The age of fish

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By Milthon Lujan

by Peta Morton, The University of Sydney 
Ageing animals is never a simple process, but a far more important one than you might first think.

Think about how hard it is to know how many candles to buy for your co-workers birthday cake without asking or sneakily checking their file.

While trying to figure out the ages of animals may seem like a somewhat trivial pursuit, accurate ageing becomes incredibly important when you delve into the world of fish.

Most fish have age-dependent fecundity, which means the older they are, the more eggs they are able to produce. This dynamic is a key factor in the sustainability of the fisheries that feed us.

Fishery limits are set by estimating how well the fish will be able to breed and repopulate the numbers lost to fishing. This is done by modelling the current population levels and their rate of reproduction, which usually includes educated guesswork on both counts.

Since the rate and variability of reproduction in many fish species is based off of their age, knowing the age and birth year of a fish can be integral to figuring out its vulnerability to extinction.

So we need a way to accurately age fish – and the good news is that we have one. It’s called otolith ageing and it’s been used in the ageing of fish since the production of the microscope (interestingly it is speculated that Aristotle may have been the first to suggest something along these lines in 320AD).

It works on a very similar basis to reading tree rings. Essentially, there is a small bone deep in the inner ear of bony fishes called an otolith. Once you have retrieved this bone, you clean it up, take it to a lab, and carefully count the rings from the centre.

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Because the bone grows throughout a fishes’ life – this should give you a fairly accurate reading of the fishes age. There’s just one problem – the fish has to die.

So if you want to use this methodology for fishery management, you have to assume that the samples you retrieve are a good indication of the population, and it doesn’t really work at all for severely threatened species where removing individuals is not a viable option.

The good news is that just recently, the CSIRO announced that they had devised a method for ageing the extremely vulnerable Murray and Mary River Cod and the Australian Lungfish. They did this by reading the code of their DNA.

When cells divide, DNA is replicated which leads to a process known as DNA methylation where a methyl group may be added or lost to the molecules along the chain of DNA.

Over the fishes’ life the methylation occurs at a predictable rate over the course of a fishes’ life, scientists can take a sample from any given individual and compare it against their reference to accurately identify the age of the fish.

All of this can be done with just a tiny clip of their fin, leaving the animal alive, meaning the process is accessible to even the most threatened populations.

This type of DNA methylation has also proven to be a successful ageing method in humans, some mammals, and even mantis shrimp.

This paper represents the first time this methodology has been used in fish and should mean that a lot less of them have to die for us to figure out how many babies they could of potentially had.

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While it’s still a long way off from being able to snatch a lock of your coworkers hair for their birthday cake, this methodology is a significant step in how we age animals.

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