Photo: Northern Australia Hub, National Environmental Research Program
By Amy Kimber
A variety of techniques are being used to detect the movements of big fish in Kakadu National Park and the Daly River region in the Northern Territory, to better understand the importance of connectivity between the floodplain, river and ocean.
Charles Darwin University researcher David Crook and his team are using a combination of tracking tagged fish and ear-bone chemistry to monitor the movements of key species.
To date there has been little scientific research on the movements of fish in the NT. Previous research showed that the floodplains are an important habitat for Barramundi (Lates calcarifer) at particular stages in their life cycle, such as when they are growing and reproducing.
While it is clear that certain species use floodplain habitats at times, there are many unknowns; for example, how long individual fish stay in the floodplain, how far they move, if there are particular habitats fish prefer, and what fish do as the water recedes and the floodplains begin to dry out.
David Crook’s research is being funded through the Australian Government’s National Environmental Research Program, which aims to improve the capacity to understand, manage and conserve Australia’s unique biodiversity and ecosystems.
Since October 2013, his team has been investigating the movements of 65 Barramundi and 55 catfish throughout the wet season. The Barramundi were caught by electrofishing in October 2013 and January 2014, while the catfish were collected by hook and line in October.
“We are investigating these species because they are important to recreational and commercial fishers and for traditional harvest,” David Crook says.
“They are also important species in the food web because they make up a large part of the biomass in Kakadu’s river systems and, because larger species such as crocodiles prey on them, they transport a lot of energy around the ecosystem.”
The fish were caught from the Yellow Waters area of Kakadu National Park, a wetland system that is part of the South Alligator River floodplain. This river system, which is the largest in Kakadu, contains extensive wetlands that include river channels, floodplains and backwater swamps.
Acoustic and radio-transmitters were surgically implanted into the fish. The acoustic tags are being detected using an extensive array of fixed receivers, while the movements of the radio-tagged fish have been tracked by boat and helicopter every two weeks since the fish were released.
Since beginning the tracking, the research team has seen a range of fascinating behaviours. Preliminary results show that the Barramundi moved up to 50 kilometres from where they were released, while the catfish moved up to 20 kilometres.
Following the first major rainfall in early December last year, there was a spike in fish movement, with some fish moving several kilometres out onto the floodplains and even disappearing altogether from the 3000-square-kilometre area being surveyed by helicopter.
Each fish has its own story. Among the fish tagged in October 2013 was a 450-millimetre Barramundi, collected by electrofishing in Mardugal Billabong, and a 410-millimetre catfish collected by hook and line.
The Barramundi made regular movements of up to two kilometres between Mardugal Billabong and Home Billabong during October and November, before moving downstream to Yellow Waters in early December after the first significant flows of the wet season.
It was detected back upstream in Home Billabong on 18 December and stayed in that area using the main channel and flooded side-channels throughout January. Following heavy rains in late January, the fish left the billabongs and moved approximately 30 kilometres downstream, where it was recorded in a side-channel near the South Alligator River on 4 February.
The catfish was detected within a restricted area (approximately 30 metres) around the same five-metre-deep hole throughout October and early November. Following the first minor flow increase in mid-November, the fish moved out of the hole and was detected 100 metres upstream in Mardugal Billabong.
As the water levels rose in early December, the catfish moved eight kilometres downstream and was detected on the inundated floodplain downstream of Yellow Waters on 4 December. It then moved back upstream and was detected near its original location in Mardugal Billabong on 18 December.
In early January the catfish was detected four kilometres downstream on the floodplain near Yellow Waters, and had moved a further eight kilometres downstream by the end of the month. It continued to move downstream and was detected in early February more than 20 kilometres from its original tagging location.
According to David Crook, anecdotal information suggests that a lot of the species in the NT may be diadromous, meaning they migrate between the ocean and freshwater during their lives.
“However, while our findings show that at least some tagged fish move between fresh and saline water, the movement patterns are much more complex than we had expected and there is a lot of individual variation in movement behaviour.
“These observations are really starting to change the way we think about fish migration,” he says.
The research team is also using ear-bone (otolith) chemistry to determine whole-of-lifetime movements of ecologically important fish species in the Daly River, such as catfish and Mullet.
Otoliths are the ear bones of fish, functioning for hearing and balance in much the same way that they do in humans. Otoliths are made of calcium carbonate and start developing in the fertilised egg even before the fish has hatched.
As fish grow, growth rings, similar to those found in tree trunks, are deposited and chemicals from the surrounding water become locked into the otolith structure.
The chemicals that accumulate in the otolith reflect the water chemistry that the fish has been living in, meaning researchers are able to determine the age of the fish and where it has been at various stages in its life.
“For example, we know that certain strontium isotopes are in much higher abundance in the ocean than freshwater, and so we can tell when a fish has moved between marine and freshwater environments by measuring the strontium isotopes in the otoliths,” David Crook says.
“Back in the lab, we extract the otoliths from the fish collected then use a saw and very fine sandpaper to expose the core of the otolith. Using a tiny laser trace from the core to the edge, we can work out where a fish has been living throughout its entire life.
“Tracking and otolith chemistry are complementary techniques. Although tracking provides very fine-scale detail of fish movements, we can only monitor the movements of fish large enough to be tagged.
“The great thing about otolith chemistry is that you can go right back to the early life history of a fish to work out where it was living as a larva and as a juvenile.
“In southern Australia, a lot of the connectivity between river channels and nearby habitats was cut off by human activities before we had any of these tools to measure the impact. Up here, we’ve still got relatively pristine systems to study the importance of connected catchments and coasts.
“We hope that this research will help us to better understand the role of fish and ecological connectivity in supporting the remarkable productivity of northern Australia’s floodplain rivers.”
The full results from this research, which are expected in the second half of 2014, will ensure that policy and management decisions regarding our fisheries and their habitats can be made using sound scientific evidence.
Photos: Northern Australia Hub, National Environmental Research Program
FRDC Research Code: 2012-050
Amy Kimber, 08 8946 7619