Love et a\ Fish assemblages around oil platforms in the Santa Barbara Channel area 



99 



calculations "estimated density" (number/m^) and 

 "estimated biomass "{kg/m-). 



This method of estimating transect length and 

 hence fish density and biomass relies on the assump- 

 tion that the submersible travels at the same speed 

 in both habitats. Although we did not have data on 

 submersible speed, every attempt was made to main- 

 tain the submersible at the same speed on all tran- 

 sects during the survey. However, because of debris 

 on the bottom and water currents in the midwater, 

 if there were differences in speed, the submersible 

 was likely to travel slightly faster in the midwater 

 habitats than on the bottoms. In this case, the sub- 

 mersible would cover more area per unit time and 

 the true fish density in the midwater may actually 

 be slightly lower than our estimated density. We 

 consider the potential bias introduced by differences 

 in submersible speed to be minor in relation to the 

 magnitude of the observed differences in fish densi- 

 ties between the midwater and bottom transects (see 

 "Results" section). 



We calculated both species richness (number of 

 species) and species diversity. We used the Shan- 

 non-Weiner diversity index (H') for all species diver- 

 sity comparisons (Shannon and Weaver, 1949). We 

 also calculated a percent similarity index (PSD that 

 quantifies how similar two assemblages are in terms 

 of their species composition (i.e. the relative abun- 

 dance of those species). The index ranges from (no 

 species shared) to 100'7f (identical composition and 

 relative abundances). The formula for PSI is 



PS/ = {^min(p„,pj)x 100. 



where, p^, and p^.^ are the proportion of the ith spe- 

 cies in habitat x and habitat y. PSI was calculated 

 for each pair of platform bottom assemblages. 



Results 



Bottom versus midwater transects 



We found that there were several distinct differ- 

 ences in the fish assemblages living in the midwater 

 and bottom habitats around all of the platforms. We 

 calculated percent similarity indices (PSI) between 

 the bottom and midwater assemblages for each plat- 

 form. These PSIs ranged from 19( to 349c (mean 

 13.3%). Although both midwater and bottom assem- 

 blages were dominated by rockfishes, platform mid- 

 waters were dominated by young-of-the-year (YOY) 

 or slightly older juveniles (<10cm). Rockfishes larger 

 than about 20 cm were rarely seen in the midwater 



cm 



E 

 o 



Density (number/minute) 



20-1 



Biomass (kg/minute) 



Figure 2 



Regressions of (A) density in terms of number per 

 minute on density in terms of number per m- and 

 (B) biomass in terms of kilograms per minute on bio- 

 mass in terms of kilogram per m- for bottom tran- 

 sects on all platforms. The regression equations were 

 used to calculate density and biomass from number 

 and kilogram per minute into number and kilogram 

 per ni'^ on the midwater transects. See "Methods" 

 section for explanation of the conversion. 



(Fig. 3). The fish assemblages around the bottoms 

 of the platforms were dominated by subadults or 

 adults (11-20 cm) and occasionally harbored very 

 large individuals (up to 48 cm) (Fig. 3). 



Average density per platform of all fishes com- 

 bined was not significantly different on the bottom 

 versus the midwater transects (bottom mean density 



