FISHERY BULLETIN: VOL. 86, NO. 3 



are not met (Burnham et al. 1980; Hammond and 

 Laake 1983). Of these assumptions, the most rele- 

 vant to this study are 1) the area must be sampled 

 randomly or the animals must be randomly distrib- 

 uted within the area; 2) all groups on the trackline 

 must be detected; and 3) group size must be esti- 

 mated without error. These assumptions will be 

 addressed below. 



To address the first assumption (random distribu- 

 tion), cruise tracks were chosen to systematically 

 cover the coast from Point Conception to Cape Flat- 

 tery. Because the surveys were designed to cover 

 the entire longshore range of harbor porpoise in this 

 area, randomly placed survey tracks were deemed 

 unnecessary. vVlthough some areas of the coast were 

 missed, these locations were determined by weather 

 and were presumably not correlated with porpoise 

 abundance. Surveys were, however, limited to a 

 very narrow strip along the 18 m isobath. Initially, 

 the choice of this survey track was based on the 

 observation that, in aerial surveys, harbor porpoise 

 were usually found within 0.5 km (0.25 nmi) of the 

 shoreline in California (Dohl et al. fn. 4). The 18 m 

 isobath was simply the shallowest reasonable work- 

 ing depth for the NOAA survey ships. In the course 

 of these surveys, it was found that harbor porpoise 

 are commonly distributed much further from the 

 coast than 0.5 km and that one survey track could 

 not adequately cover their habitat. The offshore 

 distribution of harbor porpoise is not random, but 

 is related to water depth, distance from shore, or 

 both. The model from which I extrapolated density 

 at 18 m to density at other depths was based on a 

 rather limited sample at a few locations along the 

 coast. The assumption of random search in offshore 

 areas was not met. Additional work is required to 

 evaluate the effect of this. 



The second assumption is that 100% of the 

 animals in the immediate vicinity of the trackline 

 were detected. Animals near the trackline can be 

 missed because they move away from the path of 

 the ship, because they do not surface within the 

 visual range of the observers, or because the 

 observers fail to detect animals that do surface. Any 

 of these would result in a negative bias and an 

 underestimation of porpoise abundance using line 

 transect methods. These three problems are con- 

 sidered in more detail. 



West-coast harbor porpoise are commonly said to 

 avoid vessels (Flaherty and Stark fn. 11; Szczepa- 

 niak and Webber fn. 12) and may be missed or not 

 counted in the proper perpendicular distance 

 category for this reason. On the surveys, the major- 

 ity of harbor porpoise were oriented roughly parallel 



to the ship at the time they were sighted and were 

 swimming parallel to the ship and in the opposite 

 direction (see footnote 5). This was also observed 

 in one instance from the helicopter; however, in that 

 case the group first moved perpendicular to the path 

 of the ship. These observations indicate that harbor 

 porpoise are reacting to the ship before they are 

 seen by observers. Reaction to and avoidance of the 

 ship does not necessarily mean that estimates of 

 trackline density are biased if animals are detected 

 before they travel an appreciable distance from the 

 trackline. In several instances, harbor porpoise sur- 

 faced within 50 m of the ship and directly in its path. 

 These animals appeared startled and quickly moved 

 to avoid the ship. In these cases, the rapid move- 

 ment of the animals and splashes associated with 

 that movement made the animals more visible to 

 observers. Because avoidance behavior may make 

 harbor porpoise more visible and because the 

 distributions of perpendicular distance show only a 

 single mode (at the origin), vessel avoidance prob- 

 ably does not introduce a large bias in harbor por- 

 poise abundance estimation. More work is needed 

 in this area. 



Harbor porpoise near the trackline may also be 

 missed if they either inadvertently or intentionally 

 do not surface within the visual range of the ob- 

 servers. Typical mean dive times for harbor porpoise 

 have been measured as 1.5-2.3 minutes (Glacier 

 Bay, AK; Taylor and Dawson 1984), 1.8 minutes 

 (northern Oregon; B. Taylor^^), and 0.4-1.4 minutes 

 (Bay of Fundy; Watson and Gaskin 1983). The ships' 

 speed during surveys was approximately 18.5 km/h 

 or 310 m/min; thus, in 2 minutes the ship would 

 travel 620 m. The average distance at which animals 

 were first seen was 704 m from the ship. If in- 

 dividual dive times were appreciably longer than 2 

 minutes, some trackline individuals would not be 

 detected by observers. In data collected in north- 

 ern Oregon, 16% of dive times were greater than 

 2.5 minutes (B. Taylor fn. 13). In addition, harbor 

 porpoise have been reported to increase dive times 

 up to 7 minutes in the presence of boat traffic 

 (Flaherty and Stark fn. 9). (This latter estimate is 

 considerably longer than any other published esti- 

 mate, and it is possible that those researchers missed 

 one or more surfacings). Helicopter observations in 

 Monterey Bay indicated that porpoise groups did not 

 extend dive times in the presence of the survey 

 vessel (see footnote 5). This area might not be repre- 

 sentative, however, because harbor porpoise may 



''B. Taylor, Department of Biology, University of California, San 

 Diego, CA 92093, pers. commun. August 1986. 



430 



