For the next 18 months, fisheries research assistants Jeremy Verdouw* and Joe Hutchins* expect to examine up to 7000 ear bones or ‘otoliths’ from Patagonian toothfish, to estimate the age of each fish.
It’s a big job, but their work will provide critical information that will ultimately help the Australian Fisheries Management Authority (AFMA) sustainably manage the Australian toothfish fisheries at Macquarie Island and Heard and MacDonald islands.
Otoliths have been collected by scientific observers on fishing boats in the two regions since the fisheries began in the mid 1990s. Today, the Australian Antarctic Division has a collection of about 26,000 otoliths and a database of information relating to the sex, weight and length of the fish associated with each pair of otoliths, where and when they were caught, and what fishing gear they were caught with. Using all this information, scientists can better define the age structure of the toothfish stock, the habitats that different age groups tend to frequent, and the ages of fish that are caught by different fishing gear.
'Models developed using this information allow resource management bodies such as AFMA and the Commission for the Conservation of Antarctic Marine Living Resources to adjust annual catch quotas, to ensure representative age groups in the population are maintained,' says fisheries scientist and the project’s Principal Investigator, Dr Dirk Welsford.
'This sustainable approach to fishing not only ensures that enough fish survive to be able to spawn and produce offspring that are recruited into the fishery, but to fulfil other ecological roles as well. '
Otoliths are small, hard structures made of calcium carbonate, which play a role in hearing and balance in fish. Like tree rings, they record each year of a fish’s life in thin bands. To access this information though, each otolith must be put through a series of preparatory steps.
First the chosen otolith is embedded in an epoxy resin block. Three to five 0.35mm sections are then cut perpendicularly through the centre of the otolith using a diamond-tipped saw. The alignment of the cut must be just right to maximise the visibility of the bands and the series of sections gives Jeremy and Joe options if the bands in one section are obscured. The sections are then mounted in a polyester resin on a microscope slide and viewed under a dissection microscope coupled to image capture and analysis software.
Under the microscope, when lit from below, the growth bands appear as a series of dark and light lines, with each year represented by one dark and one light band.
'The two bands together represent the seasonal change in growth rate, with the lighter band considered to represent slower growth in colder months, while the darker, more dense band, with a higher proportion of organic material, represents faster growth in warmer months,' says project Co-Investigator Gabrielle Nowara.
But the bands aren’t always easy to count. For cold, deep water fish, such as Patagonian toothfish, the seasonal change in water temperature is limited, so the bands aren’t as clear-cut as they are in fish living in temperate coastal areas with distinct seasons. As the fish get older, the bands also start to get compressed.
'It’s generally easiest to age fish between 5 and 15 years old,' Joe says.
Nevertheless, Joe and Jeremy have had great success in ageing toothfish otoliths and have achieved a personal best of 100 otoliths analysed in one week. Their work has fed into an ‘integrated stock assessment’ model developed by Antarctic Division statistician, Dr Steve Candy, the results of which ultimately assist AFMA with catch quota calculations.
*Jeremy recently completed an Honours degree through the Tasmanian Aquaculture and Fisheries Institute, looking at mercury levels in Derwent estuary fish. Joe completed a BSc at the University of Tasmania and has worked with environmental science consultants, Aquenal, sorting and identifying benthic species in Ralphs Bay and the Tamar River. He has also worked with the Antarctic Division, identifying Southern Ocean fish species.