Author Identifier

Jennah Tucker

Date of Award


Document Type



Edith Cowan University

Degree Name

Bachelor of Science Honours


School of Science

First Supervisor

Chandra Salgado Kent

Second Supervisor

Holly Raudino

Third Supervisor

Kelly Waples


Aerial surveys are commonly used to estimate abundance of marine mammal populations, to detect trends over time and inform conservation management. Estimating abundance in areas critical to a species’ survival and reproduction is important to assess that population’s status and its vulnerability. Aerial surveys allow relatively large areas to be covered over short periods of time which can be important for highly mobile species with wide-ranging movement patterns such as marine mammals. While aerial surveys are often designed for specific target species, information on additional non-target species is often recorded, as surveys are generally infrequent due to expense and logistical constraints. However, estimating absolute abundance of non-target species can have limitations. For example, in areas that have dense populations of target species, observers’ time may be limited, and information recorded for non-target species may be constrained or incomplete. This study aimed to evaluate the potential for extending aerial surveys designed for target species to estimate absolute (‘true’) abundance of non-target species, accounting for biases including those that were not available to be seen by observers (availability bias) and those that were present but missed (perception bias). A strip transect sampling approach was taken as distances to nontarget species were not measured (i.e., distance sampling could not be undertaken). This project aimed to validate these estimates by comparing observer derived estimates from a range of plausible strip-widths through simulation (as exact strip widths were not recorded for non-target species) with estimates from simultaneously collected aerial images with a known strip width, to identify the ‘best’ strip widths for which to estimate abundance. In addition, the percentage of groups containing calves was evaluated to further inform the importance of the study area. This study used existing data from aerial surveys conducted in Exmouth Gulf, Western Australia (by WA Dept. of Biodiversity, Conservation and Attractions) designed for dolphins (the target species) to produce estimates of non-target humpback whales (Megaptera novaeangliae) and dugongs (Dugong dugon). The resulting ‘best’ strip widths varied depending on the correction factor applied to account for availability bias of the informing abundance estimates, and the number of detections captured in the images across days, consequently leading to variable absolute abundance estimates where these were applied. For humpback whales, estimates were 2960.95 (CV = 44.0%) and 3915.38 (CV = 51.2%), using a ‘best’ half strip width of 637.05m with an availability correction derived from drone data and a half strip width of 833.62m with an availability correction calculated from boat-based data, respectively. For dugongs, absolute abundance was estimated as 838.42 (CVport = 114.59%, CVstarboard = 114.72%) and 1013.36 (CV = 40.46%), using a ‘best’ half strip width of 281.12m and corrected for availability as per Marsh & Sinclair (1989a) and a strip width of 259.92 with a Pollock et al. (2006) availability correction applied, respectively. This study highlighted the need for an adequate survey intensity to improve the reliability of images as a validation tool as well as access to site-specific data on species availability to produce robust abundance estimates. Despite limitations in the extension and validation of these non-target data, the estimates produced here provide the first absolute abundance estimate of humpback whales in Exmouth Gulf and contribute to informing ongoing monitoring and management needs for the proportion of humpback whale and dugong populations that use Exmouth Gulf as critical habitat. Furthermore, the results from this work highlight the opportunities and limitations and provide recommendations for extending aerial survey effort beyond target species. This is particularly important for maximising the potential knowledge gained where funding or logistics may be limiting.