Vertical distribution of saithe (Pollachius virens) aggregating around fish farms

Abstract

Abstract Floating fish farm installations attract a number of marine species, probably because they provide both shelter and excess feed from the cages. Saithe are by far the most numerous fish visitors to fish farms on the Norwegian coast, and may gather in large numbers beneath the cages, but detailed knowledge of their swimming behaviour is limited. This study examined the vertical movements and distribution of saithe equipped with acoustic transmitters at 15 fish farms over a period of almost 2 years. The saithe aggregating around fish farms displayed behavioural patterns reported from saithe elsewhere; diel rhytms in vertical distribution, reduced activity level at night and seasonality in swimming depth. Typical residence depth was from 25 to 50 m in summer and 60 to 90 m during winter. However, an anomaly was observed, as fish moved 10–20 m closer to the surface during mid-winter. The reason for this is not known, but may be associated with the use of artificial light to illuminate the fish farm sea cages. The fish were usually distributed through ∼100 m of the water column. In 38% of the observations, there was no apparent diel vertical migration pattern, but 46% of the data showed fish either moved up or down during the daylight hours. Interindividual and intersite variability, in preferred night-resting depth, is a possible explanation for the differing vertical migratory patterns. Fish also moved away from fish farms in daytime (16% of all observations), probably to feed elsewhere. The results suggest that saithe establish core residence areas close to fish farms.