Small-boat surveys for coastal dolphins: line-transect surveys for Hector's dolphins

Fishery Bulletin, July, 2004 by Stephen Dawson, Elisabeth Slooten, Sam DuFresne, Paul Wade, Deanna Clement

Observers used reticle- and compass-equipped Fujinon 7x50 (WPC-XL) binoculars to make sightings and to measure the downward angle from the land, or horizon, to the sighting. If the former, the corresponding distance to land was measured with RADAR (Furuno 1720 model), or, if within a few hundred meters of shore, with a Bushnell lightspeed laser rangefinder (tested accuracy [ or -]1 m from 12 to 800 m). We calibrated the accuracy of the RADAR by comparison with transit fixes and laser rangefinder measurements. Sighting angles were recorded by using angle boards (see Buckland et al., 1993) in the first season, and thereafter with the compasses in the binoculars. There were no ferrous metals or significant electrical fields within 6 m of the sighting platform.

Navigation was facilitated by the use of a Cetrek 343 GPS chartplotter with digitized C-MAP charts onto which transect waypoints were plotted. Depths were measured with a JRC JFV-850 echosounder (at 200 kHz).

At the start of each survey, several days were spent training observers at Banks Peninsula, where sighting rates are high. Training continued until we gained about 100 sightings (data gathered in this period were not used in the analyses). An observer manual (available from authors) specified scanning behavior and recording methods. To ensure a wide shoulder on the histograms of perpendicular sighting distances, observers were instructed to concentrate their effort within 45[degrees] of the trackline and to spend less time searching out to 90[degrees]. Observers spent about 85% of the time scanning with binoculars. Regular scans with the naked eye minimized fatigue and reduced the chance of missing groups close to the boat. To promote consistency, observers were asked to re-read the manual at least once a week throughout the survey.

While the survey was underway, exploratory data analyses were undertaken to assess data quality. These analyses showed that in the early stages of the first survey, observers were rounding angles of sightings close to the trackline to zero. The use of the angle boards was modified to minimize this problem, and they were not used in subsequent surveys. The data from these early lines were discarded and the survey lines repeated.

Survey effort was restricted to sea conditions of Beaufort 3 or less and swell heights of [less than or equal to]2 meters. Transect lines were run down-swell and down-sun to minimize pitching and effects of glare. Deviations of up to 10[degrees] from the intended course were made if needed to further reduce pitching or glare. The inshore end of each line was surveyed to just outside the surf zone on open coasts, or until a 2 m depth was reached, or to within 50 m of rocky shores. All surveys were conducted in passing mode to minimize the extent of vessel attraction.

Line-transect data were collected in three surveys in three consecutive summer seasons, each focussing on a particular coastal area (Fig 2; Motunau to Timaru, 5 January-21 February 1998; Timara to Long Point, 9 December 1998-16 February 1999; Farewell Spit to Motunau, 17 December 1999-28 January 2000).