DVTF BROWN TROUT HATCHERY

Evidence that brown trout have been successfully spawning in significant numbers along the river Darent between Eynsford and Shoreham has persuaded the club install an incubator for brown trout eggs to the design of the Wild Trout Trust.http://www.wildtrout.org/WTT/library/articles/jacuzzi.asp . The whole project is being carried out in close collaboration with the Environment Agency who met the original cost of the incubator tank and associated fittings.

Modest success was achieved during the winter of 2002/2003 when about 15% out of a total of 15,000 eggs were hatched and released as fry. However, problems with spasmodic water flow and silt accumulation led to most of the eggs failing to hatch.

As a reuslt of that experience, modifications were made to the incubator for another trial in 2003/2004. A large ca 30 gallon filter tank was installed upstream of the incubator and a second water feed pipe was introduced to effectively double the flow.

Charles Saunders collected 10,000 eggs, at the "eyed-up" stage, from Berkshire Fish Farm in a refrigerated box on Monday December 8th 2003 and by 3:00 p.m. they were safely "planted" in the gravel bed at Preston Farm. The hatching time depends on the temperature but a useful empirical guide is to record the "Accumulated Temperature Units" (ATU's). This number is derived simply by adding together the measured water temeperature in degrees Celcius each day starting on the day when the eggs first showed eyes ("eyed-up"). In fishery experiments the time for 50% hatch to alevin (yolk sac) stage from the eyed-up stage for brown trout eggs has been found to be about 230 ATU's. On this basis, and using our own water temperature measurements, we expected 50% hatch on about January 14th with a spread over several days before and after this date. In fact, Peter Farina and Tony Kallend saw the first signs of alevins on Friday 21st January 2004 but they were probably there before that. Charles Saunders observed the first swim-up in numbers on February 7th and during the following three weeks the numbers gradually increased to a maximum of about 500 on February 29th.

	Left Trout alevins
Right Ian Douglas releases some fry

By the beginning of March, however, numbers of swim-up fry began to decrease and we suspected accumulated silt might be blocking the gravel channels. After taking advice from the Freshwater Fisheries Laboratory, we decided to dismantile the incubator and release the remianing fry. About 5,000 fully develpoed fry were released that day adding to the 2,000 previously released.

Incubator 2005. For the third year running, the DVTF trout egg incubator has been running at Preston Farm. On December 11th, Charles Saunders collected 15,000 eggs from Berkshire Fish Farm at Hungerford and just a few hours later they were safely planted in the gravel-filled incubator tank. This year an improved filter was used with a more robust tank and special filter granules of the type used for filtering water for Koi Carp ponds. From water temperature measurements taken throughout the hatching period, we predicted 50% hatch to the alevin stage (i.e. still with yolk sac) by 16th January. The first alevins were indeed spotted during the previous week and were returned to the gravel.

The incubator is situated below a low weir providing about 0.6m of water head which is sufficient to give a flow of up to 50 litres per minute through the gravel.

We were hoping to be able to show some fry ready for release in time for the working party of February 6th. In the event, cold weather delayed the development somewhat and the first fully developed fry were released by Alan Gaunt on Monday February 8th. Following that, numbers grew steadily and reached a peak of 1,300 released by Tony Kallend on March 3rd. The cold weather in early March seemed to slow down the rate of swim-up but the incubator was finally dismantled on Good Friday when no further fry were found.

In total, we released in excess of 12,000 fry although we believe we grossly under-estimated numbers on some days as we had not them developed a good method of counting them. Only a handful of dead eggs were observed early on and no dead fry at all.

The above diagram shows the daily number of fry released averaged over the previous 7 days. We believe the cold weather (water temperature as low as 2 C) up to about mid-March slowed down swim-up. After the cold spell the water temperature rose to over 8 C in the latter part of March. Plotted against accumulated temperature units (ATU's), the distribution swim-up rate shows a much greater degree of symmetry.

In parallel with our own incubator, but about a month later in time, the E.A. installed a second incubator nearby with 2500 eggs stripped from "native" Darent trout and fertilised in the E.A. laboratory before being planted about a month after our own eggs. We have been looking after this as well. This incubator is smaller than ours and is based on a design used by the E.A. on the Test for hatching salmon eggs. It also uses a different design of both main filter and inlet filter both based on our experience. We may well adopt their design of upstream filter next year. Incidentally, for anyone who knows the upper reaches of the Darent, the "native" trout were caught by electrofishing just where the Darent passes under Beggars Lane (the bye-pass that goes round Westerham to Biggin Hill) in Westerham.

By the end of March we had released over 1,500 fry from this incubator including 760 on one day released by Tony Kallend. By this time we had devised a good method of counting fry fairly accurately.

Note: See also details of the Wilton Fly Fishing Club experience with incubators:

http://www.wildtrout.org/WTT/library/articles/jacuzzis.asp