Eider Somateria mollissima predation of cultivated mussels Mytilus edulis
This thesis investigates the ecology of the eider Somateria mollissima in the sealochs of Western Scotland with particular reference to their impact on mussel Mytilus edulis farms. The numbers of eiders within sealochs showed seasonal variations with numbers increasing over February and March to reach a peak in April for all years. A summer decline in numbers was followed by an increase in the early autumn of all study years. Numbers then declined in lochs during the late autumn and early winter periods - a time when numbers in the Clyde estuary showed a corresponding increase. Mussel farms are now present in many sealochs along the West coast of Scotland. Farming is by suspended cultivation using either long-lines or raft flotation. Farms hold a concentrated supply of mussels which are within the size range normally consumed by Eiders. Cultivated mussels are thinner shelled and have a greater flesh content than similar sized individuals from adjacent intertidal populations. Peak numbers of Eiders at farms tended to increase with successive years of cultivation on site. Numbers fluctuated seasonally with large numbers present at farms in spring and autumn of most years. One farm on Loch Etive did, however, have a peak in Eider attendance during the summer. Spring flocks at farms tended to consist mostly of adults, while autumnal flocks were predominantly made up of juvenile birds. There was an excess of males in spring flocks at farms and both sexes arrived and departed simultaneously. A turnover of individuals was seen to take place at farms, with successive groups of birds moving through, giving rise to peaks and troughs in attendance over short periods of time. No definite feeding routine by birds at farms was apparent. The dive duration of birds feeding at suspended ropes decreased when they were scared as did the inter-dive period. The number of dives per feeding bout was seen to alter on sane occasions when birds were scared, scared adults spent less time loafing near ropes than did unscared adults, rather, all their time near ropes was spent actively feeding. Damage to ropes was approximately 2.6 kg of mussels per bird day. It is not inferred that this quantity was actually consumed by the birds but was removed from the ropes by their actions. Damage was visible as ropes completely or partly stripped or where selective removal of particular sizes of mussel had occurred. Plastic fringes placed around rafts were seen to be a useful short-term protective technique. Where alternative food was available then the value of fringes protecting stock was enhanced. Netting was also seen to be a useful protective technique and when placed around long-lines or rafts it may reduce the profitability of feeding at farms to birds by increasing their dive duration necessary to reach the mussel stock. Trials with horizontal and vertical netting around ropes led to a decrease in predation but did not completely eliminate feeding by Eiders at one study raft. The protection strategy enployed on farms must consider the number of birds feeding at the ropes and their pattern of attendance (turnover). Scaring activities and the use of physical barriers could then be pulsed to coincide with periods of maximum likelihood of damage occurring. Farm type and size is also important in protection, with raft based units likely to be easier to protect than long-lines. Similarly, larger farms may be more cost-effective to protect than smaller units.