Marine
Biologist’s update from Shieldaig: June 2015
July 21, 2015 by Marine Scotland Communications 1 Comment | Category Marine Directorate Science
Welcome back to Shieldaig, where spring has sprung and we are surrounded by new life. The run of sea trout smolts is now at an end and we have escorted nearly 650 fish on their way to the sea. A number of these have already returned to the river and some were found to have sea lice attached. Returning to a river is a way for fish to remove these parasites, as the lice are intolerant of fresh water. We report on sea lice numbers in plankton samples at Shieldaig elsewhere in the Marine Scotland Science web pages.
Barnacles settled on rock (several years’ worth).
Spring arrives early in the sea. Before any buds have burst or birds nested, the increasing day length brings successive waves of organisms in the plankton. The reappearance of these animals is like welcoming old friends; a favourite is the barnacle. Early in May there were dense clouds of barnacle cyprid larvae dancing in quiet water at the shoreline between seaweed fronds. Within a day or so they had settled out of the plankton, covering the rocks like a sprinkling of salt. After another day or two they had transformed into the more familiar barnacle form, albeit tiny, known as spats. Barnacles are crustaceans and grow in the same manner as crabs, shedding their exoskeleton periodically, and we regularly find their moulted feeding appendages in our plankton samples.
Glass eel on arrival at the mouth of the Shieldaig.
An early-rising heron caught by trapcam
However, since conservation efforts have led to the closure of glass eel fisheries in Europe which, historically, were the source of information on eel recruitment, ICES (International Council for the Exploration of the Seas) have identified the need for new indices. Accordingly we have begun to monitor these young eels on their ascent into the Shieldaig, with a view to developing a first Scottish index of glass eel recruitment. It is amazing to think that these tiny animals are already 2 or 3 years old and have travelled several thousand miles from their spawning grounds in the Sargasso Sea. It could be another 30 years before these individuals head back out to sea to reverse the long journey and complete their life-cycle. During spring we also find juvenile flounders entering the river. Flounder and eel are unusual amongst European fish in adopting a catadromous life-style: breeding in the sea and making use of fresh water for growth. The opposite, anadromous, life habit is more familiar, and typified in Scotland by salmon and sea trout.
Spring also brings nesting birds: we are often accompanied by sandpipers telling us to keep away from their nests or young. The chicks run around in the car park and occasionally even peek in through the office door. A pair of house martins is currently nesting on our burglar alarm. Less welcome visitors during the smolt migration are herons, attracted to the trap by the concentration of small fish. We have engaged a variety of methods to dissuade their attentions including netting, bunting and scarecrows, with general success.
Less welcome still is the return of the midges. They were slightly later than usual this year, not appearing until late May, but now we’ll have the dubious pleasure of their company throughout the summer and into autumn. They can make our work ‘challenging’ at times!
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