Image of a swordfish caught and released with a satellite tag off Tasmania

Bluewater Magazine Issue 125

As anglers know, Australia’s east coast is a unique environment. Few other places in the world allow fishers to catch tropical gamefish and tuna at such latitudes and so close to shore. The south coast of New South Wales (NSW) saw the beginnings of Australia’s now prolific game fishery in the 1930s and was the centre of the development of commercial tuna fishing in the 1950s. 

The conditions that allow these tropical fish to be targeted at latitudes south of 35°S are primarily driven by the Eastern Australian Current (EAC) bringing warm tropical waters down the east coast of Australia. But it is the mixing of this warm water with nutrient rich colder waters, and the formation of large eddies that drive the productivity of the pelagic ecosystem in this area.

Game and commercial fishers alike have known for many years how to target these eddy systems and the fronts between warm and cold water. However, it is becoming increasingly apparent that the arrival of the warm waters in an area does not guarantee high catch rates of tropical tuna and billfish, and understanding why this occurs is often a subject of debate and discussion among fishers.

During the 2016-17 summer and autumn, when the EAC had pushed down the east coast, game fishing tournaments along the NSW coast reported very low tag rates, despite in some cases very high numbers of participants. Catches in the commercial Eastern Tuna and Billfish Fishery were also lower than the previous season. Several seasoned skippers, both commercial and game fishers, believe that this is the result of a strong EAC pushing faster and further south than in previous years, moving fish down the coast much more rapidly. In contrast, past seasons have sometimes shown almost opposite trends in adjacent waters between commercial and recreational catch rates. Again, the main culprit is suspected to be differences in near shore and offshore ocean conditions.

However, until we have a much more detailed understanding of how tuna and billfish interact with the ocean currents in our region, we can only guess what drives the year-to-year changes in availability of these species. 

In recent years, access to fine-scale satellite sea surface temperature, ocean current and other ocean information has significantly improved (figure 1). These days, there would hardly be a commercial longliner or serious game fisher that wouldn’t use real time, fine-scale sea surface temperature maps. The information used to generate these maps is also being stored by scientific agencies such as the Commonwealth Scientific and Industrial Research Organisation (CSIRO) to create enormous databases of oceanographic information.

It is hoped that this information, combined with data from electronic tagging studies of tuna and billfish, and data from commercial and recreational fisheries, will allow scientists to get a better understanding about what drives these species’ behaviour and more accurately predict their availability to both commercial and game fishers off Australia’s east coast.

The Tropical Tuna Resource Assessment Group (TTRAG) is the Australian Fisheries Management Authority’s main source of scientific advice on tuna and billfish, and helps to identify key research priorities for tuna and billfish fisheries. TTRAG is looking at how to better utilise oceanographic data to understand how oceanographic conditions influence catch rates off Eastern Australia. To progress this work, TTRAG is supporting the development of two project proposals.

The first proposal aims to collect comprehensive time series of recreational game fishing catch and effort data from the east coast of Australia and then examine potential relationships with oceanographic data and the commercial offshore fishery.  The project is being developed by Blue Water’s own Dr Julian Pepperell and will provide critical information enabling scientists to investigate the different factors that might cause catch rates in the game fishery to change so unexpectedly over time.

The second project is being developed by CSIRO, in collaboration with other scientists from Australia and overseas. It aims to develop mathematical models that explore the relationship between oceanography and occurrence of tuna and billfish in our region and then forecast how tuna availability might change in the short term (seasonal forecasts), and long term (e.g. changes that might occur due to climate change). Similar research has already been conducted in relation to southern bluefin tuna and mahi mahi, which is used to predict the occurrence of these species in the commercial and recreational fisheries respectively.

It is hoped that these projects will receive funding, and in future years fishers will be able to better predict where and when gamefish and tuna will be off Southern NSW, and if the EAC keeps flowing strongly, maybe Tasmania as well.

Figure 1

Understanding the variation in the ocean off eastern Australia is important for fisheries. For example, in the summer of 2016, the Eastern Australian Current (EAC) was broad and wide off Sydney (35S), and extends as far south as northern Tasmania. In contrast, the 2015 December, the EAC was narrower and extended south to the bottom of Tasmania. Cooler water close to the Australian mainland coast is similar in both years. In each of these years, the distribution of pelagic fishes would also differ.

Image source: the Group for High Resolution Sea Surface Temperature (www.ghrsst.org/)

Credit: Feature image of swordfish caught and released with a satellite tag off Tasmania by the Sean Tracey, Senior Research Fellow, Institute for Marine and Antarctic Studies (IMAS), University of Tasmania

View the article printed in issue 125 of Bluewater Magazine.