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

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 (Martien 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 (Canterbury) for one fishing season (1997-98; Baird and Bradford, 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 fishermen (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 surveys reported in the present study is 1880 individuals (CV=15.7%). Hector's dolphins are more common on the South Island west coast, where an aerial survey of similar design resulted in an estimate of 5388 (CV=20.6%; Slooten et al., in press). Thus Hector's dolphin abundance in South Island waters is estimated at 7268 individuals (CV=15.8%). The North Island subspecies of Hector's dolphin, now considered critically endangered (IUC[N.sup.4]) remains to be surveyed quantitatively.

The new abundance estimates provide an empirical basis from which to calculate levels of take that would still allow the currently depleted populations to recover (e.g., Wade, 1998). These levels of take should be seen as short-term targets for bycatch reduction in gill and trawl nets. For the management of Hector's dolphin to be put on a rational basis, a more comprehensive and wide-ranging assessment of bycatch, including statistically robust observer programs in coastal fisheries, is urgently needed.