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Fishery Bulletin 105(3) 



ally abundant jellyfish (Scyphomedusae) along the West 

 Coast of North America (Eisenberg and Frazier, 1983; 

 Shenker, 1984), where coastal upwelling creates a dy- 

 namic and highly productive ecosystem. 



The California Current ecosystem is dominated by 

 seasonal upwelling that is most intense between Pt. 

 Conception and Cape Mendocino and gradually abates 

 between July and October (Bakun et al., 1974). Previ- 

 ous studies of sighting patterns have linked leatherback 

 turtle distribution and occurrence off the West Coast of 

 North America to sea surface temperatures of 15-16°C 

 during late summer and early fall (Stinson, 1984; Star- 

 bird et al., 1993). In particular, Monterey Bay, Califor- 

 nia, was identified as an area where leatherback turtles 

 can be found during August, according to incidental 

 sighting information collected by recreational boat- 

 ers, researchers, and whale-watching operators. The 

 spatially biased nature of these observations, however, 

 precluded the estimation of overall leatherback turtle 

 density and abundance. 



In this study, we report the results of systematic 

 aerial surveys conducted over coastal California waters 

 between 1990 and 2003 and provide the first estimates 

 of abundance for foraging leatherback turtles along the 

 California coast. We also describe the density, distribu- 

 tion, and interannual variability of leatherback turtles 

 off California during the peak period of occurrence in 

 late summer and fall, examine oceanographic factors 

 related to their occurrence in this region, and evalu- 

 ate the potential significance of this foraging area to 

 the western Pacific stock. Knowledge of leatherback 

 foraging habitats is essential for the recovery of this 

 critically endangered species, and the results of this 

 study provide a basis for identifying and examining 

 other potential foraging regions in the northeastern 

 Pacific. 



Materials and methods 



Field methods 



Aerial line-transect surveys for marine mammals and 

 sea turtles were conducted between 15 August and 15 

 November in 10 of 14 years between 1990 and 2003. 

 The primary objective of these surveys was to estimate 

 abundance and trends of harbor porpoise (Phocoena 

 phocoena), a small, cryptic nearshore cetacean; how- 

 ever, turtle sightings were also recorded systematically. 

 Surveys were restricted to good weather days, defined 

 as days with clear to partly cloudy skies and winds of 

 less than about 12 kt (Beaufort sea states of 0-3), The 

 transects followed a zigzag pattern designed to survey 

 systematically between the coast and the 92-m (50- 

 fathom) isobath, located less than 30 km offshore, and 

 covering the primary habitat for harbor porpoise (Fig. 

 1). During each survey year, 26 transects between Pt. 

 Conception and the Russian River (38°27'N) were rep- 

 licated 4-8 times, depending on weather conditions. An 

 additional 17 transects were surveyed 1-3 times a year 



off northern California between the Russian River and 

 the California-Oregon border. Total transect length was 

 916 km, and under good weather conditions all transects 

 were surveyed in two days. The full set of 43 transects 

 was surveyed during the years 1990, 1991, 1993, 1995, 

 1997, 1999, and 2002. A subset of the transect lines 

 (between Pt. Sur and Pt. Arena) was surveyed during 

 2000, 2001, and 2003 to provide further information 

 on leatherback turtle occurrence off central California 

 during these years. 



Details of the survey methods have been reported 

 elsewhere (Forney et al., 1991), and only a summary 

 of key methods is provided here. The survey platform 

 was a high-wing, twin-engine Partenavia P-68 aircraft, 

 with two bubble windows for lateral viewing and a belly 

 port for downward viewing. The survey team consisted 

 of three observers (one on the left, right, and belly) and 

 one data recorder. Distances to sighted animals were 

 calculated from the declination angle to the sighting 

 when abeam of the aircraft (obtained with a hand-held 

 clinometer) and the aircraft's altitude. Surveys were 

 flown at 167-185 km/h (90-100 kt) airspeed and 213 m 

 (700 ft) altitude. Sighting information and environmen- 

 tal conditions, including Beaufort sea state, percentage 

 cloud cover, and horizontal sun position (to measure 

 glare direction) were recorded and updated throughout 

 the survey by using a laptop computer connected to the 

 aircraft's LORAN or GPS navigation system. 



Visibility of submerged leatherback turtles is de- 

 pendent upon water clarity and color contrast of the 

 animals. When viewed from the air, this species gener- 

 ally exhibits considerable contrast to the surrounding 

 waters within the California study area. To approxi- 

 mate the visibility of turtles at varying depths below 

 the surface, a calibration experiment was conducted 

 by using a set of multiple light-colored Secchi disks 

 submerged at 1 m, 2 m, 3 m, and 4 m depth. During 

 overflights, observers recorded the maximum visible 

 Secchi disk depth. The proportion of time leather- 

 back turtles spent within the visible depth range was 

 estimated from dive data obtained during 2005 on 

 free-swimming turtles, using a suction cup apparatus 

 containing a VHF transmitter and a Lotek LTD 1110 

 time-depth-recorder (TDR) (Lotek, St. John's, New- 

 foundland, Canada). The suction cup apparatus (280 

 g weight in air) was attached to the dorsal surface of 

 three leatherback turtles by using a pole from a small 

 vessel, without capturing or handling the animal. The 

 TDRs recorded depth every 5 sec at a resolution of 0.5 

 m and with an accuracy of ±1%. Tagged turtles were 

 tracked for several hours until the tag disengaged 

 from the animal or was actively removed with the 

 pole, creating little or no disturbance to the animal. 

 Potential posttagging effects were examined by visu- 

 ally inspecting the full dive profile, and by analysis 

 of variance to test for differences in the time spent 

 within the estimated visual depth range between the 

 first 30 minutes and subsequent 30-minute periods. All 

 deployments occurred during daylight hours between 

 12:00 and 16:30 local time. 



