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Does sponge distribution lead to sponging behaviour by bottlenose dolphins in Shark Bay?

Tyne, JulianORCID: 0000-0002-0676-5659 (2008) Does sponge distribution lead to sponging behaviour by bottlenose dolphins in Shark Bay? Honours thesis, Murdoch University.

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Inter and intra-population variation in behaviour can be the result of genetic, ecological or social differences. Behaviour can be passed down from generation to generation via vertical or oblique transmission, or spread throughout a population within generations by horizontal transmission. Typically, social learning is studied by excluding ecological and genetic explanations. However, before ecological factors can be excluded they require full investigation. Shark Bay bottlenose dolphins (Tursiops sp.) have been observed carrying conical shaped sponges (Echniodictyum mesenterinum) on their rostra and foraging with them, which is believed to be the only documented tool use behaviour in cetaceans. Social learning may be responsible for this “sponging” behaviour within Shark Bay bottlenose dolphins. However, the relationship between sponging dolphins and ecological factors in the Western Gulf of Shark Bay has not been investigated previously.

This study examined the distribution and density of sponges in the Western Gulf of Shark Bay, Western Australia and whether the distribution of sponges influences sponging behaviour in bottlenose dolphins. Data on the distribution of sponging dolphins were provided by Dr Michael Krützen and his students at the University of Zurich and by Dr Lars Bejder, Murdoch University. In order to determine the distribution of sponges in the study area a rapid, non-destructive and cost effective methodology was developed to sample marine benthic habitats (Chapter 2). This system comprised of a video camera, attached to the apex of a stainless steel pyramid and pointed down towards a 1m2 quadrat, linked to a database to record the video image of the substratum and other data electronically in the field. The video image was then examined to determine the precent cover of seagrass, sponge type (conical or non-conical) and the total number of sponges. This system enabled 1,380 video quadrats to be collected from a study area covering approximately 248 km2, ranging in water depth from < 1 m to 16 m. The survey was completed in 16 days and an average of 18 videos per hour was recorded.

Fourteen percent of the 1,380 video samples contained sponges and only 4% contained conical sponges. Sponges were not observed in water depths of < 10 m. The depth data for each sample were interpolated to create a digital elevation model (DEM) for the Western Gulf. This bathymetric map shows deep narrow channels in the north western region of the study area where conical sponges appeared to aggregate (Chapter 3). The distribution of conical sponges varied from west to east when travelling from north to south in the study area. Generalised linear models showed that the predicted probability of the occurrence and number of conical sponges increased with increasing depth. In contrast, the predicted probability of the occurrence and cover of seagrass decreased with increasing depth. Sponges were not observed in the seagrass beds. The mean sponge density estimated for water >10 m deep was lower along 50 m transects swims (n=24) (<0.05 ± SE 0.01 m2), than from the video quadrats (0.37 ± SE 0.03 m2).

During two field seasons to the Western Gulf of Shark Bay (in 2007 and 2008), 42 sponging dolphins have been identified by researchers at the University of Zurich. Foraging sponging dolphins were found in a mean depth of 13.3 m, similar to that of conical sponges (13.5 m) and greater than the mean depth for the foraging non-sponging dolphins (9.8 m).

Similar to conical sponges, the presence of sponging dolphins increased significantly with increasing water depth. However, no significant evidence was found to show that the distribution of conical sponges may influence the distribution of sponging behaviour in bottlenose dolphins in the Western Gulf of Shark Bay.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Biological Sciences and Biotechnology
Supervisor(s): Bejder, Lars and Loneragan, Neil
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