Structure and trophic organization of subtidal fish assemblages on the northern Chilean coast: the effect of habitat complexity

Abstract

A series of local factors such as substrate structure, presence or absence of kelp forests and environmental variables play an important role in determining both the diversity and abundance of temperate reef fish assemblages. The presence of kelp forests has been shown to increase struc- tural complexity, available shelter, and prey availability in the reef environment. This, in turn, may affect the structural organization of the fish assemblage. In this study, the diversity, abundance and trophic patterns of subtidal fish assemblages at 2 localities off the northern Chilean coast were ana- lyzed. These localities differ markedly in their spatial structure: the subtidal substratum at Caleta Errazuriz is characterized by a sloping bedrock with large rocks and boulders and a kelp forest, while at Carrizal Bajo it consists of a sharply declining bedrock with few boulders and isolated kelp stands. We hypothesized that structurally highly complex habitats would exhibit higher diversity, abundance and a more complex trophic organization in terms of more functional groups and guilds of reef fish assemblages than less structured environments. Contrary to our hypothesis, we found similar species diversity and fish abundance at both localities. However, there were clear differences in species dom- inance, number of functional groups and trophic guilds. There were also marked differences in the trophic webs between localities. In the presence of kelp forest, both carnivores and omnivores con- sumed mainly benthic prey inhabiting the understory canopy, while in its absence carnivores fed on pelagic prey in the water column. Therefore, a more complex spatial structure would seem to support a more complex trophic organization with a greater degree of connectivity, as well as interaction between the species in a reef fish assemblage.