The proposal addresses the key needs specified in FRDC investment opportunity on GoC KTF in an integrated manner, in consultation and collaboration with industry, by a team of people highly experienced in the science and logistical challenges of working in the GoC. The ‘Needs’ in the FRDC Call for R&D Investment Opportunities were (1) stock structure, including spatial and temporal connectivity between regions (i.e., movement and reproductive connectivity), (2) quantify life history information at regional scales relevant to stock assessment, and (3) factors influencing variation in the relationship between catch rate and population abundance.

The proposal addresses these knowledge gaps, which continue to bring uncertainty to the GoC KTF assessment and management. Further details are included in the Methods section.

It is critical that the inputs to the stock assessment, such as the spatial structure of the model, standardised catch rates, and life history parameters (e.g., temporal and spatial variation in growth rates, proportion mature-at-age, proportion mature-at-length) are representative of GoC stocks. Independent review of the latest KTF stock assessment (Campbell et al. 2024) concurs with this statement. Past research has included GoC samples of varying levels of spatial representativeness (Garrett et al. 1997; Welch et al. 2010; Newman et al. 2010; Moore et al. 2011; Moore et al. 2017) primarily due to due to logistical challenges. Despite the past research, there remains spatial and temporal uncertainty in how KTF populations function in this large tropical region, which has highly variable patterns in wet season rainfall and flood - the primary drivers of nutrient input to coastal GoC ecosystems.

This project will build on the 2022 FRDC National Workshop on Shark Depredation (project number 2021-038) and have four complementary phases through: (1) a detailed review of the peer-reviewed and grey literature to identify and assess the efficacy of the most recently developed mitigation methods, (2) a survey of the scientific community and fishing-industry stakeholders (i.e. fishers, industry representatives, managers) to identify lesser known methods developed and/or used by fishers, (3) a series of workshops to identify the most promising approaches for mitigating depredation and plan the design of field trials, with a stop/go point occurring at the end of this phase based on whether suitable methods are found, and if so, (4) a robust experimental trial of the most promising mitigation method(s) to assess their effectiveness at reducing shark depredation. Phase 4 would be dependent on whether suitable methods can be identified through Phases 1–3. Stakeholder involvement and outreach will be a critical part of this project across all phases. The findings of this project will be widely disseminated via presentations to fishing clubs and industry members, written articles in the FRDC and fishing magazines, a final project report and subsequent peer reviewed publications. All off these outreach activities will be conducted in collaboration with the FRDC Extension Officer network. This will ensure broad adoption of the identified depredation mitigation measure(s).

As one of the most prominent species sold in northern Queensland fish shops, Spanish mackerel is iconic to seafood consumers as well as commercial, recreational, and charter fishers. However, a recent assessment of the Queensland East Coast stock raised significant concern about its sustainability, with a spawning biomass estimate below 20% of unfished levels (Tanimoto et al. 2021). Historical analyses indicate a substantial contraction of the spawning aggregations of Spanish mackerel in North Queensland, with those off Cairns being extirpated in the 1990s (Buckley et al. 2017). Previous stock assessments have also shown a trend of declining abundance over the last 20+ years (Welch et al. 2002, Campbell et al. 2012, O'Neill et al. 2018). This historical sustained decline, combined with the recent low biomass estimates, raise substantial uncertainty over the continued viability of the fishery.

This stock assessment result indicates the need to rebuild the stock in line with State and National policy objectives to reduce the number of overfished stocks. The high social and economic importance of Spanish mackerel in Queensland has led to significant public debate around uncertainty in the stock assessment, and the issue being debated in Queensland State Parliament.

As a result, key research gaps need to be addressed to increase the precision of future assessments and reaffirm stakeholder confidence in the stock assessment process. This project aims to address the following research gaps highlighted by the stock assessment: (1) determine the feasibility of using a CKMR method for estimating abundance of Spanish mackerel, (2) quantify cryptic mortality from shark depredation and post release survival, (3) improve catch and effort standardisation by better adjusting for potential hyperstability occurring at spawning aggregations and make the catch rate robust against potential management change, (4) assess the influence of key environmental variables on abundance and recruitment variability. This project will inform more accurate Spanish mackerel stock assessment, thereby increasing stakeholder confidence in line with Outcomes 4 and 5 of the FRDC R&D Plan (‘optimising benefits for the Australian community through fair resource sharing based on evidence-based management’ and ‘transparent decision-making tools that demonstrate best practice in fisheries management’). The project will also directly address the high priority research needs identified by the Queensland Spanish Mackerel Fishery Working Group.