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

Detection functions for boat-and-helicopter sightings (Fig. 5, C and D) are relatively smooth in comparison with those presented in Turnock et al. (1995). The detection 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), confirming that densities in this zone are low.

Information on sea state is usually collected during 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 confidence 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[degrees] placement of lines facilitated this reduction in pitching because it provided two alternative sets of lines (at 90[degrees] 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 afford 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 estimate for this area by less than 6% (Gormley, 2002; Jolly-Seber model allowing different capture probabilities between first and subsequent captures).