Migration patterns of the Faroe Plateau cod (Gadus morhua, L.) revealed by data storage tags

Spawning cod were tagged in March 2002, 2003 and 2004 on the Faroe Plateau with Data Storage tags, recording temperature and depth. Migration routes were reconstructed for 23 recaptured individuals using a state-space model and analysed focusing on horizontal and vertical migration behaviour in relation to spawning, feeding and areas closed to trawling. The state-space model is constrained by homogeneous temperatures and depths however the Faroe Plateau is ideal in this regard. Although our inference is based on limited data we here demonstrate a proof of concept. Regarding horizontal movement, a log-normal distribution of daily displacement was found with a significantly higher daily displacement during spawning than during feeding season. This indicates a clear distinction between spawning and feeding behaviour. However, while food availability did not affect the migration routes cod did avoid the trawled areas. Regarding vertical behaviour, cod stayed within 10 m from the bottom more than 90% of the time. Yet, individuals caught with longline had a stronger affinity to the bottom than cod taken by other gears such as trawl, suggesting that the use of trawl as survey gear should be continued in this area. The information, available from the simulated migration routes is a great improvement compared to the information achieved from conventional tagging when studying individual fish behaviour. The results can be used to support fisheries data, i.e. survey, logbook, stock assessment or acoustic data that lack resolution to evaluate individual fish.