When Rhys Allen saws into the head of a 70cm-long dhufish skeleton, he has to get the cut just right in order to intersect the ear bone that is used to determine the age of the animal.
Mr Allen is a research technical officer with the Department of Fisheries, and spends a few days each week dissecting and collecting data from fish skeletons supplied by recreational and commercial fisheries, as part of the Send us your skeletons project.
Dhufish can live for up to 40 years, but Mr Allen said the 70cm specimen he was examining was more like 10–18 years old.
“For our dhufish stock the fish length isn’t necessarily a good indication of age, so we basically take a bone out of the head called the otolith, which is the inner ear of a fish,” he said.
“That’s got growth rings on it just like a tree, and from that we can determine the fish’s age.”
In 2007 several West Australian demersal fin fish species were identified as being overfished.
Catch limits were reduced by 50 per cent to enable stocks to recover, and the skeletons project began to systematically monitor the populations.
“The data we collect is used in the stock assessment. This is where the age is critical,” Mr Allen said.
“We determine the age structure of the stock by determining how many fish there are of each age for the year.
“That is a very good indicator as to the health of the stock. If a fish stock is heavily fished, you will see very few older fish and lots of younger fish.”
Finding ear bone via fish lobotomy
Each carcass is examined with the length, sex and spawning condition recorded, before an electric handsaw is used to retrieve the ear bone from the skull.
“We basically give the fish a lobotomy and then we can remove the brain, and the otolith sits at the base of the brain cavity,” Mr Allen said.
“As the fish grows the otolith grows with the fish, and calcium is deposited on the outside of the bone each year.
“This calcium is deposited at different rates at different times of the year, which provided us with growth rings.
“We then take a very thin cross section of otolith which is only 300 microns thick, and from that under a microscope we can count the rings and that tells us how old the fish is.”
The department said it would take about 20 years for the management changes to be reflected in a full recovery of the stocks.
In a couple of months an assessment using data collected by Mr Allen and his colleagues over three years from 2012 will be complete, giving the first data-based assessment of the management changes.
Dhufish, snapper, baldchin groper and tailor fish frames, with the head and guts intact, are collected at fisheries offices and participating stores across the state.
The department seeks 500 of each fish per financial year, and runs a number of incentive schemes and raffles to reward fishers who participate.
Mr Allen said recreational fishermen also liked to follow up on their donations.
“We generally provide the age information back to the fishermen at the end of the year once it has been processed, so a lot of people are quite keen to know how old their fish is, particularly if it is a large one,” he said.