Dawson et al.: Line-transect surveys of Cepha/orhynchus hecton 



449 



with those presented in Turnock et al. (1995). The de- 

 tection function for the duplicate sightings (Fig. 5E) was 

 more difficult to fit. Given the restricted sample size of 

 duplicates (n=33), this result is not unexpected. 



In the 1998-99 Timaru to Long Point and 1999-2000 

 Motunau to Farewell Spit surveys, robust estimation 

 of ESW was facilitated by addition of extra sightings 

 gained under similar sighting conditions at Banks 

 Peninsula (Fig. 5, C, F, G). None of the three surveys 

 showed significant evidence of larger groups being seen 

 farther away. A broad pattern of abundance declining 

 to the north and south of the Timaru-Banks Peninsula 

 area is evident (Fig 2, Table 2). We made six sightings 

 on 288 km of offshore lines (4-10 nmi offshore), con- 

 firming that densities in this zone are low. 



Information on sea state is usually collected dur- 

 ing boat line-transect surveys and sometimes used to 

 poststratify data (e.g.. Barlow, 1995). In our study this 

 was not advantageous, for three reasons. 1) We avoided 

 collecting data in conditions with whitecaps; therefore 

 only a few sightings were collected in Beaufort 3. Hence 

 variance estimates for this Beaufort state are large. 2) 

 Differences among Beaufort states for key parameters 

 such as sighting rate, average group size, and effective 

 strip width were small and showed overlapping confi- 

 dence intervals (we concede that statistical power is 

 low because of reason 1 stated above). Note that data 

 were pooled in the same way as for ESW estimation. 

 3) Stratification by Beaufort state does not produce 

 abundance estimates that match the zones of intrinsic 

 management interest (e.g., Banks Peninsula Marine 

 Mammal Sanctuary; Dawson and Slooten, 1993). 



Discussion 



The catamaran survey platform was a near-ideal vessel 

 for close inshore surveys. The sighting platform (Fig. 1) 

 was a relatively inexpensive modification (-US$2000) 

 that could be dismantled in about 10 minutes to allow 

 sailing. The vessel's minimal draught allowed coverage 

 of very shallow areas, which are an important part of the 

 distribution of Hector's dolphin and many other inshore 

 cetaceans. Although catamarans are inherently resistant 

 to rolling, pitching can be a problem when motoring 

 into a head sea or swell. We minimized this pitching by 

 arranging lines so they could be run down-swell. The 45° 

 placement of lines facilitated this reduction in pitching 

 because it provided two alternative sets of lines (at 90° 

 to one another). Further, these could be run inshore or 

 offshore, allowing a choice of four options. 



A significant advantage of vessels with low running 

 costs is that the cost of training is low. We could af- 

 ford to spend 7-10 days training before each survey. 

 Further, waiting for weather to improve is inexpensive; 

 therefore one does not need to gather data in marginal 

 sighting conditions. 



Estimated abundances (Table 4) were not significantly 

 different from those estimated in the 1984-85 strip 

 transect survey. Recent mark-recapture estimates of 



dolphin abundance at Banks Peninsula in 1996, based 

 on photo-ID data, differed from the line-transect es- 

 timate for this area by less than 6% (Gormley, 2002; 

 Jolly-Seber model allowing different capture probabili- 

 ties between first and subsequent captures). 



Our surveys confirmed previous work showing the 

 patchy nature of Hector's dolphin distribution (Dawson 

 and Slooten, 1988). Research at Banks Peninsula on 

 the alongshore range of individually identified dolphins 

 has shown a mean alongshore range of about 31 km 

 (SE = 2.43; Brager et al., 2002). Despite wide-ranging 

 surveys over 13 years, the most extreme sightings of 

 any individual were 106 km apart. These data indicate 

 very high site fidelity and indicate that even small-scale 

 discontinuities in distribution may be long lasting. Lack 

 of extensive movement along-shore, and hence limited 

 contact with neighboring populations, is likely to be 

 the mechanism by which Hector's dolphin has become 

 segregated into genetically distinct populations (Pichler 

 et al., 1998; Pichler and Baker, 2000). 



The new abundance data, in combination with the 

 genetic data indicating segregation of Hector's dolphin 

 into four populations (Pichler and Baker, 2000) and 

 modeling work indicating that the species is in decline 

 in most of its range owing to bycatch in gill nets (Mar- 

 tien et al., 1999; Slooten et al., 2000), underscore the 

 urgent need for better information on bycatch rates. 



Despite strong evidence of bycatch throughout the 

 species' range, observer coverage sufficient to estimate 

 bycatch has been achieved only in one area (Canter- 

 bury) for one fishing season (1997-98; Baird and Brad- 

 ford, 2000). During this season six Hector's dolphins 

 were observed entangled in commercial gill nets (a 

 further two were caught but released alive), resulting 

 in a bycatch estimate of 17 individuals (Starr 3 ). One 

 mortality was observed in a trawl net, but very low 

 observer coverage prevented any calculations of overall 

 trawl bycatch (Baird and Bradford, 2000). No attempt 

 was made to assess bycatch in recreational gillnetting 

 during this period, but during a more recent summer 

 (2000-01) five Hector's dolphin mortalities occurred in 

 gill nets that were probably set by recreational fish- 

 ermen (Department of Conservation and Ministry of 

 Fisheries, 2001). It is not reasonable to assume that 

 all mortalities in recreational gillnets are detected. In 

 our opinion it is likely that combined commercial and 

 recreational gillnet bycatch off Canterbury is about 

 15-30 individuals per year. 



Hector's dolphin abundance on the north, east, and 

 south coasts of the South Island estimated from the sur- 

 veys reported in the present study is 1880 individuals 

 (CV=15.7%). Hector's dolphins are more common on the 



Starr, P. 2000. Comments on "Estimation of the total 

 bycatch of Hector's dolphins (Cephalorhynchus hectori) from 

 the inshore trawl and setnet fisheries off the east coast of 

 the South Island in the 1997-98 fishing year." Unpublished 

 paper presented to Conservation Services Levy Working 

 Group, 28 p. Department of Conservation, P.O. Box 10-420 

 Wellington. New Zealand. 



