After 21 years, scientists at the Australian Antarctic Division have gained valuable insights into the biology and movement of these long-lived monsters of the deep.
Fisheries scientist, Dr Dirk Welsford, said the tagging and survey program was started by Antarctic Division scientists in 1998, a year after the fishery started. Since then the Division has collaborated with industry and the Australian Fisheries Management Authority (AFMA) to tag about 2500 fish per year.
“The fishing companies run the vessels, and AFMA provide the permits and scientific observers on each vessel to monitor the catch and deploy the tags,” Dr Welsford said.
“The Antarctic Division designs the scientific program, stores and analyses the data, and provides advice to the Commission for the Conservation of Antarctic Marine Living Resources, to assist them in setting sustainable catch limits for the fishery.”
During fishing operations, the small plastic tags are attached near the dorsal fins of roughly two fish per three tonnes caught. When the tagged fish are re-caught years or decades later, they provide information on longevity, size, age and movement.
“We’ve found that the fish generally don’t move much — no more than tens of kilometres — and often we’ll pull up fish in the same numerical order in which they were tagged, so they seem to remain in their original habitat,” Dr Welsford said.
“However a small percentage of fish do travel long distances. We’ve found some more than 3000 kilometres from their original capture and it’s still a bit of a mystery as to why.
“The fish are also very robust, as they don’t have a gas filled swim bladder, so they can survive capture from depths of over 2000 metres. They also tend to get bigger the deeper they live.”
Recently, Dr Welsford’s scientific team identified a 63 year old toothfish by the growth rings in its ear bones (‘otoliths’) — the oldest toothfish found so far.
“We’ve been collecting otoliths since the fishery started, as they help us understand growth and mortality rates, so that the stocks aren’t fished too hard,” he said.
“This is a good sign that the stock assessment strategy we’re using is sustainable, as we wouldn’t be finding such old fish if it wasn’t.”
In 2016–17 the total allowable catch for Patagonian toothfish in the HIMI fishery was 3405 tonnes and the fishery has held Marine Stewardship Council accreditation since 2012.
Dr Welsford credits the sustainability of the fishery with the collaborative approach to decision making, because “once you reach an agreement, everyone owns it”.
However, there are still many questions that need to be answered, including whether there is a difference in the movement of large, older fish compared to small, younger ones; a difference that could affect the management strategy of the fishery.
As Patagonian toothfish move between Australian and French Exclusive Economic Zones in the HIMI region, scientists in both countries are also working together to share data and stock-structure models to account for fish that move across the ‘line’.
Scientists are also looking at whether genetic technology that identifies relationships between individuals, such as parent and child (close-kin genetics), could provide additional and independent information about population structure.
“If the population is large, the chance that you’d find parents and children in a sample would be small, and vice versa,” Dr Welsford said.
“The smaller and more related a population is, the smaller your catch should be.
“So this method could provide independent verification and validation of the answers we get from tags, and that our current stock assessment method is correct.”
The Antarctic Division is now applying the knowledge gained in the HIMI Patagonian toothfish fishery to the multi-national Antarctic toothfish fishery — working with the five other countries that currently fish for toothfish, to tag and age fish.
“From our experience in the HIMI fishery, we are optimistic this pathway can lead to a sustainable fishery in the long term,” Dr Welsford said.
Australian Antarctic Division