Progressive mooring failure
Today, flexible circular tubes made of poly-ethylene (PE) are the most commonly used floating collars in Norwegian aquaculture. The major part of the flexible floating collars is moored using either a grid or a ladder mooring system. The floaters are connected to the mooring using bridles (Fredheim & Langan 2009). Traditionally the grid mooring system was oriented to minimize the total drag force on the fish farm, i.e. with the direction of the main current running through the length of the fish farm. Orienting the farm in this manner reduces the total drag as only the front nets are fully exposed to the current (the subsequent ones are in the shadow of those in front and will experience a reduced current velocity). However, when the farm is oriented in this manner the number of mooring lines in the same direction as the main current is limited. During a storm, the current velocity increases, especially in the upper water layers, introducing large forces on the nets. These forces are transferred to the mooring lines through the bridles. If one mooring line breaks or one anchor drags, the loads on the remaining mooring lines might be exceeded and they will rupture one by one, resulting in a complete failure of the fish farm. This was the cause of the complete breakdown of a fish farm in which close to 500 000 salmon escaped from a fish farm in central Norway in August 2005.
Breakdown and sinking of steel platform fish farms
Hinged steel fish farms are popular work stations as, due to their buoyancy, they allow the use of heavy auxiliary equipment and fork lifts on the farm. Due to their construction, steel farms in general have no or limited flexibility in the horizontal plane. This gives rise to structural problems when exposed to short-crested irregular waves, as the waves induce forced vertical displacement of the bridges and introduce large stresses and strains in the structure. Fatigue and crack propagation in or close to the hinges often results and can lead to separation of the bridges and tearing of the net. In January 2006, close to 150 000 cod escaped from a fish farm in northern Norway during a storm. The fish escaped after the farm collapsed under the combined loading from wind, waves and ice.
Abrasion and tearing of nets
Net failure, and the subsequent formation of a hole, is by far the dominant means of reported escape for fish from Norwegian aquaculture. Approximately two thirds of reported escape incidents and number of fish escape due to a hole in the net. Multiple reasons exist for the formation of holes. Biting by predators or caged fish, abrasion, “collisions” with boats, flotsam and cage handling procedures (e.g. lifting) are among the most common causes of holes in the net (Fig. 6c). The trend in Norway is that fish farms are moving into areas with stronger and steadier currents in order to improve water quality. As a result, forces on the net increase and net deformation increases accordingly. Several large scale escapes have occurred over the past 2-3 years due to contact (and thus abrasion between the net and the sinker tube chain (Moe 2008a, b, 2009c). In one of them more than 80000 fish escaped.