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Document Number

SC/69B/HIM/16

Full Title

Evidence of dolphin bycatch reduction with low-cost passive acoustic devices attached to bottom set gillnets

Author

Federico Sucunza, Gabriel G Larre, Antonio Barth, Daniel Danilewicz, Paulo H Ott, Lorenzo Von Fersen, Nick Tregenza And Per Berggren

Authors Summary

Mortality due to fisheries bycatch in gillnets is the main threat to marine mammals globally. Gillnet fisheries are also the main challenge for the conservation of franciscana dolphins (Pontoporia blainvillei), the most threatened cetacean in the western South Atlantic Ocean. The purpose of this study was to evaluate the effectiveness of upcycled plastic drink bottles as acoustic reflectors and low-cost mitigation method to reduce dolphin bycatch and their potential impact on target species catch. Observed trials with (treatment) and without (control) plastic bottles in bottom set trammel nets and gillnets recording dolphin bycatch and target species catch were conducted between November 2020 and February 2024. A total of 251 sets (130 control and 121 treatment) resulted in a bycatch of six franciscana dolphins and two bottlenose dolphins (Tursiops sp.) in control sets and one franciscana dolphin bycatch in treatment sets. GLM results showed a statistically significant negative effect on dolphin bycatch using plastic bottles (z-value = -1.979, p = 0.0478) and a non-significant positive effect on target fish catch using plastic bottles (t-value = 0.965, p = 0.335). Passive acoustic monitoring (using F-POD) logged 2,522 h (964 h control, 1,558 treatment) and showed that franciscana dolphins and other odontocetes were present near control and treatment sets. The results showed that upcycled plastic bottles used as acoustic reflectors in bottom set trammel nets and gillnets reduced the bycatch of dolphins (including franciscana) and had no effect on target species catch compared to control sets. Given these positive results we strongly encourage colleagues to trial plastic bottle acoustic reflectors as a low-cost bycatch mitigation method in other sub-surface gillnet fisheries.

Publisher

IWC

Publication Year

2024

Abstract

Mortality due to fisheries bycatch in gillnets is the main threat to marine mammals globally. Gillnet fisheries are also the main challenge for the conservation of franciscana dolphins (Pontoporia blainvillei), the most threatened cetacean in the western South Atlantic Ocean. The purpose of this study was to evaluate the effectiveness of upcycled plastic drink bottles as acoustic reflectors and low-cost mitigation method to reduce dolphin bycatch and their potential impact on target species catch. Observed trials with (treatment) and without (control) plastic bottles in bottom set trammel nets and gillnets recording dolphin bycatch and target species catch were conducted between November 2020 and February 2024. A total of 251 sets (130 control and 121 treatment) resulted in a bycatch of six franciscana dolphins and two bottlenose dolphins (Tursiops sp.) in control sets and one franciscana dolphin bycatch in treatment sets. GLM results showed a statistically significant negative effect on dolphin bycatch using plastic bottles (z-value = -1.979, p = 0.0478) and a non-significant positive effect on target fish catch using plastic bottles (t-value = 0.965, p = 0.335). Passive acoustic monitoring (using F-POD) logged 2,522 h (964 h control, 1,558 treatment) and showed that franciscana dolphins and other odontocetes were present near control and treatment sets. The results showed that upcycled plastic bottles used as acoustic reflectors in bottom set trammel nets and gillnets reduced the bycatch of dolphins (including franciscana) and had no effect on target species catch compared to control sets. Given these positive results we strongly encourage colleagues to trial plastic bottle acoustic reflectors as a low-cost bycatch mitigation method in other sub-surface gillnet fisheries.

SC/69B/HIM/16

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