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Fishery Bulletin 104(3) 



recruitment that occurred in the Santa Barbara Channel 

 during 2003 took place at the surveyed platforms. 



Because it is likely that most reefs both attract and 

 produce fishes, discussions of reef function have evolved 

 from the single issue of attraction versus production to 

 an evaluation of the ecological performance of fishes at 

 natural and human-made habitats. Reviews and stud- 

 ies that compare natural and human-made reefs as 

 juvenile fish habitats should address: 1) a comparison 

 of the survival rate of young fishes (including preda- 

 tion, growth rates, and survival after emigration, if 

 that occurs) at the two habitat types, 2) the density of 

 recruiting juveniles at a human-made reef versus that 

 at surrounding natural reefs, 3) the possibility that a 

 human-made reef is drawing recruiting juvenile fishes 

 from nearby natural reefs, 4) the source of juvenile 

 fishes found on a human-made reef — either from the 

 plankton or from natural reefs (discussed in a number 

 of papers including Bohnsack et al., 1994; Carr and 

 Hixon, 1997; Pickering and Whitmarsh, 1997). 



What is the survival rate of young fishes that recruit 

 from the plankton to platforms and those that recruit to 

 natural reefs? What appears to be a unique feature of 

 the midwaters of many California platforms (compared 

 to both natural reefs and to Gulf of Mexico platforms) 

 is that many may act as juvenile (particularly YOY) fish 

 refuges. The midwaters of platforms do not act as struc- 

 tural refuges. That is, they are not dotted with crevices, 

 caves, and other complexities that allow fish to hide 

 (Hixon and Beets, 1993). Rather, platforms may afford 

 spatial refuges because, in general, concentrations of 

 adult reef fishes tend to be found at the bottom of plat- 

 forms (except for those platforms that harbor almost no 

 adult fishes at all) rather than in the midwaters. Tran- 

 sitory and migrating piscivorous species, such as jacks 

 and barracuda (taxa that are abundant around Gulf of 

 Mexico platforms [Stanley and Wilson, 2000]) are also 

 not common around most California platforms (Love et 

 al., 2003). We have also not observed high densities of 

 either fish-eating pinnipeds or sea birds around these 

 structures. In addition, studies (Schroeder et al., 2005) 

 demonstrate that predation rates on small fishes are 

 lower in platform midwaters than at natural reefs. 



Several lines of evidence demonstrate that YOY bo- 

 caccio recruit to platforms from the plankton and that 

 the platforms do not attract previously settled fish from 

 natural reefs. Studies of the seasonal timing of plat- 

 form recruitment pulses and the size of the recruiting 

 individuals are consistent with the hypothesis that 

 young bocaccio settle on the platforms from the pe- 

 lagic environment (Nishimoto et al., 2005; Schroeder'^; 

 Nishimoto'). Are these platforms, then, drawing away 

 pelagic juveniles that would have settled instead on nat- 

 ural outcrops? Although it is possible that some pelagic 



^ Schroeder, D. M. 2002. Unpubl. data. Marine Science 

 Institute, University of California, Santa Barbara, CA 

 93106. 



' Nishimoto, M. 2005. Personal commun. Marine Science 

 Institute, University of California, Santa Barbara, CA 93106. 



juveniles that had settled on platforms would have found 

 natural reefs, it is likely that most of these fish would 

 not have survived in the absence of these structures. As 

 an example, Emery et al. (2006) simulated near-surface 

 drift trajectories from high-frequency radar current 

 measurements to model movements of pelagic juvenile 

 bocaccio in the vicinity of a platform off Pt. Arguello, 

 central California. They estimated that for 1999 and 

 2002, a minimum of 76% and 69% of the modeled pe- 

 lagic juvenile bocaccio would not have found natural 

 habitat on which to settle. Although the platforms we 

 surveyed were east of the platform examined in the 

 Emery et al. (2006) study, it is unreasonable to conclude 

 that these relatively small platforms are filtering the 

 majority of pelagic juvenile bocaccio from the pelagic 

 environment to the exclusion of natural reefs. Rather, 

 it is possible that some platforms provide recruitment 

 habitat for fishes that would otherwise be lost from the 

 population. 



What is the fate of YOY bocaccio that settle around 

 platforms? In general, YOY bocaccio emigrate from 

 shallower platforms within two years. At least some 

 of these fish migrate to natural reefs, because young 

 bocaccio tagged around Platforms A and B in 1978 

 and 1979 were recaptured on natural reefs as adults 

 as much as 150 km from the tagging sites (Hartman, 

 1987). Our observations of bocaccio on the deeper Plat- 

 form Gail demonstrate that many of the YOY bocaccio 

 that recruited to that platform from the plankton in 

 1999 remained in the area (migrating to the bottom in 

 2000) and matured into adults (Fig. 4). 



To summarize, the weight of evidence implies that 

 some of the platforms in southern California produce 

 bocaccio. During some years, young bocaccio settle from 

 the plankton to the platforms in large numbers. This 

 settlement may occur even when juvenile recruitment 

 to natural reefs in the same region is low. Compared 

 to the structure of most natural reefs, the structural 

 complexity and high vertical profile of platforms prob- 

 ably provide pelagic juvenile rockfishes with a relatively 

 strong stimulus to trigger settlement. Mortality on plat- 

 form YOY bocaccio from predation may be relatively 

 low because of a relative scarcity of predators. There 

 is also evidence that platforms retain pelagic bocaccio 

 juveniles that would otherwise have been carried into 

 inhospitable offshore waters. At least some bocaccio that 

 emigrate from platforms survive to populate natural 

 reefs, whereas, as in the case of one platform, some 

 YOY bocaccio remain and mature into adults. Thus, 

 there is the likelihood that many of the YOY bocac- 

 cio we observed at platforms will either emigrate and 

 seed natural reefs or will reside at the platforms and 

 reproduce. 



Clearly, our research was subject to a number of as- 

 sumptions and the ambiguities that may be associated 

 with these assumptions. These assumptions add some 

 degree of uncertainty both to our estimates of YOY bo- 

 caccio abundances and their subsequent contribution to 

 stock rebuilding. However, we consider this research to 

 be sufficiently strong to demonstrate the importance of 



