Abstract
Shellfish aquaculture is a growing food-producing sector. The blue mussel Mytilus edulis is the primary farmed shellfish and is also a main prey for various species of sea ducks. With their large density of high-quality mussels, mussel farms attract these predators, and consequent depredation by ducks represents a substantial economic loss among mussel growers worldwide. Total exclusion with nets seems to be the only method that provides complete and long-term control of bird predation. The best nets for duck exclusion must be cost effective, efficient, easy to handle, and safe for bird populations. In order to identify the best net type, we tested 8 different nets under controlled conditions using captive common eiders Somateria mollissima, the largest sea duck species in the Northern Hemisphere. We identified a net with a maximum mesh size of 6 inches (~15 cm) and large twine size to be best in excluding common eiders considering the above-mentioned criteria. Nets with thin twine and large mesh size were more likely to cause bird entanglement. In addition to using the best nets for sea duck exclusion, it is necessary to identify a target zone where such nets are the most effective. Good knowledge of the predation problem as well as collaboration among mussel growers, bird specialists, and government authorities are essential to reduce the costs and effort of installing and maintaining exclusion nets.
Highlights
Aquaculture is a growing food-producing sector, with an average annual growth rate of 8.3% from 1970 to 2008 (FAO 2010), and a total production of 63.6 million t in 2011 (FAO 2012)
Blue mussels Mytilus edulis are produced in mussel farms either from bottom culture or in suspended culture in most coastal countries of the Northern Hemisphere, both in Europe and North
Our study indicates that the best net to use to efficiently and safely exclude common eiders, is a net with a maximum mesh size of 6” with larger twine diameter
Summary
Aquaculture is a growing food-producing sector, with an average annual growth rate of 8.3% from 1970 to 2008 (FAO 2010), and a total production of 63.6 million t in 2011 (FAO 2012). Mussels are among the most produced marine shellfish, with a worldwide production of 1.8 million t in 2010 (FAO 2012). Different mussel culture techniques exist, but the most common method is the suspended culture, either with rafts (see Fig. in Rueggeberg & Booth 1989) or long lines (see Fig. in Mallet & Myrand 1995) (Spencer 2002). Productivity is better at shallow depths (0 to 3 m; Spencer 2002), but in areas subject to ice, ropes and lines may be sunk to greater depths (up to 10 m; Mallet & Myrand 1995)
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