GPS Tags Results

Previously unknown migration of cod
-Results from DST GPS Data Storage Tags-

In 2004 a total of 302 cod were tagged in Icelandic waters with the DST GPS, an archival tag that can receive and store GPS positions received from boats.  The system’s principal is that a free swimming fish can receive a position from a boat, either through a Simrad sonar, or a small and portable sounder from Star-Oddi called FPS (Fish Positioning Sounder). The position of the boat is recorded in the memory of the tag, when fish is within the transmitting range.  Temperature and depth is also recorded at user defined sampling intervals. 

The GPS tagging has been conducted by the Marine Research Institute (MRI) of Iceland, led by fisheries biologist Dr. Olafur Karvel Palsson.  Dr. Olafur Karvel recently held a presentation meeting at IMR.  The results indicate that a few cod that were tagged South of Iceland, went North of Iceland for spawning.  This behavioral pattern has previously not been observed. The objective of the research was to study the migration route of cod from main spawning grounds South West of Iceland, to feeding grounds. Furthermore, a new technology was being tested, with the cod being the first species to be tagged with the new tag called DST GPS.

Boats and vessels frequently get a position from a satellite.  The GPS signal is modulated into a Simrad sonar or a Star-Oddi FPS, and then transmitted via acoustics underwater to DST GPS. As the fish tag listens for the acoustic signal, it receives and records the GPS code (longitude-latitude). The research vessel Bjarni Sæmundsson was used for the study as it is equipped with a Simrad SP70 sonar.  The Simrad sonar has a transmitting range of ca. 5 km (radius), depending on environmental conditions in the sea. 

In April and May 2004, 199 tagged fish were released South West of Iceland from two different fishing grounds (Selvogsbanki, Eldeyjarbanki).  In October same year, 103 tags were released from another fishing ground at the South East coast (Myrabugur). Acoustics (positions) were transmitted from the vessel’s sonar in May, July-August and October 2004, and in February-March 2005.  From the April-May tagging a total of 67 tags have been recaptured (33.7%), and 8 of them had recorded positions (6 in May, 1 in Oct, and 1 in Feb-March).  These tags were mostly positioned in shallow waters off the southern and western coasts.  In the first batches of DST GPS (199 tags in May 2004), it was realized after recapture that some tags had problems, the battery lost contact, which explains the low reception of GPS codes by the tags.  In the second round of batches of DST GPS (103 tags in Oct. 2004) these technical problems had been corrected and worked as expected. 

From the 103 tags that were released in Myrabugur in October 2004, 36 tags have been recaptured (35%), all in shallow waters and most of them close to the release site. 6 of the tags were recaptured elsewhere, thereof 3 north east and north of the shore.  33 tags had recorded temperature and depth the whole period in sea, and 22 tags recorded positions. A cod recaptured in Myrabugur showed limited migration with regards to depth. It mostly stayed above 200 m, but started to increase its vertical migration in the coldest months of the year; February and March.  For most of the time, the fish stayed in relatively warm sea, 6°C and higher, although the sea temperature was considerably lower from January to March.  Two fish tagged in Myrabugur, were recaptured North of the country, one at Eyjafjordur in March 2006, and the other in Thistilfjordur in May 2006.  They showed very similar migration with regards to depth and temperature preferences, and both seem to have spawned in relatively cold waters (~3°C) in April-May 2005, although they stayed in warmer waters most of the year.  “The Myrabugur area has one of the best conditions for cod around Iceland, and the fish there has a high growth rate.  The cod in that area is relatively local, so it was not expected that it would migrate much,” Dr. Olafur Karvel said.

The main results from the cod tagging in Myrabugur is the confirmation of a relatively local fish, but approximately 10% migrate outside the area.  From the 36 tagged that were recaptured, 3 fish migrated to the North of Iceland from the South East area.  Taking this into consideration, the hypothesis for migration pattern of cod is that it migrates to the North for spawning from March to May, but stayed most of the year in Myrabugur.  “The common opinion is that cod growing up in the South, also spawns in the South.  If it migrates to the North for migration, then it’s a new pattern for cod around Iceland.  These cod were 6-9 years old, up to 90 cm in length when recaptured, which is the age and size of a spawning cod. Is the purpose of the trip to the North anything else but to spawn?  The only thing we need is to capture it in spawning condition in the North.  An interesting observation is how much of a hurry the fish is in.  The cod recaptured in Eyjafjordur had travelled 10 miles per 24 hours on average on its way from Myrabugur.  That’s quite a sprint,” Dr. Olafur Karvel said.

Questions and comments from people at the meeting included other hypotheses about the previously unknown migration pattern of cod. One was the possibility of cod migrating from South East to the North to avoid spawning by going to cooler waters.  Another hypothesis was that the cod tagged at the South East coast, is in fact a spawning fish from the North, which migrates to the South West where it dwells most part of the year, but migrates North to its ‘home-ground’ for spawning (similar to how the Salmon and other species do).  Dr. Olafur Karvel did not want to exclude these possibilities although he believed his previously explained hypothesis to be more likely.

Learn more about the GPS system.