RecRuitment of Young-of-the-YeaR fishes to natuRal and aRtificial offshoRe stRuctuRe within centRal and southeRn califoRnia wateRs, 2008–2010

many of the oil and gas platforms off california harbor high densities of young-ofthe-year (YoY) fishes, primarily rockfishes (genus Sebastes). one decommissioning option for these platforms is “topping,” cutting the structure some distance below the water line and leaving the rest of the jacket in place. Questions have arisen regarding how this option would affect the nursery function of the remaining structure. Between 2008 and 2010, we surveyed the densities of YoY fishes at seven intact platforms, as well as six reefs and three shipwrecks that did not crest near the surface and thus might act as surrogates for topped platforms. Rockfishes of about five species dominated the deeper parts of the platforms, reefs, and shipwrecks, while YoY blacksmith, Chromis punctipinnis (cooper, 1863), were most characteristic of the shallow portions of platforms. over the course of the study, we observed large fluctuations in the recruitment success of fishes; recruitment of rockfishes was particularly successful in 2010, while particularly poor for blacksmith in the same year. This interannual pattern was observed throughout our study sites. in general, the YoY fish assemblages around the deeper parts of the platforms were similar to those around the reefs and shipwrecks, implying that removing the shallow parts of california platform may have relatively little effect on juvenile rockfish recruitment; however, blacksmith recruitment to platforms will likely decrease. although offshore oil and gas structures have been present in us waters since the mid-1900s (schroeder and love 2004), new and forthcoming projects include marine renewable energy installations, offshore liquid natural gas ports, and openocean mariculture facilities. all of these types of installations add fixed, shallowwater structure into an offshore environment where none existed before, and thereby introduce novel habitat into the ecosystem. novel habitats may modify local physical and ecological processes. for example, novel habitat may enhance colonization rates of non-indigenous species that in turn alter trophic structure (Page et al. 2006, 2007), or provide unique habitats favored by sensitive species (clynick 2007). The extent to which novel habitats in the offshore environment affect the ecology of local species is not well understood. managers require this information to predict and mitigate impacts of an offshore project through its life cycle, including decisions about what to do with the installation once its original purpose has concluded, a process called decommissioning. There are 27 offshore oil and gas platforms off the coast of california; these are distributed in both state and federal waters from just north of Point arguello southward to off long Beach (fig. 1). These platforms are located between 1.9 and 16.9 km from shore and at bottom depths ranging from 11 to 363 m (details regarding platform dimensions and placement are found in love et al. 2003). Previous research documents that some of these offshore structures may function as nursery habitats BULLETIN OF MARINE SCIENCE. VOL 88, NO 4. 2012 864 for juvenile fishes (sensu Beck et al. 2001) as determined by increased growth and survivorship rates when compared to rates on natural reefs, and the observation that during some years, hundreds of thousands of juvenile rockfishes (Sebastes spp.) inhabit the midwaters of these structures (e.g., love et al. 2003, 2006b, 2007, 2010b, love and schroeder 2004) and this nursery function may be important in maintaining or increasing some rockfish populations (love et al. 2006b). as platforms become uneconomical to operate, understanding this nursery function is important both to the department of the interior and the state of california, as these entities assess the ramifications of different platform decommissioning options (Bernstein et al. 2010). Platform decommissioning alternatives fall into four general categories: complete removal, toppling (laying the structure on its side), partial removal, and leave-in-place (schroeder and love 2004). in particular, partial removal or “topping,” where a platform would be cut off at some distance below the sea surface, is often mentioned as one of the preferred alternatives. The effect of removing part of a platform on young-of-the-year (YoY) rockfish recruitment is unclear. carr et al. (2003), utilizing surveys of intact platforms, speculated that platform topping might decrease the number of YoY rockfishes of at least some of the species that recruit to the topped structure. however, Bernstein et al. (2010) estimated that, Figure 1. Map of the study area. Scuba surveys were conducted at sites from Avila Beach, central California to off Long Beach, Southern California, during the summers of 2008–2010. Circles demarcate all of the platforms off California. The seven platforms surveyed in this study are designated by darkened circles and by name. Surveyed reefs and shipwrecks are labeled by name and are demarcated by stars and squares, respectively. love et al.: recruitment to natural and artificial structures 865 again based on indirect evidence, removing perhaps the top 25 m of the underwater portion of a platform might have relatively little effect on rockfish recruitment. no studies have specifically addressed the issue of how partial removal might impact the rockfish nursery function of platforms. The most rigorous approach for making such an assessment would be to compare rockfish recruitment strength and composition at platforms that have been topped with those that are still intact. however, there are no topped platforms off california. a second method would compare structures that might act as surrogates for topped platforms with intact platforms. we apply this approach here using rocky reefs and shipwrecks that crest well below the sea surface as surrogate structures. Thus, the goal of the present study was to fill gaps in information about the spatial and vertical depth variability in recruitment of fishes to platforms and topped platform surrogates (natural reefs and shipwrecks) off the california coast and particularly to provide data that can be used in future analyses of the environmental consequences of partial platform removal on local and regional fish populations.