FISHERY BULLETIN: VOL. 86, NO. 3 



found within 0.25 nautical miles (nmi) of the shore- 

 line. We therefore designed our aerial surveys to 

 cover a very narrow coastal band. Subsequent in- 

 formation from ship surveys (Barlow 1988) has 

 shown their distribution to extend considerably far- 

 ther from the coast. Therefore, estimates of porpoise 

 density from aerial surveys apply to a relatively 

 small portion of harbor porpoise habitat. For this 

 reason we do not estimate population size by ex- 

 trapolating aerial density estimates to the entire 

 area inhabited. The density estimates presented 

 here are used to corroborate estimates based on ship 

 surveys and to estimate density for areas that were 

 too shallow to be surveyed by ship. 



Based on previous studies of dive times (Watson 

 and Gaskin 1983; Taylor and Dawson 1984) we ex- 

 pected a proportion of the harbor porpoise to be 

 diving and therefore missed by aerial observers. 

 Shore-based studies were conducted in September 



1985 from cliffs in northern Oregon to determine 

 average dive times for west-coast harbor porpoise. 

 Helicopter observations were made in April and May 



1986 in Monterey Bay and near Bodega Head, CA 

 to gather dive time information and obtain a direct 

 measure of the fraction of time that harbor porpoise 

 groups are visible from the air. These two samples 

 did not differ significantly from previous samples 

 of harbor porpoise in Alaska, so all samples were 

 pooled to adjust estimates of porpoise density from 

 aerial surveys to account for the probability of miss- 

 ing submerged animals. 



METHODS 



Aerial Survey Methods 



Strip transect methodology (Seber 1973) was used 

 during the aerial surveys. This method assumes that 

 all individuals within a transect strip are detected. 

 Transect lines were flown parallel to the coast line 

 at distances of 0.61 and 1.85 km (0.33 and 1.0 nmi) 

 offshore. Transect strips of equal width were sur- 

 veyed on both sides of the aircraft. The margins of 

 the strips were denoted by tape marks or streamers 

 on the wing struts. Strips were divided into inside 

 and outside swaths of unequal width (Fig. 1) by a 

 third tape mark or streamer between the other two. 

 When porpoise were sighted within the transect 

 strip, the pilot was directed to leave the transect 

 line and circle over the porpoise to obtain an ac- 

 curate count of the number within the original group 

 that was sighted. If additional groups or individuals 

 were sighted during this circling, they were ex- 

 cluded from density estimates. Porpoise density, x, 



was calculated as the number of individuals sighted 

 within a transect, n, divided by the product of the 

 transect width, w, times the distance, d, that was 

 flown: 



X = n/iW  d) 



(1) 



We used both single and twin-propeller, high- 

 wing, 4-passenger aircraft in our surveys. The 

 search team consisted of two observers seated in the 

 right and left passenger seats. A data recorder sat 

 in the copilot's seat and did not search. If the 

 recorder sighted animals that were missed by the 

 observers, these were noted but were not included 

 in density estimates. The planes were flown at an 

 altitude of 213 m (700 feet) and at an airspeed of 

 158-167 km/h (85-90 knots). The original survey 

 plan called for all sections of the coast to be covered 

 twice on each survey. This was accomplished in 

 1984, but poor weather in 1985 resulted in the 

 Washington coast and part of the Oregon coast be- 

 ing covered only once. The dates flown and areas 

 covered are given in Table 1. 



Table 1.— Dates, areas covered, and observer teams during 

 aerial surveys for harbor porpoise. Geographic regions refer 

 to those shown in Figure 2. Observer team refers to a pair of 

 individuals. 



Data gathered on both the 1984 and 1985 surveys 

 were similar in format. Recorded data on sighting 

 conditions included Beaufort sea state, a measure 

 of cloud cover, a code indicating the presence of haze 

 or fog, sun position relative to the aircraft, and a 

 subjective measure of the observers' ability to see 

 into the water through turbidity, surface reflection 

 and diffraction. The latter was called surface pene- 

 tration and was recorded separately for each of the 



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