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Fishery Bulletin 89(3). 1991 



Figure 1 



Flight transects for aerial survey of harbor porpoise in cen- 

 tral California, 1986-90. Transect 7 was combined with 

 transect 8 after 1986 and is not shown. 



per kilometer. Standard ANCOVA F-ratio tests were 

 applied to determine whether a significant trend with 

 year is present. Monte Carlo methods were used to 

 determine the power of this test to detect known trends 

 in abundance. 



Methods 



Field methods 



The surveys were established to monitor changes in 

 abundance within the range of porpoise/gillnet fishery 

 interactions (Point Conception to Russian River, Cali- 

 fornia). Surveys were started only when there was a 

 good likelihood of completing at least half of the survey 

 (Point Conception to Monterey, or Monterey to Rus- 

 sian River) under good weather conditions. They were 

 halted when viewing conditions deteriorated below ac- 

 ceptable levels (sea state higher than Beaufort 4 or 5, 

 excessive dark cloud cover, rain, or fog). 



A series of predetermined locations marking the 

 beginning and end of each transect was entered into 

 the aircraft's LORAN C navigational receiver to give 



the pilot a course to follow. The transects zigzagged 

 in a generally northward progression between shore 

 and roughly the 50-fathom (91-m) contour (Fig. 1). 

 Sightings at the endpoint of a transect were very rare, 

 and duplication at the start of the next transect did not 

 occur. The transect lengths ranged from 5.2 to 44.8km 

 and averaged 24.8 km. The aircraft maintained an 

 altitude of approximately 213 m and speeds of 90-100 

 knots (167- 185 km/hour). To reduce sun glare, surveys 

 were conducted only from south to north. 



The surveys were flown in a twin-engine, high-wing, 

 seven-passenger aircraft with the rear seat removed 

 (Partenavia P68). Two observers sat behind the pilot 

 and copilot seats and looked out the side windows; a 

 third observer (belly observer) lay on the floor on 

 his/her stomach just behind the right-side observer's 

 seat and surveyed the water below the airplane through 

 a 25 x 30-cm rectangular viewing port. Starting in 

 1988, the side windows were fitted with plexiglas 

 bubble-type windows, allowing the side observers to see 

 from the horizon to directly under the plane. This 

 created an overlap with the belly observer's field of 

 view; however, this did not result in double counting 

 because the observers were in constant communication 

 and discussed all possible sighting duplicates as they 

 occurred. 



The data recorder sat in the copilot position and 

 recorded flight information, including location (latitude 

 and longitude), time, weather (% cloud cover, Beaufort 

 sea state, and sun position), viewing conditions, and 

 porpoise sighting information. The data recorder en- 

 tered weather and viewing conditions at the start of 

 each transect and whenever conditions changed. Each 

 observer subjectively evaluated viewing conditions as 

 excellent, good, poor or "off effort," depending on 

 estimated viewing depth into the water, sun glare, and 

 sea state. To simplify the recording procedure and 

 enhance accuracy of the data, a lap-top computer con- 

 nected to the LORAN C navigational receiver replaced 

 the hand-written flight log during the 1988-90 surveys. 



The pilot, recorder, and observers communicated 

 through headsets and voice-activated microphones. All 

 communication was recorded on a central tape re- 

 corder. Additionally, each observer used a hand-held 

 tape recorder for storage of individual sighting infor- 

 mation. The two side observers used hand-held in- 

 clinometers to measure declination angles in degrees 

 to the animals sighted. Due to space limitations, the 

 belly observer could not use an inclinometer and 

 estimated angles using marks applied to the viewing 

 port. The observers changed positions approximately 

 every 1-1.5 hours and between flights. 



The observers actively searched (were "on effort") 

 from start to finish of a transect, except when circling 

 or when they declared themselves "off effort" because 



