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

Abstract--Management of coastal species of small cetaceans is often impeded by a lack of robust estimates of their abundance. In the Austral summers of 1997-98, 1998-99, and 1999-2000 we conducted line-transect surveys of Hector's dolphin (Cephalorhynchus hectori) abundance off the north, east, and south coasts of the South Island of New Zealand. Survey methods were modified for the use of a 15-m sailing catamaran, which was equipped with a collapsible sighting platform giving observers an eye-height of 6 m. Eighty-six percent of 2061 km of survey effort was allocated to inshore waters (4 nautical miles [nmi] or 7.4 km from shore), and the remainder to offshore waters (4-10 nmi or 7.4-18.5 km from shore). Transects were placed at 45[degrees] to the shore and spaced apart by 1, 2, 4, or 8 nmi according to pre-existing data on dolphin density. Survey effort within strata was uniform. Detection functions for sheltered waters and open coasts were fitted separately for each survey. The effect of attraction of dolphins to the survey vessel and the fraction of dolphins missed on the trackline were assessed with simultaneous boat and helicopter surveys in January 1999. Hector's dolphin abundance in the coastal zone to 4 nmi offshore was calculated at 1880 individuals (CV=15.7%, log-normal 95% CI=1384-2554). These surveys are the first line-transect surveys for cetaceans in New Zealand's coastal waters.

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Several international workshops on cetacean bycatch problems have stated that a key impediment to the conservation of coastal and riverine small cetaceans is the lack of quantitative data on abundance (e.g., IWC, 1994). An important reason for this lack of data is that line-transect surveys are often conducted from large (>50 m) vessels (e.g. Barlow, 1988) and hence are extremely expensive ($US 10,000/ day). Such costs usually put high-quality surveys such as those conducted for harbor porpoise in the U.S. (e.g., Carretta et al., 2001) beyond the reach of less affluent nations. The need for abundance estimates is especially great for the coastal and riverine species found in Asia, Africa, Australasia, and South America (Table 1). Several of these species have apparently small populations and restricted distributions, and all suffer from being taken as bycatch in fishing gear, principally in gill nets (IWC, 1994). In addition, it is difficult or impossible for large vessels to work close to shore, in shallow waters, where some of these species are most common.

The work described in this contribution had two aims: 1) to adapt ship-based line-transect methods (e.g., Barlow, 1988) to a 15-m catamaran, and 2) to provide an updated estimate of the abundance of Hector's dolphin (Cephalorhynchus hectori). Hector's dolphin, a small delphinid found only in the inshore waters of New Zealand, is subject to bycatch in gill nets throughout its range (Dawson et al., 2001). At least in the Canterbury region, and off the North Island west coast, recent catch levels are clearly unsustainable (Dawson and Slooten, 1993; Martien et al., 1999; Slooten et al., 2000; Dawson et al., 2001). Studies of mt-DNA indicate that the very small North Island population is distinct and that there are at least three separate populations in South Island waters (Pichler et al., 1998; Pichler and Baker, 2000; see also Baker et al., 2002). At the time of the present study the only quantitative population estimate was from a strip-transect survey conducted in 1984-85 (Dawson and Slooten, 1988), in which the offshore distribution, as well as the proportion of dolphins detected within the strip, was estimated. A current, more robust estimate is needed for management. This study describes line-transect boat surveys conducted to estimate Hector's dolphin abundance on the north, east, and south coasts of the South Island of New Zealand.

Materials and methods

Vessel choice and field methods

Displacement catamarans are inherently suitable for inshore surveys because of their resistance to rolling and their ability to sustain reasonably high cruising speeds with modest power. We based our surveys from a 15.3-m sailing catamaran (RV Catalyst), which is powered by two 50-hp diesel engines, and cruises at 9-10 knots while using <10 liters of fuel per hour. We fitted a collapsible aluminum sighting platform (~6 m eye height; Fig. 1) to increase the resolution with which observers could measure the downward angles to sightings (sec Lerczak and Hobbs, 1998, for details) and to allow the observers to see animals farther away. The surveys were conducted with a crew of six (five observers, one skipper).

Three people stood on the platform at any given time; one scanned the surface waters to the right of the platform, and the other scanned to the left, and a third person (the recorder) recorded sightings dictated by the observers. Sightings made by the recorder were not used in our analyses because his or her sighting effort was unavoidably uneven (the recorder could not make sightings while recording another sighting). The recorder did not point out sightings to observers. Observers and data recorder rotated every 30 minutes to avoid fatigue. Although Hector's dolphins are easily identified from other species, and group size is typically small (usually 2-8; Dawson and Slooten, 1988), in order to maintain even sighting effort on both sides of the trackline, observers did not confer during a sighting. Sighting information was entered into a custom-written program on a Hewlett-Packard 200LX palmtop computer on the sighting platform. Data recorded included horizontal sighting angle, downward angle to sighting (in reticles), species, group size, orientation of the animals when first sighted, depth, Beaufort sea state, swell height, glare, GPS fix, date, and time. The program also recorded survey effort by storing a GPS fix every 60 seconds. Weather conditions were recorded at the start of field effort, and whenever they changed.