| SS9.01 Ecosystem Science Practiced in an Urbanized Estuary: South San Francisco Bay |
| Mooney, T, A, University of Hawaii / Hawaii Institute of Marine Biology, Manoa, HI, USA, mooneyt@hawaii.edu |
| Au, W, W, University of Hawaii / Hawaii Institute of Marine Biology, Manoa, HI, USA, wau@hawaii.edu |
| Nachtigall, P, E, University of Hawaii / Hawaii Institute of Marine Biology, Manoa, HI, USA, nachtiga@hawaii.edu |
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| TARGET STRENGTH AND PREDICTED BIOSONAR DETECTION RANGES OF THREE EXPERIMENTAL GILLNETS: A METHOD TO REDUCE MARINE MAMMAL BYCATCH |
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| Small cetaceans are often caught as bycatch in gillnet fisheries. One potential method that has been shown to reduce harbor porpoise bycatch is the use of barium sulphate (hypothesized acoustically reflective) enhanced gillnets. To determine acoustic reflectivity, this study compared the target strength of two experimental gillnets; a barium sulphate and an iron oxide gillnet, with a comparable control monofilament gillnet at 0º, 10º, 20º, 30º and 40º. At normal incidence, all nets measured were not different in target strength and biosonar detection ranges are estimated to be relatively similar. As the angle of incidence increased, the experimental nets’ echo strength remained the same while the control net echo strength decreased at a greater rate. Iron oxide nets maintained the highest target strength respective of incident angle. Biosonar detection ranges are estimated to be greatest for the iron oxide net and shortest for the control net at increased angles. Echolocating odontocetes may utilize acoustic reflections of experimental nets to a greater degree but due to the weak target strength of both nets, other mechanisms may also mitigate reduced odontocete bycatch. |
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