Echo-Location Loggers (POD) to Assess Bottlenose
Dolphin Interactions with Trammel Nets

Silvia Scali ⁽¹⁾, Manel Gazo ⁽¹⁾, Nick Tregenza ⁽²⁾, and Alex Aguilar ⁽¹⁾
(1) GRUMM, Department of Animal Biology and Parc Cientìfic de Barcelona, University of Barcelona, 08071, Barcelona, Spain.
(2)Institute of Marine Studies, University of Plymouth. Drake Circus. Plymouth,PL4 8AA UK.

This Poster Presentation won 2nd Prize in the 2002 Student Poster Contest.

In the Balearic Islands, bottlenose dolphins (Tursiops truncatus) adversely interact with the artisanal fishing, particularly when trammel nets are used. This results in economic loss to fishermen and in deliberate and incidental kills of dolphins. The extent and frequency of such interactions is unknown. Within the framework of a project to assess the effectiveness of acoustic deterrent devices (pingers) to mitigate the conflict, we tested the utility of the POD, a submersible computerized hydrophone that logs dolphins echo-location clicks (Chelonia T-POD, UK), to detect the presence of dolphins around trammel nets. The study was conducted in September-October 2001 during the red mullet (Mullus surmuletus) season in Majorca. 13 trials were made. The POD was attached to the floating line of the net and, in order to optimize the rejection of other ultrasonic noises (boat sonars, propeller noise, sand movement noise and unknown sources) we used 4 different settings by changing the center frequency and sharpness of the two hydrophone filters, the minimum intensity of the higher filter, and the ratio energy between the two filters. The POD detected click trains in 7 of the monitored fishing operations. In 6 of these detections, dolphins were sighted in the vicinity of the nets. In 3 of the cases, detection coincided with dolphin predation and damage to nets. Most detection were registered at night, when visual sighting would not have been possible. Although further studies are needed to improve the detection of dolphins and to better understand echo-location patterns, the POD is considered to be a useful tool in the study of interactions between dolphins and fishing.