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.