FEATURE STORY
November 2013
By: Adam Junner, Hydrographic Surveyor Teledyne RESON
Ocean News & Technology
10
Biologists from Aberdeen University wanted to find out if sonar equipment could be used to learn more about the movements of basking sharks. Most of the current knowledge about basking sharks is based on what is visible at the surface and, as such, an investigation was sought to determine if surface observations matched what was happening below the surface. In particular, the scientists were interested to know if the sharks were ‘stacking’, and whether there was evidence of young sharks.
Figure 1. The vessel
To view videos and more images of the survey, please visit www.teledyne-reson.com/basking-sharks
Figure 2. Teledyne RESON SeaBat 7128 multibeam imaging system
Figure 3. The dry end
Figure 4. Basking shark within a few metres of the vessel
Figure 5. Dorsal fin, nose and tail visible at the surface
Figure 6. Basking shark within 15m range. The pectoral fins are visible here
11
Figure 7. Four basking sharks on the SeaBat interface Figure 8. Three basking sharks following each other nose to tail. The speckled/cloudy substance may be plankton
A cloudy/speckled substance occasionally appeared on the SeaBat interface. Its density varied, and the location of it often corresponded with shark sightings. This substance may be the plankton that the sharks feed on (Figure 8). Acknowledgements: Dr Les Noble Reader, Institute of Biological and Environmental Sciences, University of Aberdeen; Lilian Lieber PhD Student, Institute of Biological and Environmental Sciences, University of Aberdeen; Innes Henderson, Skipper, FV Tarka.
Ocean News & Technology
To support this research, Teledyne RESON supplied Aberdeen University with a SeaBat 7128 forward-looking multibeam imaging system. The project was carried out from 7-9 August 2012 off the waters of Coll in the Inner Hebrides of Scotland. Basking sharks are known to reside here and sightings are particularly common at this time of year. The SeaBat 7128 was tied to the port side of a small fishing vessel (Figures 1-2). The dry end consisting of the sonar processor and monitor was contained in a shipping case (Figure 3). Sightings were common, and in most cases the boat was able to travel to within 30 m of the sharks. The sharks varied from 5 m to 10 m in length. On the surface, the dorsal fin was always visible, while the nose and tail were sometimes visible (Figures 4-5). The sharks showed clearly on the SeaBat and the pectoral fins were distinguishable (Figure 6). The tail could be seen moving from side to side on the SeaBat interface. The head width provided an indication as to whether the mouth was opened or closed. The strength of the return varied depending on whether the mouth was opened or closed. When the mouth was open, a strong narrow return appeared down the center of the head, with weaker returns on either side, and stronger returns at the edges. Below the surface, the sharks often left a distinctive wake in their paths. When the background noise was low, this made it possible to clearly track the movements of the sharks. Up to 5 sharks were visible simultaneously on the SeaBat (Figure 7). Nothing out of the ordinary was discovered as far as the research was concerned. Indeed, what appeared at the surface closely matched up with what appeared to be happening below the surface – the sharks were not ‘stacking’ as the research hypothesized. Cases where the shark was visible on the SeaBat but not at the surface were extremely rare.
November 2013
INSET