Sea Trout survey

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Juvenile sea trout populations in Orkney
The OTFA has been monitoring several key sea trout populations in Orkney since 2004.  This work has been augmented by ex-committee member Malcolm Thomson in his PhD research.  Malcolm previously reported on the mature sea trout population migrating into the Burn of Eyrland.  However, extensive data are now available on Orkney juvenile sea trout populations inhabiting burns across the Orkney Islands.  This report provides some detail on these populations, particularly on growth rates and density of young trout and the smolts which migrate to sea each spring. 


Which burns?
Around 80 burns in Orkney have now been surveyed by electrofishing.  Brown trout (Salmo trutta) were found in 35 of these burns.  This figure can be broken down into the following groups:

20 burns:  Presence of sea trout confirmed, either by presence of smolts and/or adult
sea trout.  Resident burn trout also present. 

Eyrland *
Waulkmill/Kirbister *
Wideford *
Rossmyre *
Burness *
Desso/Aikerness *
Stromness Mill
Whaness *
Ore *

* indicates which burns are monitored annually by OTFA

7 burns:    Only resident burn trout found, more than 2 age groups.  Evidence of migratory trout may yet be found.

Maitland (Finstown)
Hullion (Rousay)

8 burns:    Less than five burn trout were sampled, it is suggested that the presence of trout in these burns is tenuous.

Stennadale (Finstown)
East Rennibister
St Mary’s
Lean (Rousay)
Rennibister Farm

Therefore, a total of 20 burn systems in Orkney currently support sea trout populations with the possibility of a few more being identified in the future.  Orkney’s largest freshwater system, comprising the lochs of Harray and Stenness, will add to this number, for sea trout are known to migrate into this system in late summer/autumn and presumably spawn in the some of the various feeder burns.

Ore Burn Hoy
The upper reaches of the Ore Burn in Hoy

This leaves 46 burns where no trout were found.  Given the effectiveness of the electrofishing method, it is likely that trout are absent from these systems.  This was a slightly surprising result as it had previously been thought that more systems would support trout.  There was no obvious reason, such as gross pollution, that trout were absent from these systems.  It may be that water flow is simply too low to support trout in some cases, although the presence of trout in some very small burns, e.g. the Burns of Maitland and Stennadale in Finstown, indicates the ability of the species to cope with this situation.  Other reasons such as channel modification, poor sea access and poor water chemistry may also be to blame, but more investigation is required to know for sure.
The identification of burns with no trout has allowed the OTFA to undertake some targeted stocking work in an attempt to establish new sea trout populations.  To date (spring 2009), three burns with suitable looking habitat have been stocked with unfed fry.  These are being monitored in order to determine the success of the stocked fish, but survival rates appear to be reasonable at least over the first year.

Density of juvenile trout
Results of OTFA electrofishing surveys are shown in Table 1.  This shows that the juvenile density in sea trout burns varies between sites and between years, but tends to occur within the range of 0 to just over 3 trout per square meter of burn.  The surveys are carried out each year between July and October.  Trout numbers in spawning burns naturally decline through this period.  Trout fry emerge in large numbers from gravel in spring but this is followed by a high mortality rate as numbers settle down to the carrying capacity of the burn.  Therefore, some caution is required when comparing results gathered at different times through the survey period.  It also needs to be remembered that the results relate to only one or two sites in each burn and density may differ in other areas.  The assumption is made that trends from one site reflect the situation over the catchment and if that site is surveyed at the same time each year then trends in juvenile trout density can be tracked.

Table 1: OTFA Electrofishing results for sea trout burns, 2004 – 2008.



Trout per m2












































































*Based on single run survey

Trends in density
While results for some sites fluctuate, clear trends in density are apparent at others.   Significant increases in density were noted at sites on the burns of Desso, Wideford, Burness and Rossmyre.  These results underpin the case that the Orkney sea trout population has experienced a recovery in recent years, with a greater number of mature fish returning to spawn in these burns.  Why these should have shown such significant increases while others have not is unclear.  It could be that these burns, being slightly smaller than the others, are more sensitive to changes in numbers of spawning sea trout and therefore the best indicators of sea trout abundance.   
Another interesting feature is the modest and very steady results obtained from the two Hoy burns, Whaness and Ore.  Trout density at each site was similar and there was relatively little variation between years.  This contrasts with the higher and more variable results obtained from mainland sites. 
The results obtained in Orkney, especially in recent years are indicative of are very healthy populations.  They certainly compare well to results from sea trout systems elsewhere in Scotland, where densities rarely exceed 0.5 fish per square meter in late summer surveys. 
Data on juvenile trout in spawning burns is extremely useful, particularly in Orkney where there is no other system in place to monitor the health of the sea trout fishery.  The OTFA’s dataset is therefore a crucial part of sea trout conservation work in Orkney and has formed the basis of the Association’s input to local aquaculture development. The impact of other activities has also been gauged.  The Burness burn in Rendall suffered a pollution event in October 2006 and a new sewage pipe was laid under the burn (right through the OTFA’s site) as part of the new Finstown sewage treatment project.  Electrofishing demonstrated that fortunately, neither event seemed to have a major impact on the burn, indeed, the density of juvenile trout increased through the monitoring period.

Growth of juvenile trout
Using scale samples it has been possible to examine growth rates of juvenile trout in their first year of life.  In the period 2004 – 2006, the average length of a one year old trout, sampled in spring was 92.4mm.  The largest one year olds were sampled from the Binscarth/Wasdale system while the smallest were found in the Whaness burn in Hoy.   Indeed, the average length of one year old trout in all the Hoy burns was significantly shorter compared to those in Mainland burns.  This situation has been reflected by the OTFA surveys, where trout of less than 40mm in length are routinely sampled each year in October.

Two small trout (~40mm) sampled from the Whaness burn in October 2004.

The exact reason for the slow growth observed in the Hoy burns is unclear, but is possibly related to differences in land type.  Hoy is mainly heather moorland with peaty soil and largely unfarmed.  Its burns are probably nutrient poor in comparison with mainland burns, which drain an intensively farmed landscape.  Increased nutrient inputs probably stimulate invertebrate growth, providing more food for trout.  It is rather ironic to think that in Orkney’s most pristine burns (in Hoy) growth of young trout seems to be limited, while the burns on the mainland, many of which have been straightened and affected in other ways by agriculture, seem to provide a better growing environment.  It seems that there is a balance to be struck here, some enrichment may well promote trout growth, but obviously too much enrichment, e.g. organic pollution from silage effluent, is bad.  An interesting point to remember when trying to get farmers enthusiastic about trout burns.

Smolting trout
Eventually, many juvenile trout in these burns will smolt and migrate to sea.  The average size of smolts sampled across all burns in 2004-2006 was 154mm although they ranged in size from around 120mm to just over 200mm.  This time however, no difference was observed between the length of Hoy smolts compared with mainland smolts.  This was interesting, given the slow first year growth described above.  However, scale analysis revealed that smolts from Hoy were significantly older than those produced in mainland burns.  Hoy smolts were mostly 2 or 3 years old (average 2.7yrs), while mainland smolts were mostly 1 or 2 years old (average 1.7yrs).  These results lend weight to the argument that trout must reach a minimum size before they will smolt, and because first year growth is much slower, Hoy trout take longer to smolt compared to their mainland counterparts.  This pattern is illustrated in Graph 1 which shows annual growth in 1, 2 and 3 year old smolts.

After two surveys, no smolts have been found in the Pegal burn.  A large waterfall underneath the road bridge may prevent the upstream migration of spawning sea trout.  Although some may spawn between the road bridge and the sea, it is likely that smolt production is limited, if any.  There appears to be the remains of some structure at the waterfall – was there a fish ladder here in the past?  Access past this waterfall would open up a large area of burn for spawning and could be an interesting project in the future! 
Neither were any smolts found in the Caldale burn which runs past Scapa distillery.  This burn also has a large waterfall right at its mouth, which probably prevents sea trout access most if not all the time.  There was a very healthy population of resident trout though, and mature female burn trout were more common here than in other burns.  The presence of the waterfall may have caused an adaptation towards residency in this population, as migrating fishes (smolts) may not get back into the burn to spawn as adults. 
In terms of smolt production, extensive data is only available for the Burn of Eyrland where a trap has been in operation since 2004.  Annual results are shown in Table 2.  Trapped smolts are tagged, either by VIE mark (red line behind right eye) and/or by clipping the adipose fin.  Recaptures by anglers of these fish have occurred at Stromness, the Bush, the Bay of Eyrland, Orakirk and Houton.  A previous report on the sea trout returning to spawn in the Eyrland burn noted that many carried these tags, indicating their faithfulness to their natal burn.

Table 2: Smolt catches from the Eyrland trap 2004 – 2008.


No. of trout













*New trap installed





The smolt trap on  the Eyrland burn, where over 600 smolts were caught in spring 2007.  Screens are a combination of steel rod and larch strips.

The trap results from the Eyrland burn suggest that many Orkney burns will be producing hundreds of smolts each per year.  Only a guesstimate can be made for the overall figure, but it will certainly be in the thousands.  The big gap in our knowledge so far is the contribution of smolts from the Stenness/Harray system.  Given the size and number of burns, it could potentially produce more than all the rest combined, but has been impossible to sample so far.  It is hoped that some attempt can be made in the future to assess the importance of this system to Orkney’s sea trout population.

Orkney contains a large number of burns and it was previously thought that many would contain trout and produce sea trout.  We can now say with some confidence exactly which burns are producing sea trout – a much better situation to be in when trying to look after these populations for the future.  A number of interesting features arise from the work carried out so far:

It’s not many years ago though that sea trout here were almost non-existent and it was feared that they might not recover.  However, from late 2005 there were signs of a come back, with large numbers of small fish reported.  Much better fishing materialised in 2006 and now, two weeks into the 2009 sea trout season, Orkney enjoys an amazing sea trout fishery, capably of producing high numbers of pristine, hard fighting fishes, rivalling, if not beating anything else the UK has to offer.  Indeed, our fishery is a rare jewel, in a depressing picture of sea trout decline in many other parts of the UK.

What it’s all about – a muscular Orkney sea trout.

A number of problems still affect the survival of sea trout at sea.  Salmon farm production is on the increase again after a slump in production, which interestingly coincided with the sea trout recovery.  Unfortunately a small number of anglers continue to kill more than their fair share of fishes.  However, the spawning burns around Orkney are fundamental to maintaining the fishery.  While in the past we could only guess, we now have robust information on exactly which burns support spawning sea trout and therefore which burns we should be most concerned about looking after.  Such knowledge would be important for sea trout conservation anywhere in the world, but in Orkney where there is no catch return system, it provides the OTFA with an alternative weapon to help conserve sea trout numbers for the continued benefit of the community.

Grateful thanks goes to the Atlantic Salmon Trust, the Crown Estate, Scottish Natural Heritage  and the Wild Trout Trust for their funding contributions, without which most of the above work would not have been possible.  Thanks also go to the funding support of Orkney Islands Council and the Orkney Trout Fishing Association.

The many volunteers, both OTFA members and other members of the Orkney public, are also to be thanked for their invaluable help during fieldwork.

If anyone has any queries on this report you can contact Malcolm Thomson on:


OTFA Electrofishing Trap Survey

Eyrland Sea Trout Survey


Sea trout being released upstream with trap in the background

Research into Orkney’s sea trout populations continued through 2007. Committee member Malcolm Thomson is now well through his PhD and 2007 was largely spent in sampling work of juvenile and smolting trout in four of Orkney’s sea trout systems. However, through his work at SULA Diving and with funding support from the Atlantic Salmon Trust and the Wild Trout Trust, he was able to carry out an extremely interesting study of the mature sea trout population running up the Burn of Eyrland, Stenness, in autumn last year.

Between the 23rd September and the 22nd December, a trap was set by the fish ladder a short distance up from the beach. The trap was checked each day, often twice, and any trout caught moving upstream were recorded and released. Water level and temperature were also recorded.


The big one, 635mm and between 6 and 7lb.

By the end of the project, a total of 83 trout had been caught in the trap. This included 8 burn trout, leaving a total of 75 sea trout. The first sea trout were caught in late September, shortly after the trap was installed. The main run occurred over a four-day period from the 28th to the 30th October 2007 when 50 sea trout were caught in the trap. This run coincided with a rise in water levels after about one month of low water conditions. A small number were caught after this, with the last trout being caught on the 30th November.

Sea trout ranged in length from a modest 240mm, up to a very impressive 635mm. This largest fish was estimated at between 6 and 7lb in weight! In common with many other populations, hen fish were the most abundant, making up 79% of the sample. All but one was full of eggs at the time of capture. Most of the males were finnock in their first year at sea. Only three older male sea trout were among the sample.

Scale analysis revealed the age of the trout. The sample included:

• 17 trout in their first year at sea (0+), finnock, smolted spring 2007
• 32 trout in their second year at sea (1+), smolted spring 2006
• 19 trout in their second year at sea (2+), smolted spring 2005
• 3 trout in their third year at sea (3+), smolted spring 2004


Scale from a sea trout in its second year at sea (1+ sea winters).

No complete scales were obtained from the largest trout. Partial readings indicated that it was at least 2+, though it’s size would suggest it was older. Most of the trout had smolted and left the burn at two years of age, although some returning fishes had smolted at one or three years old.

The presence of tags on 31 sea trout indicated that these fish were coming home to spawn, having been previously tagged as smolts when leaving the burn. The improved trap installed in spring 2007 caught around 600 smolts and of the 16 finnock caught in the autumn survey, 11 were tagged.

This project has thrown up some very interesting facts about the sea trout spawning population using the Burn of Eyrland:

• The size of the sea trout population using the Burn of Eyrland numbered at least 75 trout in 2007. This is almost certainly an underestimate as the electrofishing surveys revealed that sea trout had run upstream before the trap was installed.
• The main component of the spawning population are female sea trout in their second or third winter after smolting. Older fish are less common.
• The number and size of the females caught in the survey would produce an estimated 85,000 eggs. The actual figure may be well over 100,000.
• Male sea trout are uncommon – do males spawn with multiple females? Do male burn trout, which are common, breed with female sea trout?
• Tagging shows that sea trout remain faithful to their natal burn, returning there to spawn.


Tagged sea trout

This is the first attempt to quantify the size of a sea trout spawning population in an Orkney burn. The numbers and age structure suggest a fairly healthy population although it is difficult to say using just one year’s data. However, these results are better than many larger systems elsewhere and may reflect the recent upturn in the sea trout fishery seen here in 2006 & 2007.