Smolt traps 2012

For the last few years the Spey foundation has operated smolt traps in the Tromie and Truim, two important salmon spawning and nursery tributaries in the upper Spey catchment. The Tromie trap has been operated during the smolt run season (March to end of May) since 2009, whilst the Truim trap has been operated since 2010. Operating the smolt traps involves considerable resources in terms of equipment, staff time and transport. The focus on these two tributaries is because the water flow in both is abstracted and regulated as part of the Tummel Hydro scheme. Scottish & Southern Energy (SSE) have submitted a proposal which involves increasing the water abstraction from both tributaries hence the concern regarding smolt production. SSE contribute towards the running costs of the traps as does Hon. Micheal Samuel, a landowner on the Truim.

In 2012 both traps were deployed on the 5th March;  a date which based on historical catches should be early enough to trap the first of the smolts. The first smolts trapped this year were recorded on the 9th March when two salmon smolts were recorded in each trap.

Constructing and installing the traps is quite an undertaking but with the assistance of the Spey bailiff team both traps were built and deployed very efficiently.

Smolt trap construction

Truim smolt trap

Tromie smolt trap

The traps used are known as Rotary Screw Traps (RST). They consist of a rotating cone and box trap with two pontoons for floatation. The cone faces upstream and the baffles cause it to rotate leading any smolts, or anything else coming downstream, into the trap at the back. They sample only a proportion of the flow and the efficiency of each trap will vary from site to site. Our traps are two different sizes; the larger 6′ diameter trap is used in the Tromie with the 4′ diameter trap in the Truim.

In order to assess the efficiency of the traps and to obtain an estimate of the total smolt run a proportion of the smolts captured are marked then released back upstream. To mark the fish we use is a “tattoo” using a blue dye. This mark is temporary but remains visible long enough to allow recapture rates to be calculated.

Marking technique

Trout tattoo

Condtions were very good during the trapping season this year with only three or four days missed due to high water. These periods of high water occurred at the end of the smolt run so we can be confident the results obtained in 2012 provide a good indication of the run.

High water in the Truim, one of the few days when the trap was not operated in 2012.

In the Truim trap 2012 we captured 5487 salmon smolts, 3654 of which were marked with 1110 recaptured. The trap efficiency was calculated at 30.38% giving a smolt production estimate of 18061.

For the Tromie we captured 9045 fish, 2990 of which were marked with 2033 recaptures. The trap efficiency was calculated at 68% giving a salmon smolt estimate of 13301.

The 2012 smolt output figures are considerably higher than those recorded in previous years. The previous highest recorded salmon smolt output from the Truim was 8268 in 2010, whilst for the Tromie it was 8810 in 2011. In the Tromie we captured more salmon smolts in 2012 than had been estiimated for the total smolt output in previous years. Full analysis of the data in not yet complete; it takes a lot of time to enter the data into the spreadsheet and to prepare and read the scales, but it is clear that the smolt outputs we recorded in 2012 were at the upper end of what can be expected from upland tributaries. Based on estimates of habitat availablity in each tributary the smolt production per 100m2 was 9.2 in the Truim and 7.1 in the Tromie.

So how can we account for what appears to be a much improved smolt run from both the Truim and the Tromie? I think the biggest factor has to be the stable river levels that persisted during the trapping season. Our biggest single catch in the Tromie trap this year was over 1800 salmon smolts (20% of the total) in a 24 hour period. That happened when the river level rose by about 10cm overnight. The trap can continue to fish effectively during increases in river level of that magnitude. If there had been a 50cm rise in river levels the trap would have had to be lifted and we would have missed what is likely to be a significant proportion of the run. In each of the previous trapping years there have been periods, often lasting for several days, when river levels meant the trap had to be lifted. It is likely that the 2012 results have provided the best indication so far of the true smolt output from these tributaries with previous years results providing a cruder estimate of smolt productivity.

As well as salmon smolts we also captured many trout. The Truim and Tromie are considered not to be significant for sea trout spawning (they tend to use the lower burns) so most of the trout captured are parr moving downstream rather than smolts. However each year we do record some that appear to be smolting, an example is shown below.

The upper one a trout smolt?

Historically the Tromie has produced more juvenile trout, perhaps due to abundance of loch habitat upstream, whereas the Truim also provides the occassional big adult trout. We had trout up to 4lb in the Truim trap this year. A trout doesn’t get to that size feeding on flies, so its little wonder the smolts tend to be one the opposite side of the trap when there is a big trout in with them.

The traps were removed on the 22nd May this year, by then the smolt run was over. The peak of the salmon smolt run occurred a bit earlier this year than normal. In the Truim 50% of the total salmon smolts trapped were caught by the 10th April; in 2010 it was the 1st May before the same proportion had been trapped. No doubt the exceptionally good March weather had a bearing as water temperature has been shown to have a major influence on run timing.

The factors controlling smolt run timing are not fully understood but there is likely to be a significant element of genetic control. In order to find out more about this issue we are supplying genetic samples from salmon parr taken from the Truim and Fiddich, two tributaries at either end of the catchment, to researchers at Aberdeen University. I’ll report on the findings when they are available.


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