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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.