Quantifying shark depredation and post-release survival in the Queensland East Coast Spanish mackerel Scomberomorus commerson fishery

Abstract

Context : Fishers report high levels of shark depredation and post-release mortality in the Queensland East Coast Spanish mackerel fishery, yet little empirical data exist for these sources of cryptic mortality, limiting the precision of stock assessments.

Aims : This research aimed to quantify shark depredation and post-release mortality in this fishery.

Methods : Depredation rates were recorded across 92 commercial and recreational fishing trips. To estimate post-release survival, 32 Spanish mackerel Scomberomorus commerson were tagged with survival pop-up archival tags.

Key results : Shark depredation rates were 6% during commercial trips and 36% during recreational trips, with substantial daily variation. Highest depredation occurred in the Lucinda region and lowest in the Far North region. Statistical modelling indicated that sector and latitude had a significant effect on depredation rates, with higher depredation rates for recreational fishing and increasing depredation with more southerly latitude. Post-release survival of S. commerson was 53% and mortality was solely caused by predation shortly after release, likely by sharks. Post-release survival varied regionally, with the highest rates in Sunshine Coast (67%) and lowest in Lucinda (20%).

Conclusion : Shark depredation and post-release predation lead to substantial extra mortality in the Queensland East Coast Spanish mackerel fishery, particularly in the recreational sector.

Implications : Shark depredation and post-release survival rates were incorporated into the 2025 stock assessment, improving the precision of fishing mortality assessments and overall stakeholder confidence in the stock assessment outputs. These sources of mortality should continue to be monitored over time. Modifications to fishing practices and adoption of different fishing gears can mitigate depredation.