Abstract
Platform Hidalgo is located in 123 m of water, approximately 10 km SW of Pt. Arguello, California. A pilot study was conducted to examine the distribution and abundance of rockfishes around Hidalgo (by scuba and by ROV photosurveys) and at eight adjacent natural reefs (by ROV photosurveys). To test for differences in fish assemblages among reefs using factorial ANOVA, the natural reefs were classified according to two levels each of three factors: “low” (0.2–0.5 m) versus “high” (>1 m) relief height; “nearfield” (<3 km from Hidalgo) versus “farfield” (>3 km from Hidalgo) proximity to the platform; and “shallow” (113–160 m) versus “deep” (195–213 m) reef depth. Fishes were also tagged over natural reefs with breakaway hook tags. Surveys and tagging studies were conducted from July to October 1990. During surveys in July and August, large numbers of young-of-the year (YOY) rockfishes (primarily Sebastes entomelas, S. flavidus and S. hopkinsi) were found in the surface and midwaters around Hidalgo. At the base of the platform, 1+ yr rockfishes (mainly S. entomelas and S. flavidus) were very abundant. When the platform was resurveyed in October, following the first gale of the year, nearly all the YOY rockfishes had disappeared, though the 1+ yr fishes were still abundant at the platform's base. While the ultimate fate of the YOY rockfishes is not known, our preliminary data suggest the hypothesis that Platform Hidalgo acts as a producer of fish biomass by providing recruitment habitat for pelagic larvae to settle and grow before dispersing as small juveniles. Both multivariate cluster analysis and univariate ANOVA show that the rockfish assemblage around Hidalgo is different in species composition and abundance from those over the natural reefs, although several species co-occur at the platform and the reefs. Three-way factorial ANOVA (without replication) suggests that among the natural reefs there are no statistically significant differences (at P ≤ 0.05) in abundances of rockfish populations or in diversity of rockfish assemblages due to the effect of relief height, depth, or proximity to the platform. However, patternrecognition techniques (cluster analysis) reveal distinct groupings of these fauna that are related foremost to reef depth, followed by proximity to the platform and finally relief height. Fish density differed significantly (P ≤ 0.05) between sampling times at selected sites. We propose that the factor relief height be dropped in subsequent definitive studies of these offshore reef assemblages. By doing so, the effects of remaining factors, including possible platform effects, on the distribution and abundances of these fishes can be tested with a more powerful replicated factorial design. Sampling also should be conducted on a number of occasions to examine patterns and sources of natural temporal variability.