University of Tasmania
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Assessment, evaluation and mitigation of marine mammal bycatch in commercial fishing gear

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posted on 2023-05-28, 12:17 authored by Hamilton, SA
Marine mammals, particularly cetaceans and pinnipeds, are killed as incidental bycatch in fisheries around the world. Management measures to reduce this bycatch and improve the conservation outcomes of affected bycatch species include the implementation of temporal and spatial fishery closures, operational protocols and the use of technical mitigation measures, such as the installation of devices or adjustments to operational gear. In my thesis, I first undertake a global review and assessment of technical mitigation measures used to reduce the bycatch of marine mammals, particularly cetaceans and pinnipeds, in commercial trawl, purse seine, longline, gillnet and pot or trap fishing gear. For some gear types and taxa, there are currently limited technical options that show strong evidence for effectively reducing bycatch. Research and development is urgently needed into effective measures to reduce small cetacean bycatch in trawl nets, the bycatch of some small cetacean and pinniped species in gillnets and the entanglement of large baleen whales in pot or trap buoy-lines. While there are promising results from options such as loud pingers to deter dolphins from trawls and rope-less pot or trap systems, continued research and development in these areas should be a high priority. Few technical mitigation measures have undergone robust testing to determine their effectiveness in reducing mortality of the bycatch species while maintaining operational efficiency and target catch quality and quantity. Examples of effective measures include acoustic devices (pingers) that have reduced the bycatch of some small cetacean species, particularly harbour porpoise Phocoena phocoena, in gillnets, and appropriately designed exclusion devices which have reduced pinniped bycatch in some trawl fisheries. As a case study, my thesis focused on a 'Sea Lion Exclusion Device', or SLED, developed to reduce bycatch of the endangered New Zealand (NZ) sea lion (Phocarctos hookeri) in trawl fisheries. The SLED, installed before the trawl codend, has a stainless steel upwardly inclined grid that directs entrapped sea lions to a top-opening escape hole covered with a backward-facing hood. Target species are able to pass through the grid and into the codend. Research, development and implementation of the SLED was undertaken to reduce NZ sea lion bycatch in the squid trawl fishery operating near the Auckland Islands in sub-Antarctic New Zealand. Observed bycatch has been greatly reduced following SLED implementation. However, there has been scepticism and concern that significant 'cryptic' or unaccounted mortality may be occurring, particularly sea lions actively exiting the net but exceeding their breath holding capabilities before they reach the surface. I assessed the effectiveness of SLEDs by reviewing and evaluating trials and tests of sea lion interactions with SLEDs in trawl nets. I complemented this assessment by developing and fitting population models for the two largest NZ sea lion populations on the Auckland Islands and Campbell Island to evaluate the impact of fisheries bycatch on population growth, particularly after bycatch mitigation implementation. The available evidence shows SLEDs are effective in reducing NZ sea lion bycatch, sea lions are able to escape via SLEDs and are unlikely to sustain life-threatening injuries, and SLEDs contribute to reduced rates of observed sea lion mortality in trawl fisheries. Sea lion carcasses are also unlikely to passively drop out from a SLED top-opening escape hole. Further compelling evidence that cryptic mortality is unlikely to be significant is provided through NZ sea lion demographic parameters, population viability assessments and current population trajectories. Modelling of both the Auckland Islands and Campbell Island populations showed that current bycatch estimates from relevant trawl fisheries are sustainable following effective bycatch mitigation, and population growth rates are positive although slow. Modelling also indicated that disease events causing reduced pup production may greatly impact the population growth at the Auckland Islands, and management actions that reduce pup mortality would lead to increased growth rates for both populations. The development, refinement and testing of the SLED is an example of effective mitigation resulting in encouraging bycatch reduction and conservation outcomes. The conservation of NZ sea lions over the past decade should be regarded as a 'good news' story, with fisheries bycatch effectively mitigated, the population decline observed at the Auckland Islands halted, all other breeding populations increasing or stable, and the species breeding range expanded. While effective bycatch mitigation implementation has significantly improved the conservation status of the NZ sea lion, the focus should remain on addressing other threats, such as disease, which appear to be slowing population recovery.


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Copyright 2020 the author Chapter 2 appears to be the equivalent of a post-print version of an article published as: Hamilton, S., Baker, G. B., 2019. Technical mitigation to reduce marine mammal bycatch and entanglement in commercial fishing gear: lessons learnt and future directions, Reviews of fish biology and fisheries 29(2), 223‚Äö-247 Chapter 3 appears to be the equivalent of a pre-print version of an article published as: Hamilton, S., Baker, G. B., 2015. Review of research and assessments on the efficacy of sea lionexclusion devices in reducing the incidental mortality of New Zealandsea lions Phocarctos hookeri in the Auckland Islands squid trawl fishery, Fisheries research, 161, 200-206 Chapter 4 appears to be the equivalent of the pre-peer reviewed version of the following article: Hamilton, S., Baker, G. B., 2016. Current bycatch levels in Auckland Islands trawl fisheries unlikely to be driving New Zealand sea lion (Phocarctos hookeri) population decline, Aquatic conservation: marine and freshwater ecosystems, 26(1), 121‚Äö-133, which has been published in final form at This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Use of Self-Archived Versions Chapter 5 appears to be the equivalent a pre-copyedited, author-produced version of an article accepted for publication in ICES journal of marine science following peer review. The version of record, Hamilton, S., Baker, G. B., 2019. Population growth of an endangered pinniped ‚Äö- the New Zealand sea lion (Phocarctos hookeri) ‚Äö- is limited more by high pup mortality than fisheries bycatch., ICES journal of marine science, 76(6), 1794‚Äö-1806 is available online at:

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