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Mitigating discard mortality from dusky flathead Platycephalus fuscus gillnets
Matt K Broadhurst1, Russell B Millar, Craig P Brand
1New South Wales (NSW) Department of Primary Industries, Fisheries Conservation Technology Unit, PO Box J321, Coffs Harbour, New South Wales 2450, Australia. mbroadhurst@nmsc.edu.au
Diseases of Aquatic Organisms
|August 22, 2009
Summary
Discarded bycatch in Australian dusky flathead fisheries experiences high mortality, influenced by water temperature and fish length. Reducing discard rates requires management changes and careful handling to improve survival.
Area of Science:
- Fisheries Science
- Ecology
- Marine Biology
Background:
- Commercial gillnetting targeting dusky flathead (Platycephalus fuscus) in southeastern Australia results in significant bycatch.
- Discarded fish and invertebrates face immediate and short-term mortality, impacting ecosystem health and fishery sustainability.
Purpose of the Study:
- To estimate mortalities of key species discarded during commercial gillnetting.
- To identify contributing environmental and biological parameters influencing discard survival.
- To assess physiological stress responses in discarded yellowfin bream (Acanthopagrus australis) and luderick (Girella tricuspidata).
Main Methods:
- Onboard assessment of immediate mortality for 1470 bycatch individuals (16 species).
- 4-day caging experiments for 570 individuals (11 species) to monitor short-term fate.
- Physiological analysis of plasma cortisol and glucose in stressed and baseline fish.
Main Results:
- Immediate mortality varied by species, reaching 70% for undersize dusky flathead.
- Water temperature and total length (TL) significantly influenced immediate and short-term mortalities.
- Discarded yellowfin bream and luderick exhibited elevated plasma cortisol levels, indicating significant stress.
Conclusions:
- Overall discard mortality for key species ranged from 5.9% to 76.9%.
- Mitigation strategies include retaining some undersize dusky flathead, restricting fishing in high temperatures (>16-17°C), and improving catch handling.
- Understanding discard mortality is crucial for sustainable fisheries management.
