The photographs on the left were shot on 15/03/2011 on Stronsay, where the tank water temperature has been on average 2 to 3 degrees (at around +9°C) warmer than on Hoy where the pictures on the right were taken two days later (17/03/2011). Actually, the water temperature in the Hoy shed is much more similar to the one measured in the Orkney burns at this time of year (around +6°C). Look at how the yolk-sac has been nearly used up on the Stronsay fry while the Hoy alevins still have their whole sacs.
There was a slight delay in releasing our alevins, as we were advised by Scottish Natural Heritage to apply for a permission to Marine Scotland (Scottish Government) under the Aquaculture & Fisheries (Scotland) Act 2007. The application form for “Single Introductions” (accessible on http://www.scotland.gov.uk/Topics/marine/Licensing/fishintros) was actually relatively straightforward to fill in; it asked for details regarding the purpose of the release, grid references of the origin of the brood stock, details about the water where the introduction is going to take place, species of fish, number, age, etc… It was dowloaded, filled in and emailed to Marine Scotland last night and we received the consents to stock unfed fry in Meikle Water and the Ore Burn by mail today before noon! A very speedy service! So, if all is well, the Stronsay fry should be released on Wednesday 23rd March and the North Walls fry on Thursday 24th.
In the meantime, we were getting worried about our alevins’ yolk-sacs which have been shrinking considerably over the last couple of weeks (especially the Stronsay ones, which hatched earlier). Fortunately, we checked them today and by the size of their yolk-sacs, it’s safe to say that our alevins have still a few days left in the tank! (excuse the pun…) The photograph above was taken today and will give you a good idea of the stage of development our “peedie troot” are at.
Clockwise from the top left corner: Testing tank water for Ph. - Ova in the tank - First "eyed" ova at North Walls - Newly hatched alevin - 3 week-old alevin hatched on Stronsay.
If you’re at a loose end and that your Internet connection is fast enough (hope it’s faster than mine ;)), click on the link below to see the Stronsay alevins swimming in the tank.
Yesterday, there were only two eggs visible on the mesh frame and only a couple of alevins left… On closer inspection though, you could see there was activity at the bottom of the tank, underneath the mesh. It was time to check how the alevins were doing and we removed the mesh frame on which the ova had been lying and … they were all there, about 160 alevins, newly hatched! They look incredibly frail and yet, about 15 mn after we removed the mesh frame and left it to drip outside in the sun, Grace and Sarah spotted two alevins on the ground, still healthy as can be, and they were returned to the tank! We also rescued another two alevins which were trapped in the mesh and were returned, safe and sound, after spending 20mn on dry land!
On Monday, Jamie and Oscar had a unique experience: as they removed what they thought was a dead egg, they checked it closely in their hands but noticed a tail and then, the alevin hatched in front of them as they were holding the egg!
It was on Valentine’s Day, 2 weeks ago that our ova hatched into alevins at the Stronsay School. The pupils have been visiting them every single day, making sure the pump is working, removing the dead alevins (3 casualties over the 2 weeks) and recording water temperature. The yolk-sacs on the alevins have visibly shrunk and the small fish are turning darker and a lot more active. Jim Adams let us use the picture above which shows an alevin about the same age as ours just now – all the other pictures were taken at the school.
The first alevins have hatched today at the North Walls school! They were very difficult to spot through the Hoy peaty water! However, the clue was revealed this morning when Jamie and Oscar said they had noticed foam in the tank, a sure sign of the alevins hatching. Class 1 and 2 went to have a look and when looking very closely, Callum noticed some of the eggs rocking as the alevins were wriggling inside their shells! Oscar even spotted an ova with a tail sticking out!
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.
This is it! Eventually, the alevins have hatched! The very cold weather back in November/December has certainly been responsible for the delay but we got there in the end! The death rate has been very reasonable too with probably a dozen ova and 4 alevins out of 130 eggs. The grid at the bottom of the tank has been removed to give way to gravel which provides the alevins with shelter from the current generated by the water pump. We have even been able to observe the hatching process as an alevin was wriggling out of its egg shell!
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