The Importance of Macroalgae on Rocky Reefs: A Critical Aspect for Fish and Epifauna of the East Otago Coastline

The East Otago coastline north of the Otago Peninsula, New Zealand, is an area of rocky macroalgal dominated reefs, which contain Macrocystis pyrifera kelp forest and dense mixed macroalgal beds. These areas are considered crucial habitats for finfish and epifaunal invertebrates, providing habitat complexity, abundant food sources, protection from predators and kōhanga (nurseries) habitats. The present study concentrated on three distinct macroalgal habitats, including those dominated by Carpophyllum flexuosum and Cystophora spp. (0-3 m depth), Macrocystis pyrifera kelp forest (3-10 m) and Ecklonia radiata beds (10-15 m). The work was conducted within a locally managed fishery called the East Otago Taiāpure, a 24- km2 area of coastline established in 1999. The primary objective of this study was to gain a better understanding of reef fish and epifaunal associations with macroalgal habitats. This was achieved through the use of three primary methods: Firstly, SCUBA stationary visual point surveys of finfish populations in different macroalgal habitats. Secondly, the deployment of novel epifaunal invertebrate collectors, which simulate rocky coralline turf or macroalgal habitats (known as coralline cobble and seaweed simulators); and thirdly food web studies through the use of stable isotope and gut contents analysis performed upon reef, soft sediment and pelagic associated fish species. The results from this study indicate strong associations of reef fish and epifaunal species with macroalgal habitats. Ninety-one percent of all fish observed in this study were encountered within M. pyrifera kelp forests and E. radiata beds. Epifaunal species showed tight associations to macroalgae with gammarid amphipods found in high abundances within the M. pyrifera canopy. Small snails and crab species dominated coralline cobble simulators, while amphipods dominated seaweed simulators within all macroalgal habitats sampled. Food web analysis revealed that reef fish use a combination of both macroalgal and phytoplankton primary productivity in food sources. Furthermore, for soft sediment and pelagic associated fish species, over 90% (on average) of primary productivity comes from macroalgal primary productivity. This study shows that macroalgal habitats, especially Macrocystis pyrifera kelp forests, are important to reef fish and epifauna. This study also reveals that macroalgal productivity also plays an important role in driving food webs within soft sediment and pelagic associated species in coastal areas. This indicates that rocky reefs are extremely valuable to coastal areas and should be protected for healthy finfish populations to exist