Marine

Biologist’s update from Shieldaig: December 2015

December 14, 2015 by Marine Scotland Communications No Comments | Category Marine Directorate Science

Four different ages of trout from a single section of the Shieldaig River

Late summer and autumn are busy periods at the Shieldaig field station. The two main tasks are running the river trap for sea trout (ascending) and silver eels (descending) and carrying out electrofishing surveys of the catchment. After a wet spring and summer, September was reasonably dry, allowing the electrofishing to be conducted without hindrance. This annual survey is designed to assess fish populations in the river and monitor the performance of stocked fish.

The first photo shows a selection of trout captured at one of our sites. The large fish is a mature male which was expressing milt and ready to mate. Below this is a younger fish, in its third year (or, in the terminology of fish scale age analysis, a 2+ fish, indicating that the fish has grown through two summers and is part way through the growth of the third). Below this is a 1+ fish and at the very bottom is one of this year’s fry.

A Finnock

Many years of electrofishing the same sites in the same river yield, eventually, an important long term data set on the performance of fish in fresh water, providing essential context in which current trends in populations can be assessed. An additional benefit of the repeated electrofishing surveys is a growing familiarity with the minutiae of river and bank habitats, making the process more efficient. Navigation begins to proceed by familiar rocks, structures and trees. In fact, during routine communication, the sampling sites lose their formal designations and become known by their individual features, for example ‘the cleft rock’ or ‘the holly tree’. This summer, while working around a site known formally as Shieldaig 36, but informally as ‘the culvert’, there was some unexpected company: a large adder some 50 cm long sunning itself on the remains of an old road bridge. A second, smaller, adder was heading for cover, while nearby half a dozen slow worms, legless lizards rather than snakes, were curled around each other in twos or threes.

An nathair, a nadder

Snakes are rarely seen in Glen Shieldaig, so spotting these adders at close range was an unexpected pleasure. ‘The culvert’ was revisited several times over the summer and autumn and the snakes were present most days, sunning themselves on a south-facing bank. Adders incubate their eggs internally and give birth to live young, usually in late summer or autumn.

The adder is the only snake found in the Highlands. Its Gaelic name is ‘an nathair’, which is of the same origin as the old English name for serpent, ‘nadder’; over time ‘a nadder’ became ‘an adder’. Will there be young nadders on visits to this site next year? If so ‘the culvert’ will be in danger of losing its former nickname for a more serpentine moniker.

More Information

• Freshwater Fisheries Science
• Previous Blog from Shieldaig (June 2015)
• Previous Blog from Shieldaig (April 2015) 

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