Assessing the ecological importance of red tree coral thickets in the eastern Gulf of Alaska

Abstract

Abstract Red tree corals (Primnoa pacifica), the largest structure-forming gorgonians in the North Pacific Ocean, form dense thickets in some areas. These thickets are a dominant benthic habitat feature in the Gulf of Alaska (GOA), yet little is known about the ecosystems they support. In 2005, we used a submersible to study the ecology of thickets inside or near five small areas of the eastern GOA later designated in 2006 as habitat areas of particular concern (HAPCs)―areas closed to all bottom contact fishing. We show that red tree corals are keystone species in habitats where they form thickets (mean density 0.52 corals m−2)—the densest and largest thickets documented anywhere. Measured sponge densities (2.51 sponges m−2) were also among the highest documented anywhere. The corals and sponges in the study areas provide essential fish habitat for some fish species, and we show with logistic regression models modified with a scaled binomial variance that bedrock, while important habitat for some fish, is even more important when paired with corals and sponges. Red tree corals were not equally distributed with regard to habitat characteristics, and we show that their presence was correlated with bedrock substrate, moderate to high seabed roughness, and slope >10°. Most corals and sponges are vulnerable to disturbance from longlining, the principal bottom contact fishing in this region, but the larger corals and sponges are the most vulnerable. We observed evidence of infrequent recruitment events and a strong pulse of predation, apparently from fishing gear-induced trauma, that could exacerbate slow recovery of red tree corals from disturbance. Some red tree coral thickets are provided protection within designated HAPCs and some are not. Modifications to longline gear and an expanded network of HAPCs could help preserve these keystone species and the ecosystems they support.