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Every week, young people from schools in WA visit the Department of Fisheries’ Naturaliste Marine Discovery Centre and take part in a practical lesson on how to dissect a fish.
What they aredoing is one of the most basic and important tasks carried out in fisheries research. In order to set up computer modelling of the make up of fish populations, scientists first have to get their hands dirty, or at least a bit bloody.

One of the ways in which fisheries scientists can tell if the stock of a particular fish species is healthy is to look at the numbers of fish of different ages in that group.

The technical term for the numbers of fish of different ages within a stock is called the “age structure”. With species that have varying degrees of success in spawning or of young fish surviving to join the breeding stock, there can be big gaps in the age structure. In other words, there may be few fish of a certain age and more that are either younger or older.

Knowledge of the age structure is important, because if the fish is a popular recreational or commercial target, this means that heavy fishing, combined with a period of low numbers of fish joining the breeding stock, could lead to the population being “fished down”, possibly to the extent that it is hard for it to recover.

Being able to tell the ages of the fish in a population is therefore vital to scientists, but how do they do it?

The simple answer is fish dissection, which not only provides the fish’s age (usually to a specific age in years) but also lots of other handy information, such as gender, spawning condition, what it has been feeding on and even, through further analysis, what water conditions the fish experienced during its life.

Fish inner-ear bones, called otoliths, show bands or rings that, like the rings in a tree stump, represent one year of growth. By extracting the otoliths, slicing them with a high-precision saw and examining the cross-section under a microscope, scientists can count the bands and work out the exact age of the fish.

They can also conduct chemical analysis of the sections to work out other stuff, such as whether the fish lived in an estuary earlier in its life.

At this point, you may be thinking: “Fish ears? Fish don’t have ears.”

Well, not on the outside like us, but inside their head they do. The ear bones function much as those of the human inner ear, playing a role in hearing and balance.

Extracting the ear bones requires skill for some species but for many fish, such as Australian herring, it is something that kids (and their mums and dads) can learn to do. I have learnt how to do it and regularly help in centre demonstrations.

It is an interesting exercise, though admittedly rather messy.

With the school holidays just started, there is an opportunity to give fish dissecting a go, among a number of other interactive, educational and fun sessions being run at the centre, on the north side of Hillarys Boat Harbour. Come down and peer through the windows at the scientists working with otoliths.

Bookings are essential for school holiday activities, most of which cost $12 per child (which includes entry) and a minimum age can apply.

Phone 9203 0339 for more details or visit www.nmdc.com.au.

GLEN JAKOVICH