# Distance Sampling

Distance sampling is widely used for estimating the abundance of wild animal populations. St Andrews provides software implementing our methodologies for designing and analysing distance sampling surveys. This software has been downloaded by over 35,000 users in over 110 countries. The two most widely used forms of distance sampling are line and point transect sampling. In line transect sampling, an observer travels along a line, and records distances from the line of detected animals. A model for the probability of detection as a function of distance from the line is fitted, from which abundance is estimated. In point transect sampling, an observer visits points, and records distances of detected animals from each point. In the mural, the equation Pa = ... describes the average detection probability in the surveyed area for line transects.

### How many dolphins are in the sea?

One of the big question in wildlife conservation is 'how many are there?' If you would like to know, for example, how many spinner dolphins are in the eastern tropical Pacific, it is easy to see that you cannot count them all. Hence, we use distance sampling methods to estimate the number (or abundance) of these dolphins. The way this works can be described in short (and very simplified): a ship with a team of observers travels on pre-defined transect lines across the study area covering a proportion of the total area. The observers scan the ocean all day during daylight hours and record each school of dolphins that they detect along with information on weather and sighting conditions. For each school of dolphins they also record how many dolphins are in the school, what species it is and, very importantly, how far these dolphins were from the transect line. Back in the office, we use this information on the distance of the schools from the line to estimate how many schools we missed in the area that we covered with our search and arrive at an estimate of how many dolphins there were in the covered area. We also know the proportion of the total area we covered and, hence, can scale up our our estimate of numbers in the covered area to an estimate of dolphins in the study area.