High connectivity of salmon farms revealed by aggregation, residence and repeated movements of wild fish among farms

Marine fish farms are widespread in coastal waters throughout the world, yet how they modify the movement patterns of wild fish species is largely unknown. We determined the spatio-temporal distribution of saithe Pollachius virens in a fjord system with intensive salmon cage aquaculture in Norway. Abundances of 8000 to 18 000 saithe were estimated around 2 salmon farms in the fjord using an underwater video system. Residence of saithe around fish farms and movements among farms and throughout the fjord were studied using implanted acoustic transmitters and an extensive array of automatic receivers. Of the saithe equipped with acoustic tags, 63% were observed daily at one or more of the 3 farms in the fjord over a 3 mo period. When resident at a farm, saithe spent 8 to 10 h d(-1) close to the sea-cages. Periods of residence at specific farms were interspersed with rapid and frequent movements to adjacent farms 1.6 to 4.7 km away. Of 24 tagged saithe, 15 moved among farms 2 to 21 times during the 3 mo period. If the movement patterns of the tagged fish are representative of the movements of untagged saithe, we estimate that fish from 2 different farms resulted in a total (+/- SE) of 167 112 +/- 41764 and 7768 +/- 1831 inter-farm movements during the 3 mo period. Thus, fish farms should be considered as connected, not only through ocean currents, but also through wild fish movements. If saithe share pathogens with farmed salmonids, their behaviours imply that they have the potential to act as vectors for diseases and parasites among salmon farms.