How big is the recreational Southern Bluefin Tuna catch?

As part of efforts to monitor and rebuild international stocks of Southern Bluefin Tuna (SBT, Thunnus maccoyii), Australia has agreed to account for its recreational catch of the species.

A new report from the Australian Bureau of Agricultural and Resource Economics and Sciences (ABARES) outlines how this might be done.

SBT is prized by commercial and recreational fishers alike. Strong international and domestic demand for the species over many years has put the stock under stress.

With current spawning stock biomass estimated to be as low as nine per cent of unfished levels, the migratory species is subject to an international rebuilding plan. This is led by the Commission for the Conservation of Southern Bluefin Tuna (CCSBT). Australia is a member of the CCSBT, which sets the international quotas for SBT.

In Australia, the Australian Fisheries Management Authority oversees domestic commercial fishing of SBT in line with the quota allocation from the CCSBT.

In 2014, the CCSBT agreed to a common definition of "attributable catch", under which members are expected to have a process to account for all sources of mortality, including commercial, discards and recreational catch within their allocation by 2018.

ABARES researchers led by Andy Moore collaborated with Fisheries Victoria and the Institute for Marine and Antarctic Studies to develop and test methods for collecting this data. They reviewed recreational fishing survey practices and also modelled different fishing scenarios to estimate how many days of on-site surveys would be needed to build an accurate picture of the data.

The simulation modelling has helped to identify the most cost-effective way to obtain a robust estimate of the national SBT recreational fishing catch.

In some states this was through on-site surveys. In others, where a targeted "sampling frame" of fishers identified through fishing or boat licence databases was available, off-site phone surveys were preferred.

But Moore says that determining recreational catch rates of SBT has particular challenges. One is the transient nature of the species, which travels through different jurisdictions covering 6500 kilometres of Australia's southern coastline from Western Australia to New South Wales. This makes standard survey techniques difficult to implement cost-effectively.

In Victoria and Tasmania catch data are relatively easy to gather because SBT tend to congregate in a few places and on-site surveys at popular boat ramps near these congregation points can quickly and effectively gather the required data.

However, in others, such as South Australia and NSW, where SBT move quickly along the coast, fishing tends to be more episodic and diffuse, making sampling difficult and costly. Staff doing surveys at key boat ramps could entirely miss the fish, Andy Moore says.

In WA and NSW off-site surveys were found to be the most cost-effective techniques, as they are in Tasmania where they are already in use. On-site surveys were preferred in Victoria and are the only current option in SA, where there is no available fisher-related databases that could be used.

The modelling indicated that in SA over a fishing season of eight months, a minimum of 500 survey days would be required. In NSW, 330 survey days over a two-month season would be required if phone surveys were not an option.

In all states, the survey information could be combined with data from charter boats and game fishing competitions to develop a national estimate.

The project has estimated that in the short term the total cost of a national SBT recreational fishing survey would be $2.3 million, in addition to resources already allocated such as WA's biennial recreational fishing survey. These costs could be reduced in the future if an off-site sampling frame for SA could be developed.

Over the longer term, a national "sampling frame" could reduce annual costs to $0.4 million.

"This whole project is about how to obtain an accurate catch figure in the most cost-effective way," Moore says. "The simulation and scenario testing allowed us to hone it right down to a realistic sample size."

The project had an advisory committee comprising representatives from the game fishing community, charter boat sector and state fisheries organisations. This was funded by the FRDC, the Australian Department of Agriculture and Water Resources, the NSW Saltwater Recreational Fishing Trust and the research collaborators.

Moore says if, in the long term, there was a way to identify people who planned to target SBT, perhaps through a specific SBT harvest tagging or permit system, it would increase the accuracy and precision of surveys while reducing costs.