Publication Date

3-2008

Series Number

173

Publisher

Department of Fisheries, Western Australia

City

Perth, Western Australia

ISBN

1 877098 98 1

ISSN

1035 - 4549

Abstract

Final FRDC Report – Project 2004/068

“Savewave” acoustic pingers were trialed on one vessel to determine if they would deter dolphins entering the trawl net. During daylight hours, the pingers were deployed on alternate shots in the configuration recommended by the manufacturer. On the other alternate shots, no pingers were deployed. A video camera in an underwater housing was attached to the headrope facing downstream for 18 trawl shots. On the 14 shots where usable video footage was obtained (11 with pingers deployed and 7 with pingers not deployed) the number of dolphins viewed in the camera frame was counted for the 1.5 hours filming time, with no account being taken for double counting. The results indicated there was no reduction in the numbers of dolphins inside the net when pingers were deployed.

A flexible grid, constructed from polypropylene pipe joined by nylon rope was trialed for 5 shots. Although the grid was set according to the manufacturer’s recommendations, the catch was almost zero. The digital camera footage showed the water and fish streaming out through the escape opening rather than through the grid. The trials of this grid were consequently abandoned.

A semi-rigid grid, which could be retrieved onto the net drum was trialed. It was constructed from stainless tube joined by braided stainless wire with articulated joints at the top and bottom. It had a bottom escape opening with a Kevlar/rubber mat just upstream of the escape opening. The mat was designed to improve water flow and assist in the escape of large objects. The selection grid was placed at the beginning of the extension, 10 m from the end of the net. During the trials, it was immediately obvious to the skipper and crew that large sharks and rays as well as large sponges were not coming on deck, which they saw as a benefit. They also perceived that scalefish catches has not decreased. The video footage showed large animals passing out through the escape opening and the dolphins backing down to a position about 4 m upstream of the grid and then swimming upstream out of view of the camera. During the period of the project, the dolphin catch with grids not deployed was 29 dolphins in 2616 shots (11 per 1000 shots) compared to 3 dolphins caught in 295 shots with the grid deployed (10 per 1000 shots). Two vessels trialed the grid voluntarily for most of these 295 shots with zero dolphin catch (albeit with no observers on board for most of the shots) and toward the end of the project, a third vessel trialed the grid for one trip and caught all three dolphins.

The grids appear to be successful at reducing dolphin catches on two vessels, but there is a need for trials to be conducted with a power sufficient to detect differences in the dolphin catch 6 Fisheries Research Report [Western Australia] No. 173, 2008 with grids deployed and not deployed. If 400 shots were observed with the grid deployed and 400 shots with the grid not deployed, at the current capture rate 10.2 per 1000 shots, a dolphin catch of 2 or less would indicate, with a power of 0.9, that grids reduce the dolphin catches.

Number of Pages

44

Keywords

Acoustic pingers, Exclusion grids, Dolphin, Turtle

Disciplines

Aquaculture and Fisheries | Marine Biology | Natural Resource Economics | Natural Resources and Conservation | Natural Resources Management and Policy | Sustainability

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