Love and York Relationships between fish assemblages and bottom horizontal beams of oil platforms 543 



Materials and methods 



Field sampling 



Between 1996 and 2002, we surveyed fish assemblages 

 around seven oil and gas platforms in southern and 

 central California (Table 1, Fig. 1) using the Delta 

 research submersible, a 4.6-meter, 2-person vessel, 

 operated by Delta Oceanographies of Oxnard, Cali- 

 fornia. Aboard the Delta, we conducted belt tran- 

 sects about two meters from the platform while the 

 submersible maintained a speed of about 0.5 knots. 

 Surveys were conducted in fall, in order to optimize 

 good weather and water clarity. 



Submersible surveys were conducted during daylight 

 hours between one hour after sunrise and two hours 

 before sunset. During each transect, a researcher 

 made observations from a viewing port on the star- 

 board side of the submersible. An externally mounted 

 hi-8 mm video camera with lights filmed the same 

 viewing field as seen by the observer. The researcher 

 identified, counted, and estimated the lengths of all 

 fishes and verbally recorded those data on the video. 

 All fishes in a volume two meters from sea floor up- 

 wards and two meters from the submersible outwards 

 were counted. Fish lengths were estimated by using 

 a pair of parallel lasers mounted on either side of the 

 external video camera. The projected reference points 

 were 20 centimeters apart and were visible to both the 

 observer and the video camera. 



We defined the amount of beam exposure on a scale 

 of 0-4: = the beam was completely covered by shells 

 and soft sediment and it was not visible; l=only the 

 top of the beam (usually encrusted with invertebrates) 

 was visible; 2=the beam was partially exposed (top 

 and sides) — the bottom of the beam remaining in 

 contact with the sea floor; 3=the beam was completely 

 exposed and formed an open crevice less than 0.5 m 

 high; and 4=the beam was completely exposed and 

 formed an open crevice more than 0.5 m high (Fig. 

 2). For each fish we recorded the size of the gap with 

 which the fish associated. 



An environmental monitoring system aboard the 

 submersible continuously recorded date, time, depth, 

 and altitude above the sea floor of the vessel. These 

 environmental data were overlaid on the original 

 videotape upon completion of each survey. Transect 

 videos were reviewed aboard the research vessel or 

 in the laboratory and observations transcribed into 

 a database. 



Statistical analysis 



We were interested in broad patterns of species' dis- 

 tribution among small-scale habitats. Because rare 

 species may prefer some (nonmeasured) extreme habi- 

 tat and thus potentially would have skewed a general 

 picture, we did not use those species where fewer than 

 40 fish were seen or those that were not seen on at least 

 five dives. This left us with 27 species (of the original 



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