Mother Nature hasn't been too cooperative this month, as the aerial survey work for the joint project with the Cawthron Institute on estimating the abundance and distribution of Hector's dolphin along the south coast of the South Island, drew to a close. Bad weather had interrupted the surveys throughout the project, causing constant delays and forcing the survey period to keep getting extended, although fortunately, we got there in the end.
The plan had been to conduct aerial line-transect surveys across the entire area of interest once, then conduct additional surveys in a known high density area (Te Waewae Bay) to get a sufficient number of sightings for the distance sampling analysis. The general rule of thumb is that a minimum of 60-80 sightings is required for distance sampling methods to work well. Based on previous Hector's dolphin surveys in the area, it was expected (or more accurately, it was hoped) that a sufficient number of dolphin groups would be sighted if Te Waewae Bay was surveyed a total of five times. We also had a back up plan of conducting an additional two surveys of Te Waewae Bay if there were an insufficient number of sightings after the five surveys. The team managed to squeeze in four complete surveys of Te Waewae Bay and were part way through their fifth before survey work was halted due to weather. At this point we had to decide whether we had enough data, or whether to keep the team on standby to wait for a break in the weather, and then conduct the two extra surveys.
A quick preliminary analysis using a mark-recapture distance-sampling method we developed for early Hector's dolphin projects was required as the number of groups sighted was close to the minimum. Did we have enough data to get a reasonably accurate abundance estimate? After a quick flurry of data verification, coding and analysis, the preliminary results suggested that the precision of our abundance estimate from the available data was about what we had expected to get for the project. The decision was made to end the survey, which the observers were very happy about after having to endure a few rather bumpy flights during the past 4 weeks. We'll be sure to post the final results of this survey work once they are made public.
