Vertical migrations of saithe (Pollachius virens) in Icelandic waters as observed with data storage tags

Knowledge of fish behaviour is important for general understanding of fish life history and responses to environmental and biological conditions. Vertical migrations can affect bottom trawl indices of the fish through their availability to the trawl and acoustic indices through changes in target strength. Here we present results based on data sets of hourly recordings of depth from 51 saithe. The data were analysed with respect to diel and seasonal differences in saithe vertical movement. Duration and extent of vertical movements were analysed with 1 min resolution, and the daily vertical range (DVR) was analysed with respect to potential use of the free vertical range (FVR). Our results show diel and seasonal differences in hourly depth changes of saithe, indicating reduced activity during nighttime and in winter. Individual saithe are capable of rapid depth changes in,1 min, often of the same order as the maximum observed in movements of longer duration. The DVR often exceeded the FVR, and in some instances exceeded critical limits, indicating that saithe may at times only maintain neutral buoyancy near the upper limit of their daily vertical range.