Scales tell the story of a farmed rainbow trout

Scales tell the story of a farmed rainbow trout

There are common grumbles during my Marine Biology degree (sometimes from my direction, often from others) that there’s not enough practical work and too much theory.
While both are necessary, the most visually interesting is obviously those good prac classes that people go away raving about, such as yesterday’s look at methods of ageing farmed (and somewhat sickly-looking) rainbow trout.
This  photo is taken from a scale reader – showing scales from below the lateral line on a 1+ year old trout, with growth lines (annuli) helping indicate the age.
The trick is picking scales from the fish that haven’t regenerated and so have been on the fish for its lifetime – not an easy task on a small fish brought up in a farmed environment!
My ancient phone doesn’t take the best shots (I drowned my smartphone on another field prac earlier this year and reverted to the old Nokia) but you can still make out the circular lines on the larger right portion of the scales.
The lecturer also showed us how to remove the otolith (ear stones) from the fish to assist with age identification by measuring the growth rings, and also measuring the teeth and intestine to determine feeding category (such as herbivore, omnivore, predator).
I’m in the middle of writing an essay on the age, diet and reproductive capacity of southern bluefin tuna (thunnus maccoyii), so practical work such as this helps to give an appreciation of the work that goes into academic research on those topics.

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Posted on August 29, 2013, in Deakin University, fun, photography, science, sea life and tagged , , , , , , , , . Bookmark the permalink. 2 Comments.

  1. Nature is a strange and fascinating thing. So the scales are like tree trunk rings? Only tree rings are years, so what are the scale rings, months?
    Also, are you saying that farmed trout are like Dr Who? Awesome.

    • when it comes to some types of larval fish, otoliths can be used to measure their growth in days! non-regenerative scales (i.e. those that have stayed with the fish for its life til capture) can show their age in years – but as the photo shows, they can be tricky to read.
      Simplistically, the thicker growth rings you can spot on the scale are the annuli (yearly marks). You need to collect several from at least three different spots on the fish to make sure the age is accurate

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