Survey design

OSC’s PhD-qualified scientists are well-versed in survey design and can offer statistically-robust design of (i) distance-sampling surveys using software ‘Distance’, (ii) Before After Control Impact (BACI) surveys to investigate impacts of operations on local marine mammal population, investigations of environmental/operational drivers of animal presence, etc.

Distance sampling

Distance sampling is a survey method consisting of recording distances of detected animals from a survey transect or point, from which animal density and/or abundance can be estimated (Buckland et al., 2001; 2004). The typology of distance sampling implemented most commonly is line transect sampling, where a survey region is sampled by placing a series of evenly spaced parallel lines with a random start point (Thomas et al., 2010). A Marine Mammal Observer (MMO) aboard a survey vessel is transported along each line, with animals detected within a set distance (w) of the line recorded systematically, alongside their distance (m) from said line. In distance sampling, all animals on the line are assumed to be detected with certainty, although detection probability decreases with increasing distance from the line. In the instance of animals occurring in welldefined clusters, detections refer to clusters rather than to individual animals. High-quality survey design, like that offered by OSC, enables animal/population density estimates to be extended to the entire survey region, thus allowing for estimation of animal abundance as well as influence of oceanographic drivers on presence.

Before After Control Impact (BACI)

Before After Control Impact (BACI) survey design is a statistically powerful and effective method to assess magnitude of anthropogenic effects on ecological variables (e.g. marine mammal presence and abundance) whilst isolating such impacts from natural variability (Smith, 2014). In other words, the purpose of BACI surveys is to (i) establish whether a stressor, such as offshore operations, has altered the surrounding environment, (ii) identify components impacted adversely, and (iii) quantify magnitude of alterations. To compensate for occurrence of natural change, the survey area is paired to another site referred to as control area. The principle behind this survey design is that the ‘impact’ activity will cause a change in the ‘impact’ area from before to after it commences that differ from natural change in the control location. Patters of change and significance of response are analysed using statistical tests. Samples are collected typically at random, replicated time intervals before and after impact occurrence, preventing chance temporal fluctuations in either sites from confounding detection of an impact.

Schematic representation of process behind BACI survey design. © OSC (2020)


Buckland, S.T., Anderson, D.R., Burnham, K.P., Laake, J.L., Borchers, D.L., and Thomas, L. (2001): Introduction to Distance Sampling: Estimating Abundance of Biological Populations. Oxford University Press, Oxford, UK.

Buckland, S.T., Anderson, D.R., Burnham, K.P., Laake, J.L., Borchers, D.L., and Thomas, L. (2004): Advanced distance sampling: estimating abundance of biological populations. Oxford University Press, Oxford, UK, pp. 416.

Smith, E.P. (2014): BACI design. In A. H. El-Shaarawi, and W. W. Piegorsch (Eds): Encyclopedia of Environmetrics, Vol. 1. John Wiley & Sons, Ltd, Chichester, pp. 141-148.

Thomas, L., Buckland, S.T., Rexstad, E.A., Laake, J.L., Strindberg, S., Hedley, S.L., Bishop, J.R., Marques, T.A., and Burnham, K.P. (2010): Distance software: design and analysis of distance sampling surveys for estimating population size. Journal of Applied Ecology 47, 5-14.

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