Although the oceans provide a variety of valuable goods and services, societies sometimes fail consider the damage that such resource exploitation may cause to marine ecosystems over time (Jackson et al. 2001). Examples of anthropogenic impacts and over-exploitations of these ecosystems are numerous, and hard continental shelves and rocky reefs are among marine ecosystems the most impacted (Lotze et al. 2006; Halpern et al. 2008). Fisheries using bottom gear such as trawls and dredges are by far the most damaging for the seafloor, acting like forest clear-cutting (Watling and Norse 1998). Due to technological improvements during the last decades, bottom-fishing gears are now used from polar to tropical waters on every type of seafloor; only few places on the world’s continental shelves remaining non-impacted (Watling and Norse 1998; Halpern et al. 2008). Other human uses of the oceans like aquaculture, mining or tourism activities threaten continental shelf ecosystems (Rossi 2013) and their effects, both direct and indirect, are synergistic (Jackson et al. 2001; Kaplan et al. 2013). Although the intensity and extent of effects on seafloor communities by marine renewable energy developments, like wave energy or offshore wind farms are as yet uncharacterized (Henkel et al. 2014) past studies of oil platforms have shown that these installations can affect invertebrate communities locally by providing surface for fouling invertebrates to establish, and in some cases, facilitating species invasions (Page et al. 2006). Wave and/or wind installations could similarly alter the habitat since they could act as artificial reefs with large surface area for new colonies of sessile invertebrates to establish (Wolfson et al. 1979). In addition, bringing new colonies of sessile invertebrates could also alter the ecological niches and change food web dynamics (Langhamer et al. 2009).
One of the major threats to continental shelf ecosystems is a reduction of habitat complexity and heterogeneity by damage to or smothering of slow-growing structure-building organisms like sponges or gorgonians (Watling and Norse 1998; Kaiser et al. 2006; Sheehan et al. 2013), typically found on rocky outcrop, as well as damage to or sedimentation of a rocky outcrop or reef itself. The preferred wave energy installation sites are sedimentary habitats with flat or low relief. As currents flow around installed devices, greater volumes of sediments will be sent into the water column, possibly exposing nearby rocky habitats to increasing sedimentation. Increasing sedimentation in some coral reefs have shown to exert negative effects by smothering the colonies, which reduces recruitment, decreases net productivity, and decreases calcification (Rogers 1990). If the rate of sedimentation on nearby reefs increased due to offshore installations, this could pose a threat to sponges, gorgonians, and crinoids, as their colonies could be smothered by increased sedimentation rates. Habitat heterogeneity can be a major driver of variability in the abundance and diversity of marine species (Benedetti-Cecchi and Cinelli 1995; García-Charton et al. 2004), supporting global species diversity by increasing niche availability and community complexity and facilitating the formation of distinct species assemblages (Cerame-Vivas and Gray 1966; García- Charton et al. 2004; McClain and Barry 2010).
The Pacific Northwest (PNW) continental shelf, especially the northern part (i.e. off Oregon and Washington), is mostly characterized by mud and gravel habitats, but rocky outcrops and reefs occur in several areas (Romsos et al. 2007), supporting structure-building invertebrates that increase the habitat complexity of the seafloor (Strom 2006). This region has a long history of fisheries with a variety of fleets using bottom gears dedicated to groundfishes, dermersal rockfishes, crabs and shrimps. Moreover, it is becoming a focus area for offshore wave and wind energy installations on the continental shelf and slope. However, despite the abundance (and some documentation) of invertebrate bycatch, little is known about mega-invertebrate assemblages on this part of the continental shelf. Hixon and Tissot (2007) and Hannah et al. (2010, 2013) compared trawled versus untrawled mud assemblages at two location sites on the Oregon continental shelf, and Tissot et al. (2007) described the invertebrate and fish assemblages at a single outer continental shelf reef off Oregon. Only Strom (2006) has summarized the distribution of structure-forming invertebrates at multiple sites along the continental margin off Oregon. Off southern California, different invertebrate assemblages have been distinguished based on the physical structure of the habitats: habitats composed of high-relief consolidated rocky outcrops are associated with greater densities of sessile and structure-forming mega-invertebrates including sponges and gorgonians while habitats composed of unconsolidated fine sediments are associated with motile mega-invertebrates including sea stars, crustaceans, bivalves, and sea cucumbers (Allen and Moore 1996; Allen et al. 1997; Stull et al. 1999; Tissot et al. 2006). Large sized, structure-forming mega-invertebrates such as sponges, corals, crinoids, and basket stars have been suggested to provide shelter and additional resources for both fish and other invertebrates by increasing the availability of microhabitats through their large surface area (Tissot et al. 2006).
See Benthic Habitat Characterization Offshore the Pacific Northwest Volume 1: Evaluation of Continental Shelf Geology for the first volume of this report.