New modelling of sub-surface ocean conditions off Australia’s east coast is identifying factors that influence the catch of key tuna and billfish species, with real-time ocean forecasts now being trialled.
Ocean conditions far below the sea surface offer new insights into factors that influence where and when commercially important tuna and billfish species are caught, according to FRDC-funded research.
These insights have been used to provide ocean forecasts for the Eastern Tuna and Billfish Fishery (ETBF), to help commercial fishers optimise their efforts. Project partners with FRDC are CSIRO, Bureau of Meteorology, University of Sunshine Coast and the Australian Fisheries Management Authority’s Tropical Tuna Resources Assessment Group that includes representatives from the commercial fishing sector.
The recently completed five-year project (2017-004) has focused on Bigeye Tuna (Thunnus obesus), Yellowfin Tuna (Thunnus albacares), Albacore (Thunnus alalunga), Striped Marlin (Kajikia audax) and Swordfish (Xiphias gladius).
These key migratory species are targeted in Australia’s ETBF, which is a comparatively small part of the much larger international fishery managed by the Western and Central Pacific Fisheries Commission (WCPFC).
The Australian portion of the fishery has a gross value of production of about $35 million a year, and stretches from Cape York in Queensland to the South Australian/Victorian border within the Australian Exclusive Economic Zone, with some fishing also on the adjacent high seas.
Lead researcher for the FRDC-funded project, CSIRO fisheries scientist Jason Hartog, says over recent years the ETBF has experienced unexplained variations in catches – both more and less than normal seasonal variability would account for.
Investigating catch correlations with standard international climate and ocean data, such as satellite-derived sea surface temperature and surface ocean currents, has failed to identify conclusive factors contributing to these catch “pulses”.
Seeking further information, Jason says the approach of this latest research was “to go deeper”.
“The target species are part of much larger international fish stocks, and some species are known to spend time at depths of 300 to 500 metres,” he explains. “We needed to look at ocean conditions beyond the extent of the EEZ or fishery and go deeper into the water column to examine potential ocean influences.”
To do this, Jason and his team gathered fisheries catch data on the Australian ETBF from the Australian Fisheries Management Authority and from 13 other Pacific nations and territories that are members of the WCPFC, and also target the same species.
They combined this information with two historical ocean reanalyses developed by CSIRO and the Bureau of Meteorology to examine historic conditions and correlation with the catches of the target species.
Both models incorporate subsurface ocean data from the Argo project, an international initiative that has a global fleet of more than 3,500 robotic floats capturing information on both sea surface and subsurface temperatures, along with salinity levels, the depth and structure of ocean layers and currents. Australia operates 301 Argo floats, through the national Integrated Marine Observing Systems, with others in New Zealand and across the Pacific.
“Our key finding is that sub-surface ocean state variables are important in explaining the variance in catch, although they’re not the whole story,” says Jason.
The modelling also found that ocean factors affecting catch per unit of effort (CPUE) varied between species and between the four sub-regions created for the project, based on dominant ocean conditions. These are the East Australian Current; Coral Sea and equatorial region; Western-Central Pacific; and New Zealand (Figure 1).
For example, modelling across all regions for Yellowfin Tuna found that sea surface salinity, kinetic energy of ocean eddies integrated down to 300 metres and water temperature at 500 metres, were the most influential environmental factors on catch per unit effort.
In the East Australian Current sub-region, the depth of the mixed water layer (affected by surface conditions), water temperature at 500 metres and sea surface temperature, were the best environmental predictors of Yellowfin Tuna catch per unit effort.
Through the Tropical Tuna Resource Assessment Group, the Bureau has provided industry members with access to real-time, quarterly forecasts of surface and subsurface ocean conditions off Australia’s east coast to trial.
Jason says the forecasts in some regions are more accurate than others. The dynamic and unpredictable nature of eddies in the EAC, for example, make accurate forecasts there more challenging compared to regions such as the Coral Sea, where ocean conditions are generally more stable.
And while modelling climate change was not a specific driver for the research, FRDC research portfolio manager Toby Piddocke says the forecasts will help fishers adapt to shifting distributions of target species that may be caused by climate change.
“Ocean environments and ecosystems are changing quite rapidly and many fisheries, including the Eastern Tuna Billfish Fishery, will need strategies to adapt. The ocean forecasts will help provide this, and there’s a potential application for this approach to be used for any species influenced by oceanographic variables.”
He points to the Southern Bluefin Tuna (Thunnus maccoyii), wild prawn fisheries and Atlantic Salmon (Salmo salar) aquaculture, all of which are already using such modelling.
By identifying important subsurface influences on the distribution of target species, Toby expects this research will also help to improve stock assessments, that are often reliant on catch information in lieu of more independent fisheries data.
“The findings will help standardise catch per unit effort measures for these species internationally and provide a clearer picture of the health of fish stocks in Australia and across the Pacific region.”
Related FRDC Projects
2017-004: Investigate oceanographic and environmental factors impacting on the ETBF