Recreational fishers help to build the billfish picture

A project partnering with recreational fishers to better understand the billfish population off the Northern Territory will help build scientific knowledge and support an emerging tourism industry.

By Melissa Marino

When Bomber Farrell tagged and released his 1000th billfish, it was notable for more reasons than one.

Not just a celebration for the passionate recreational fisher, the milestone catch would play a significant role in a landmark FRDC-supported project to better understand iconic gamefish in northern Australia.

One of several Sailfish (Istiophorus platypterus) attached with satellite and acoustic tags as part of FRDC Project 2021-001, this specimen would take the researchers on quite a journey.

When the tag self-released five months after being fitted and data transmission commenced, it revealed an extraordinary and previously unknown pattern of movement.

"That fish travelled well over 1200 kilometres from Groote Eylandt, towards West Papua and into Indonesian waters," says project principal investigator Dr Keller Kopf, an ecologist from Charles Darwin University. "And that was important because, while we know they are a migratory species, it really expanded our knowledge about the movement patterns of billfish in this region."

Keller says increasing understanding about Sailfish movements will help build a picture around the species in the fishing grounds of northern Australia and international waters. This will inform management decisions to support both the sustainability of the species and an emerging tourism industry for catch-and-release billfish fishing in the Northern Territory.

Economic and environment payoff

Little is known about billfish populations in northern waters, including their age range, genetic diversity and what drives their movement: reproduction, feeding opportunities or environmental conditions.

Most research on billfish in Australia has focused on the east coast, where there are commercial catches of Swordfish (Xiphias gladius) and Striped Marlin (Kajikia audax), and an established fishing tourism industry driven largely by the quest for Black Marlin (Istiompax indica) and other species of gamefish.

"Building knowledge around the distribution of billfish in the north is important from both an environmental and economic perspective", Keller says.

"The basis of this project is to develop a better understanding of billfish for the recreational fishing community, but also for the fisheries management and science communities so that they can be managed sustainably," he says.

FRDC Research Portfolio Manager Dr Toby Piddocke says the project will provide new information about billfish biology, ecology, population structure and movements. This will feed into stock assessments to inform management of the fishery and, ultimately, to help to ensure ongoing fishing opportunities, he says.

With an approximate total economic value of AUD$3,185 per charter fishing trip, "developing the billfish recreational fishery could be an important economic opportunity for the Northern Territory as we come out [of] COVID", he says.

Citizen science

Keller says the project will help promote tourism in the Top End. It is also important because recreational fishers themselves instigated the project after identifying an opportunity to grow the catch-and-release game fishing sector, if more was known about species movement and fishing hotspots.

"It's been a 'ground-up' project," Keller says. "And the timing was right because I had done past research on billfish, and when I moved to Darwin three years ago, I discovered a growing community of people that were really enthusiastic about it."

Catch-and-release game fishers are actively involved in the project. They have been assisting the research effort led by PhD candidate Matt Hammond, by supplying local knowledge and vessels to tag fish. They are also helping to collect biological tissue samples from fish they catch and release that will contribute to genetic research as part of an affiliated project.

Two fishers, including Bomber Farrell, have also undergone advanced training, enabling them to tag fish and collect biological data independently.

"It is a fantastic example of citizen science and public engagement with recreational fishers working with scientists and everyone pitching in," Toby says.

Underwater journeys revealed

The tagging effort is ongoing and to date, at least 15 Sailfish have been fitted with tags that, when released, will transmit their data via satellite directly to the researchers' inboxes. Acoustic tags will transmit when fish swim within range of listening stations anchored at sea.

Data, recorded every two minutes while the satellite tag is attached, shows researchers where the fish has travelled, to what depth and the temperature of the water.

Keller says this style of tracking produces "fishery-independent data", which is an important advance on past tagging techniques because data is gathered continuously while the fish are underwater, rather than only when they are caught.

"The knowledge benefit of this in terms of the fishery is knowing where the fish are going and what they're doing at times of the year and in places where people are not fishing," he says.

While Bomber Farrell's fish provided a sneak preview into these movements, most of the tags are due to detach towards the end of 2023 and 2024, revealing movement patterns over 12 months.

If more fish are found to be travelling long distances, this could inform international efforts to manage the fishery and ensure it remains sustainable, he says.

The big picture

In the meantime, all existing billfish catch-and-release data from the recreational sector has been collated, and biological tissue samples are being analysed to establish a fuller picture of the fishery.

This has revealed significant numbers of juvenile Black Marlin in the fishery, which indicates it may be a nursery area, populating stocks in Australia and even Asia. This unusual concentration is interesting from a scientific perspective and tantalising for game fishers, Keller says.

"There are not many places [in the] world where there are such consistent numbers of these really small Black Marlin," he says. "So, the questions that sit in the back of rec fishers' minds are 'where is the spawning ground?' [and] 'where are the big adults?'."

Keller and Matt provide regular updates on the project through the Amateur Fishermen's Association of the Northern Territory (AFANT) and game fishing club meetings. Recreational fishers can also get involved and keep across developments through Facebook and Instagram pages established specifically for the project.

"The fishers who are the keenest to collaborate with us want to know what's going on with the fish, so we want to ensure they get the latest information," Keller says.

Related FRDC Projects

2021-001: The emerging Billfish fishing grounds of northern Australia: fisheries description, movements, and hot-spots

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