high numerical abundance, and the results of the herbivore exclusion experiment suggest its importance in maintaining the low abundanc e of foliose macroalgae and the high relative cover of brown and red crusto se macroalgae, characteristic of many low to mid rocky intertidal areas alo ng the coast of central Chile.