Reproductive ecology and body burden of resident fish prior to decommissioning.

Abstract

Significant Results:

Task 1:

Surveys were conducted at platforms and natural sites between 2004 and 2009 aboard the research submersible Delta. Natural sites were comprised of both high and low rocky reefs. We conducted 803 tran­sects around 20 platforms, encompassing 144,022 m2 of habitat. The habitats of almost all platforms were surveyed at least once and some platforms were surveyed in a majority of years. In addition, we made 134 natural site dives (422 transects, 377,851 m2), at bottom depths of 17–343 m. A total of at least 110 unique natural sites were assessed and some sites, such as North Reef, were surveyed in more than one year. Over all habitats, we observed 687,142 fishes, comprising a minimum of 128 species. Of these, 317,583 fishes, of 95 species, inhabited platforms and 369,559 fish, of 114 species, lived on natural sites. On average, fish densities were over twice as high at platforms (257.4 individuals/100 m2) compared to natural sites (104 individuals/100m2). Rockfishes, of 45 species (at least 45 species at platforms and 43 species at natural sites) dominated the survey, as they comprised 85.8% of all fishes observed (83.8% at platforms and 87.5% at natural sites).

Among the highest density species or species complexes, squarespot, halfbanded, and shortbelly rock­fishes, and a complex of young-of-the-year (YOY) rockfishes dominated both platform and natural sites. Blacksmith, widow rockfish, jack mackerel, unidentified Sebastomus rockfishes, blackeye goby, and calico rockfish rounded out the top ten platform species. A somewhat different suite of species, including pygmy, blackeye goby, unidentified Sebastomus rockfishes, swordspine rockfish, blacksmith, and blue rockfish com­prised the top ten species by density at natural sites.

We observed three distinct fish assemblages around each platform: midwaters, bottom, and shell mound. These assemblages did not appreciably change over the course of the study. There was a tendency for densities of fishes to increase and peak in deeper midwater depths or at the bottom. Fish densities over shell mounds were usually lower than those at the adjacent platform bottom. In addition, fish densities varied greatly between platforms at similar depths. However, there appeared to be no geographic pattern to these differences. There was a tendency for fish densities on the bottoms and shell mounds of the deepest platforms to be lower than those at shallower structures.

Midwater assemblages were similar across platforms, while bottom and shell mound assemblages var­ied with platform bottom depth. In general, all of these assemblages were at least somewhat different from the assemblages observed on natural sites. There tended to be higher densities of young-of-the-year fishes, particularly rockfishes, around many platforms than at most natural sites. Older juveniles and adults of eco­nomically important species were also more likely to be found at higher densities at some platforms than at most natural sites. This latter may reflect 1) an extensive and complex bottom habitat around the bottoms of some platforms that serve as sheltering areas for economically important species and 2) the lower fishing effort (a de facto marine reserve effect) of platforms as many of these structures appear to be rarely fished.

The shell mounds surrounding California platforms are a unique feature of these structures and are composed primarily of living and dead mussels, and associated marine life. They form an extensive web of low, but rugose, sea floor. The relatively small crevices created by mussel shells deter large numbers of many high-relief species from venturing onto these areas. Rather, most shell mound species are either the juveniles of larger species, whose juvenile stages require small sheltering sites, or somewhat generalist species that live over 1) soft sea floors, 2) the ecotones between soft and low-relief hard bottom, and 3) low-relief reefs. While shell mound assemblages in shallow and middle depth waters tend to be different from those of natural sites of the same depths, deep depth shell mound assemblages more closely resemble those at natural sites. This is likely because reefs in the deeper waters of California tend to be low relief and thus more like shell mounds.

Task 2:

We observed 13,609 fishes of at least 32 species at Platform Eureka and 2,980 fishes of at least 20 species at Platform Gail. Total average fish density was much higher at Eureka (135.9 individuals per 100 m2) than at Gail (28.7 individuals per 100 m2). Rockfishes (genus Sebastes) dominated both assemblages, comprising 99.5% and 96.7% of all fishes observed at Eureka and Gail, respectively. A minimum of 28 rockfish species (28 spe­cies at Eureka and 14 at Gail) inhabited the platform midwaters. Those species with highest densities at Eureka included squarespot, widow, speckled, and blue rockfishes, while squarespot and widow rockfishes and bocac­cio dominated the midwaters of Gail. Fifteen species were unique to Eureka and four species were found only at Gail. Of the species shared by the two structures, the densities of almost all species were higher at Eureka, sometimes by a factor of 10 or more. The number of species around the crossbeams varied with depth (ranging from 6-11 at Gail, and 14-18 at Eureka) and tended to be highest around the deeper members. Between the two platforms, species numbers were higher at all depths at Eureka and usually 2-3 times that of Gail.

Thus, while the assemblages of both platforms were dominated by rockfishes there were also signifi­cant differences between them. Compared to Gail, Eureka harbored 1) higher densities both of all species combined and of most species held in common, 2) far more mature individuals of most species, 3) greater species richness, and 4) much higher densities of species that live over complex high relief. We propose that from a fish’s perspective, the complex midwater jacket of Eureka, with its many sheltering sites, mimics rugose natural reefs. This research both re-enforces the conclusion that many reef species have quite specific habitat requirements and that the platform decommissioning process must examine each platform individually.