Fishery Bulletin 93(1). 1995 



istration (NOAA) vessel McArthur to assess the abun- 

 dance of cetaceans in California waters. Primary 

 cruise tracks were drawn for a unifirm survey of the 

 814,900 km 2 area between the 18-m (10-fathom) 

 isobath and approximately 555 km (300 nmi) offshore 

 (Fig. 1). 



Primary observation team 



The basic survey method was that which was devel- 

 oped and used to estimate the abundance of small 

 cetaceans in the eastern tropical Pacific (Holt and 

 Powers, 1982; Holt, 1987; Holt and Sexton, 1989; 

 Wade and Gerrodette, 1993). The primary observa- 

 tion team consisted of three observers who searched 

 from a viewing height of 10 m above the sea surface: 

 two observers searched with 25x pedestal-mounted 

 binoculars; the third observer searched with unaided 

 eye, and (occasionally) 7x binoculars, and also served 

 as data recorder. Observers rotated among these 

 three duty stations every 1/2 hour, and two observer 

 teams alternated work and rest periods every two 

 hours. Sighting effort was maintained from dawn to 

 dusk whenever weather conditions allowed, and 

 searching covered the entire region from directly in 

 front of the vessel to 90 degrees left and right and 



Figure 1 



Transect lines (thin solid lines) completed during the survey. The 

 bold polygon indicates the limit of the main study area. 



out to the horizon. Data were recorded on a lap-top 

 computer that had direct input from the ship's GPS 

 (Global Positioning System) navigation system. Re- 

 corded data included sighting conditions (sea state, 

 cloud cover, sun position, etc.), observer positions, 

 the beginning and end of effort, and information per- 

 taining to sightings. 



When a sighting was made, all observers were 

 made aware of the animals' location. The perpendicu- 

 lar distance from the trackline to the center of the 

 group was estimated from the initial bearing and 

 distance. The initial bearing of a cue (a blow, a splash, 

 or a sighting of animals) was measured relative to 

 the bow of the vessel by means of a calibrated collar 

 on the base of the yoke of the 25x binoculars. The 

 initial distance was typically estimated from a cali- 

 brated reticle scale in the oculars of both the 25x 

 and 7x binoculars with the formula derived by Smith 

 ( 1982) and was calibrated by using radar-measured 

 distances to inanimate objects (Barlow and Lee, 

 1994). If a shore horizon was closer than 11.1 km (6 

 nmi), distance was estimated by comparison with the 

 radar-measured distance to shore. Occasionally, for 

 very close animals seen only by the third observer, 

 sighting distances and angles were estimated by eye. 

 If a cue turned out to be a cetacean, effort was inter- 

 rupted and the ship was typically diverted 

 towards the animals in order to obtain esti- 

 mates of species composition and group size. 

 The vessel was not typically diverted for ce- 

 taceans that were greater than 5.55 km (3 nmi) 

 perpendicular distance from the trackline. 



Species identification was made collec- 

 tively by the team, but quantitative estimates 

 of species composition and group size were 

 made independently by each observer. For 

 estimation purposes, a group was defined as 

 a collection of closely associated individuals 

 (typically within several body lengths of each 

 other) that exhibited cohesive behavior. In 

 the field, however, a single distant sighting 

 might prove to be two behaviorally distinct 

 groups upon closer inspection. In such cases, 

 when it was impossible to determine which 

 was the original group sighted, both groups 

 were pooled to estimate group size and spe- 

 cies composition. For mixed-species groups, 

 species composition was recorded as an 

 observer's estimate of the percentage of each 

 species present in the group. The observers 

 recorded species composition and group-size 

 data in confidential personal notebooks, and 

 the data were transcribed at the end of the 

 day into the computer data record by the 

 cruise leader. 



