Completion of fish assemblage surveys around manmade structures and natural reefs off California

Abstract

We conducted 87 transects around 11 platforms, encompassing 32,816 m2 of habitat and we made two dives (13 transects, 5469 m2) on natural reefs in shallow (46–47 m) and somewhat deeper (145 m) waters.
Over all platforms, we observed 158,129 individuals, comprising a minimum of 56 species. Rockfishes dominated platform fish assemblages; we observed 155,239 individuals (98.2% of all fishes), of at least 36 species. Squarespot and halfbanded rockfishes, and small, unidentified rockfishes (primarily young-of-the-year and likely mostly squarespot and widow rockfishes) dominated the surveys. Other abundant rockfishes included widow and bank rockfishes and bocaccio. Among non-rockfish species, blacksmith, painted greenling, California sheephead, and sharpnose seaperch were frequently observed.
Fish densities were extremely variable among platforms and between depths within platforms. Fish densities were lowest in the shallower depth strata, particularly in 0–30 m, and densities generally increased with depth. For those platforms whose midwaters, bottoms, and shell mounds were all surveyed, fish densities were always highest around the platform bottom. Fish densities over shell mounds were lower than those at the adjacent platform bottom.
Generally, the midwater fish assemblages across all platforms were similar. YOY fishes (e.g., squarespot and widow rockfish, bocaccio, and painted greenling) dominated a number of platform midwaters. Also important around some platforms were such nearshore reef species as garibaldi, blacksmith, cabezon, sheephead, and white and sharpnose seaperches. Platform Eureka, with a more structurally complex midwater platform jacket, harbored relative high densities of such typically deeper-dwelling rocky-reef rockfishes as bank, speckled, rosy, dwarf-red and starry leading to a unique midwater assemblage of the platforms surveyed.
At the bottom of all platforms other than Gail, halfbanded and/or squarespot rockfishes dominated the habitats. Other important species included calico, flag, rosy, vermilion and widow rockfishes, and lingcod. At a few platforms, notably Edith and Elly, YOY rockfishes (halfbanded and unidentified), comprised a substantial part of the assemblage, otherwise young fishes were not abundant. At Platform Gail, with a bottom depth considerably deeper than the other structures, a different suite of species dominated, including bocaccio, cowcod, pinkrose and greenblotched rockfishes. Overall, depending on platform, rockfishes were the most important group in the bottom assemblage, they comprised as much as 99.9% of all fishes observed.
The assemblage of fishes occupying the low-relief shell mounds was composed of 1) juvenile fishes of larger species and juveniles and adults of dwarf species that utilize small sheltering sites (e.g., juvenile cowcod and lingcod, blackeye goby, and calico rockfish), 2) ecotonal species that favor soft sea floor-low, hard-relief bottom (greenstriped and stripetail rockfishes), and 3) a few schooling taxa (notably halfbanded rockfish) that are habitat generalists.
The shallow reef was dominated by blackeye goby and halfbanded rockfish; squarespot, blue, and vermilion rockfishes and lingcod were also frequently observed. At the deeper site, squarespot, pygmy, swordspine, and halfbanded rockfishes and unidentified rockfishes (likely primarily YOY swordspine and squarespot rockfishes) were very abundant.
Overall, the data from 2010–2011 supports the observations of previous surveys that: 1) There are three distinct fish assemblages around each platform: midwater, bottom, and shell mound. Within these assemblages, those of the midwaters tend to be the most similar across platforms although there may be substantial interannual and some geographic differences. The bottom and shell mound assemblages tend to be somewhat similar within a platform (but are still distinct), but are often quite dissimilar among platforms and 2) During at least some years, platforms serve as nursery grounds for a variety of rockfishes and other taxa