In November Wester Ross Fisheries Trust hosted a sea trout and arctic charr discovery week in the Carron catchment. The aims of the week were to try to assess the arctic charr populations in the catchment and to assess how far up the river sea trout were travelling to spawn and also the extent to which they were using the two large lochs, Dughaill and Sgamhain.
The week was a great success, recording good populations of charr in Loch Dughaill, where two different forms were thought to exist, and also in Loch Sgamhain where they have not been recorded for some years. They were also found in a loch high in the hills where they had never previously been recorded.
Arctic charr are of great interest to biologists. In contrast to other populations around the world Scottish populations are non-migratory and have been isolated in their lochs since the ice age, leading to differentiation between populations, much like Darwin’s finches. They even form different populations within single lochs, and it is thought that the surface feeding, pelagic fish and bottom feeding benthic fish in lochs such as Dughaill may be separate populations, showing genetic and morphological differences. They are thought to be particularly vulnerable to global warming and acidification, and despite North-West Scotland being one of their strongholds, their distribution is largely unknown.
The discovery week also helped illuminate the ways in which sea trout use the Carron catchment, and sea trout and finnock were captured in fyke, sweep and gill nets in Lochs Dughaill and Sgamhain. Fyke nets at the very top of the catchment captured fish which appeared to have been to sea and returned to spawn (pictured below). Scale analysis will hopefully reveal whether these large fish have migrated to sea or grown large feeding in the lochs for several years.
In 1936 and 1937 G. Herbert Nall assessed the sea trout population of Loch Dughaill, capturing in excess of one thousand fish. Wester Ross Fisheries Trust swept one of the bays used in Nall’s study and were pleased to record numbers of finnock and sea trout at the lower end of the loch. It was noticeable that a high proportion of these fish were immature finnock (pictured below), which may over-winter in the loch before returning to sea to feed in the spring. Scale samples were taken from all trout and will be analysed over the winter to try to assess which fish were genuine sea trout and which were loch trout.
2011 was another good year for the rotary screw smolt trap, as flows allowed efficient uninterrupted trapping through the majority of the smolt season. A total of 6757 salmon smolts were caught in the trap of which 293 could be identified as clearly being of stocked origin. 8 of these were fin-clipped smolts, and had originally been released as tagged fry in 2008. Most of these fish left the river in 2009 and 2010 as S1 and S2 smolts. The remaining 285 stocked smolts could be identified by their appearance, as they had been released into the river quite recently and were still distinguishable from river-reared fish. Of the 6464 smolts not identifiable as stocked fish, an unknown percentage would have been of stocked origin released at earlier life stages. Once stocked fish have spent some time in the river they can no longer be distinguished from wild fish. It is hoped that future research work on the Carron will allow us to accurately calculate the contribution of stocked and wild-spawned fish to the smolt run. The salmon smolt run peaked on the 27th of April when 843 smolts were found in the trap.
Although it is hard to correlate smolt trap captures with flow data as the trap becomes inefficient at higher flows and cannot be fished at all during periods of heavy rainfall, it is noticeable that in both 2010 and 2011 the peak of the smolt runs coincided with periods of lower flows, and in both years the main smolt run took place in the period immediately leading up to the new moon, with few fish being captured in the days either side of the full moon.
By releasing a known number of identifiable salmon smolts from upstream release ponds it was possible to estimate the efficiency of the trap in 2010 and 2011, allowing an estimation of the total size of the smolt run. The re-capture data suggested the trap captured about 17% of the smolts leaving the river. Using this figure, the actual smolt output of the River Carron during the 27 day 2011 monitoring period would have been close to 40,000.
A lesser number of sea trout smolts were caught in 2011 (346) and it appears that the trout smolt run takes place over a longer period than that of their salmon cousins. It is also possible that the in-river behaviour of the trout makes them less likely to enter the trap than the salmon. This would be the case if they were either inclined to stay closer to the river bed than the salmon, rather than travelling with the main current, or if the trout wander more in their descent, increasing their chances of avoiding the trap.
A number of other species were also caught in the trap. Most common of these are the eels. Declines in the number of European eels have been dramatic all across Britain in recent years, and it is noticeable that the screw trap has caught fewer eels per day in each consecutive year that it has been fished. Eels and elvers are an important food source for a wide variety of creatures and their decline may well increase predator pressure on the remaining fish.
Other creatures caught in the trap this year include a sea lamprey (pictured below), a number of stickleback and flounder and also minnows, a non-native invasive species in the Highlands which is proliferating in Lochs Sgamhain and Dughaill and certain parts of the river. The effects of this introduced species on salmon and trout populations are unknown.
