This is a little bit technical but does make sense when you put your mind to it: it is recognised that from the stage of “eyed” ova, brown trout eggs need about 230 ATUs (Accumulated Temperature Units) to hatch into an alevin. 1 ATU = 1 day at a temperature of +1°C or 10 ATUs = 1 day at 10ºC.

The first eyed ova were observed on Stronsay around 20th January and hatched 26 days later; the water temperature in the shed has been averaging +9ºC so, to work out the number of ATUs we just need to apply the formula: number of days x temperature in ºC – 26 x 9 = 234 ATUs

This does explain why the ova at North Walls haven’t hatched yet. The water temperature in the shed there is about 2ºC colder and the eyed ova appeared about the same time (20/01). If we want to know when the eggs are due to hatch, we can use the number of ATUs needed for the eggs on Stronsay to hatch (234) and divide that number by the average water temperature on Hoy (+7ºC): 234 : 7 = 33.4 (about 33 days which is roughly a week later than on Stronsay).

So, as I’m typing this, if everything works according to plan, our alevins must be wriggling out of their shells right now! Don’t forget to look for the foam on the surface which is caused by enzymes breaking down the egg shells.

Let me know about any new developments on Hoy!