BARLOW: SHIP SURVEYS OF HARBOR PORPOISE 



Survey ^ was from 24 April to 5 May 1986 and 

 was designed to investigate factors which affect har- 

 bor porpoise density estimation. The surveys con- 

 centrated on several areas of high porpoise density 

 in central California. The vessel's activities were 

 coordinated with a helicopter to gather information 

 on the avoidance of the ship by harbor porpoise. Ex- 

 periments were also conducted on survey 4 to deter- 

 mine whether an independent team of 3 observers 

 would sight any porpoise that were missed by the 

 primary team of 5 observers. Data from this survey 

 were not used for density or abundance estimation. 



Typically, 8-10 observers were used on each 

 survey, with a rested observer starting every half 

 hour and rotating through 5 primary observation 

 positions at half-hour intervals. The 5 positions con- 

 sisted of port and starboard inboard observers, port 

 and starboard outboard observers, and a recorder 

 positioned amidship. The inboard observers 

 searched with 7 power (7 x ) binoculars from straight 

 ahead to 90° (survey 1) or to 45° (surveys 2, 3, and 

 4) on their respective sides of the vessel. On survey 

 1 the outboard observers searched with 25 x, 

 pedestal-mounted binoculars. Although sightings 

 could be made at great distances from the vessel 

 using the 25 x binoculars, these distant sightings 

 contributed little to the estimation of trackline den- 

 sity, and use of 25 x binoculars was discontinued. 

 On subsequent surveys, both the inboard and out- 

 board observers used 7 x binoculars. The outboard 

 observers searched from straight ahead to 90° on 

 their respective sides of the vessel. The recorder 

 searched in the immediate vicinity of the ship using 

 unaided eyes and (intermittently) 7x binoculars. 



On survey 2, only 3 observation positions were 

 used from Point Conception to Point Sur, CA and 

 from Point Reyes, CA to Cape Flattery, WA. When 

 effort was reduced to 3 observers, the inside obser- 

 vation positions were eliminated. 



On survey 4, a second team of 3 observers was 

 added to monitor the effectiveness of the principal 

 team. This monitor team searched using unaided 

 eyes and (intermittently) 7x binoculars from the 

 pilothouse deck (viewing height approximately 7 m 

 from sea surface). The principal team and the moni- 

 tor team did not communicate sighting information, 

 and independent records were kept. 



Data were noted by the recorder on data coding 

 forms. Data on search effort included the beginning 

 and ending times and positions for continuous legs 

 of effort, the ship's heading and speed, personal 

 identification codes for the observers, sea surface 

 temperature, water depth, Beaufort sea state, sun 



position relative to the ship, and codes indicating 

 the presence of rain or fog within 5 km. The ship 

 position was determined from a Loran navigational 

 system or by triangulation using coastal landmarks 

 and dead reckoning. Ship speed was recorded direct- 

 ly from the Omega/Loran system or was calculated 

 based on time and distance traveled between suc- 

 cessive position fixes along straight transect lines. 

 Water depth was measured using a 38 kHz acoustic 

 depth sounder. 



Data for sightings consisted of the above ele- 

 ments, plus estimated group size, distance to shore, 

 an estimate of the angle between the trackline of 

 the ship and the group, and an estimate of the 

 distance from the ship to the group. Group size 

 refers to all the individuals associated with a sight- 

 ing event. In most cases, groups were closely asso- 

 ciated individuals that surfaced together (mean = 

 2.92, median = 2.0). In two cases (Point Arena and 

 Monterey Bay, CA), groups consisted of 50-80 loose- 

 ly associated individuals that were organized in 

 subgroups of 4-10). Group size was estimated and 

 recorded independently by each observer; the mean 

 of these estimates was used in subsequent analyses. 

 The angle from the trackline to the porpoise was 

 estimated visually with the aid of a pelorus mounted 

 in front of the observer stations, or, when 25 x 

 binoculars were used, from a calibrated collar on the 

 pedestal mount. On surveys 1 and 2, distances to 

 harbor porpoise were estimated visually using the 

 radar distance-to-shore as a reference, or, when 25 x 

 binoculars were used, distances were estimated 

 using calibrated reticles in the oculars. On surveys 

 3 and 4, distances were estimated using calibrated 

 reticles in the oculars of 7 x binoculars. Data were 

 also collected on the porpoises' direction of travel 

 relative to the ship. 



The length of a transect was estimated as the 

 product of ship speed and elapsed time. To stratify 

 density estimates by sea state, rain, and fog, the 

 effort record was divided into segments during 

 which the sea state, rain, and fog codes did not 

 change. 



In five areas, information was collected on varia- 

 tion in harbor porpoise density with water depth. 

 During survey 3, three sections of the coast were 

 surveyed intensively (Fort Bragg to Cape Vizcaino, 

 CA; Cape Blanco to Coquille Point, OR; and Cape 

 Lookout to Tillamook Head, OR), with transect lines 

 following the 18, 56, 92, and 185 m isobaths. On 

 survey 4, the 18 and 46 m isobaths were surveyed 

 in Monterey Bay, CA and in the vicinity of the Rus- 

 sian River, CA. These data formed the basis of a 

 model (below) to extrapolate porpoise density from 



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