click on images to enlarge.
In Scotland there has for many years been considerable debate about the impacts of hydroelectric schemes on migrating fish species. Here on the Kyle of Sutherland we are impacted on three out of the five rivers within our catchments. The Shin is perhaps the most heavily hydro influenced, and has water diverted from the Brora, Cassley, Naver and Shin all going through the Shin turbines.
Politically, renewable energy is high on the agenda of the Scottish Executive so it is important that the Board’s, Trust’s and other fisheries interests work closely with S&SE on restoration and conservation projects. The Shin has been heavily modified since the mid to late 1950’s so we have been in partnership for a long time. Traditionally the negative effects of the Shin hydro scheme have perhaps been mitigated by stocking. Fry raised in the hatchery have been planted into the tributary rivers flowing into Loch Shin for several decades. Why? Because it was regarded as possibly the best way to compensate for the loss of salmon spawning upstream of the dams at Lairg.
Let us look at some of the facts. The fish counter (which is said to be approximately 80% accurate) in the Lairg dam shows an annual return of about 150 salmon every year. Based on data from neighbouring catchments the numbers of fry / ova planted should be generating a return upwards of 500 returning adult Atlantic salmon. We have to ask ourselves why is there such a discrepancy in these figures?
Lets consider the jigsaw piece by piece. We know from continued electro fishing (partially funded by S&SE) that we have a healthy juvenile population in the rivers. Therefore the planting operation works well. Also from work undertaken in the 1990’s, it was established that 90% of main spring migration of smolts travelling through the turbines survive with the correct loading of the turbines (revs per minute). It is assumed that the lift mechanism within the dam is appropriate for returning adults. If it did not work at all we would be seeing a zero upstream count. So why is there such a “hole” in our data. If we were ticking boxes, we have;
It is reasonable to deduce that the “hole” in the data is one of smolt survival between leaving their natal rivers and arriving at the dam. This may explain the 150 adults going through the dam rather than the expected 500 + (from calculations). There is undoubtedly enough riverine habitat to allow 500 adult fish to spawn.
We may be losing smolts in Loch shin for a number of reasons. These could included predator impacts such as ferrox trout or fish eating birds or alternatively there may be some physical problem that prevents the smolts finding their way past the dams.
So the recent challenge has been to design a project that will answer the question posed by poor smolt survival. We have been thinking about this for the last four years and finally we are in a position to begin the three-year project starting in the spring of 2005. This is largely due to the recent formation of the Kyle of Sutherland Fisheries Trust. This organisation has taken on the responsibility of running the project.
The river Tirry is where the fieldwork will be undertaken. There are a number of criteria that lead us to this decision. Potentially there should be sufficient smolts migrating to generate a sample size that should be large enough to deliver some meaningful conclusions. Also access from the road will enable us to easily get to the river. As the Tirry is the tributary that is nearest to the dams it is reasonable to assumption that whatever the impacts are then they would likely be greater for the smolts leaving the other tributaries.
Technically this project is quite
difficult. We will be employing a rotary screw trap, which is Canadian built.
This device allows us to catch part of the smolt run. Once the smolts have been
captured they will be “PIT (Passive Integrated Transmitter) tagged”. PIT tags
are used in many other industries, such as “micro tagging” pets. This
technology allows us to uniquely identify each fish. The tags are read using a
decoder, and each tag has a unique identification series of number and letters.
Therefore we will be able to track individual fish. Approximately 3000 smolts
will be tagged each spring, and the diversion dam at Lairg will have a permanent
decoder fitted into it. This decoder will be able to read both smolts migrating
down stream and adults returning to spawn.
click on images to enlarge.
What we are trying to establish “is there a difference in survival between a population of smolts that swims through the Loch and dams against a population that does not swim through the dam”. To achieve this half the tagged smolts will be placed in the river downstream of the trap and half will be transported to the river Shin and released below the dams. We will identify the populations because of the unique identifications read-off from the tags. The moment of truth will be upon us when the adult salmon start to return, perhaps there will be more adults from the transported population than from the river released population. Also we will know the numbers of smolts that survived the journey through loch Shin, as the decoder in the dam wall will record them as they migrate downstream.
Once we have collected three years worth
of data what will we do? Well its simple really, if there is a significant
differential survival to adult returning stage between the transported
population of smolts and those that swim through the Loch and dams then we will
have to alter our management strategy. One solution may be to divert resources
into a long term trapping and trucking program. The objective within the Shin
system is to restore a self-sustaining run of adult fish destined for the
tributaries above the dams. From our existing data it is clear fish born and
breed below the dam walls support the majority of the current shin population of
Atlantic Salmon.
