Aquatic & Fisheries Vertical Migration of Wild Faroe Plateau Cod

Not to be cited without prior reference to the author
Microprocessors and Things that Swim in the ocean:
Smart Tags in the study of Marine Life.
International Council for the
Exploration of the Sea CM 1999/AA:02

Studies of vertical migration of wild Faroe Plateau cod by use of Data Storage Tags

Petur Steingrund
Fisheries Laboratory of the Faroes
Nóatún, FO-100 Tórshavn, Faroe Islands



Eighty eight wild cod were in October 1997 tagged with data storage tags placed inside the buccal cavity. The release site was approximately 12 nautical miles north-west of the Faroe Islands at 62o19’N 7o39’W (100 m). By August 1999 forty-one (47 %) were recaptured, all from Faroese waters. All cod stayed exclusively in warm (> 5 oC) Atlantic water. The cod usually stayed at shallow (90-130 m) and nearly constant depth (presumably at the bottom) during winter. Thus vertical migration was rare, but two very clear examples were observed at exactly the same time, probably feeding in the upper layers. The onset of spawning migration could in some cases be detected by a change in depth.


Keywords: Gadus morhua, vertical migration, spawning migration, temperature and depth, Data Storage Tags.


Tagging experiments with conventional tags reveal horisontal migrations of fish, but normally no information about vertical migration of individual fish. Underwater telemetry has been used since the 1950s (Baras 1991), but is very time-consuming and expensive (Godø 1995) and normally has a short time span (less than a week). During the last decade or so, Data Storage Tags (DST) have been used to study vertical and horisontal migration of different fish species (see ICES 1997/ICES 1998 for a list of references). Data Storage Tags (or archival tags) have a very great potential to study vertical migration of individual fish, since they can operate for several months.

In the north-east Atlantic, DSTs have been used to tag cod at Iceland, Norway (Barents sea), and the Faroe Islands within the last five years, partially financed by Nordic Council of Ministers (Nord Fiskeri, 1998). The Icelandic cod were tagged at the spawning grounds, and were thus fairly large, usually more than 80 cm (Thorsteinsson 1995). The Faroe Plateau cod were tagged outside the spawning time (october) and were considerably smaller (55-70 cm). The reason for the use of different time periods or attachment procedures (Iceland and Faroe Islands used internal tagging but Norway external tagging) was, among other things, that different aspects had to be investigated in order to reveal the potentials in tagging cod with DSTs.

Simultaneous depth and temperature recordings from DSTs show, that some Icelandic cod stay in shallow (< 150 m), warm water (> 4 oC) during May and June (Thorsteinsson 1995), while some fish migrate to deep water (250-500 m) where the temperature fluctuates very much (0-8 oC) – supposedly feeding on blue whiting near the Faroe Icelandic ridge (Nord Fiskeri, 1998).

The objectives of the Faroese tagging program were to investigate vertical and horisontal migrations of wild Faroe Plateau Cod which in turn could reveal important biological features as e.g. feeding preferences (bentic or pelagic preys) and spawning activities. In order to reduce variability when tagging at different locations (Thorsteinsson 1995), all DSTs were used in one, fairly uniform area with respect to depth and temperature. This area, 12 nm north-west of the Faroe Islands, was also chosen since it was in the heart of the main cod distribution (highest cod catches), hopefully giving a representative sample of Faroe Plateau cod.




Data Storage Tags (DST 300) produced by Star-Oddi 1997. It was able to record temperature, depth and tilt angle of the fish. The tempereature range was set at 0-12 oC, the usual temperature range in Faroese waters. The depth range was set at 0-800 m, in order to reveal possible migrations into deep water. The DST was started by removing at trigger magnet from the tag and the first recording of temperature, depth and tilt angle was set at 40 min afterwards. The recordings were made in two cycles: A short cycle with 40 min intervals (36 recordings during 1 day), and a long cycle with 4 hours intervals (36 recording during 6 days). The recordings started with the short cycle, then the long cycle, then the short cycle etc.

In order identify a fish with at DST, a spaghetti tag was attached to the DST and hung out through the body wall of the fish.

Specifications of the DST:

Type: DST300.

Dimensions: length 46 mm and diameter 13 mm.

Weight: 8 g (in air), 1 g (in fresh water).

Depth range: 0-800 m, resolution: 4 m, accuracy: +- 4 m.

Temperature range: 0.0 – 12.0 oC, accuracy: +- 0.1 oC.

Tilt range: +-60 o.

Memory capacity: 2700 simultaneous recordings of temperature, depth and tilt angle (8100 recordings in total).

First recording after removal of magnet: 40 min later.

Time between recordings:

40 min (short cycle: 36 measurements during 1 day),

4 hours (long cycle: 36 measurements during 6 days).

Lifetime of battery: ca. 10 months.

Sutures used of closing cuts in abdominal wall: synthetic absorbable sutures, Coded Vicryl nr. 9321, ETHICON. LTD.UK.

Fishing (tagging) area

The tagging with DSTs was a part of a tagging experiment with conventional tags as well as DSTs. The conventional tags were used in four locations: north, west, south and east of the Faroe Islands. The DSTs were used in the northern area located about 12 nautical miles north-west of the Faroe Islands at approximately 62o19’N 7o39’W (100 m).

Treatment of fish

The fish were caught by bottom trawl (research vessel Magnus Heinason) using 135 mm in the cod-end. The hauls lasted 1 hour. Viable cod were immediately transferred to tanks with running sea-water and kept there for at least 30 minutes in order to judge if they were suited for tagging.

The fish were tagged internally with the DSTs in much the same way as in Thorsteinsson 1995. Each fish was taken from the tank and laid upside down on a "table" with a slit. The total length was measured. Then a tube with running sea-water was placed into the mouth in order to secure oxygen supply. A cloth was wrapped around the head protecting the eyes, calming the fish and directing the sea-water to pass the gills. No anaesthetics were used. A slit was cut with a scalp through the body wall on the right side, allowing the DST to be placed into the peritoneal cavity. The DST was started by removing the thrigger magnet. The spaghetti tag, attached to the DST, was let throug a small hole (located about 2 cm in front of the anus) to the exterior. Both ends of the DST were attached to the body wall by absorbable sutures. The slit was then closed by sutures. The slit and the hole were treated by a disinfecting spray. Thus the DST was located at the most ventral place in the peritoneal cavity, and the length axis of the fish and the tag were almost identical. The procedure of internal tagging took normally about five minutes and all 88 cod were tagged and released 12.-13. October 1997.

After tagging the fish was let into a tank with running sea-water and observed for at least 30 min. If the fish did not behave normally it was killed and the tag placed into another individual. The fish was then released. All cod dived at once, indicating good condition.

Getting information from recaptured DSTs.

The DST was opened and the batteries removed. The chip was then connected via an interface box to an IBM compatible computer (MS-DOS with a RS232 serial port). A computer program from the manufacturer was used to copy the information from the tag. Before the copying, the time of removal of the magnet had to be given. The file with the recordings could be opened by usual speadsheet programs (e.g. Microsoft Excel).


The tagging return of DSTs was 41 out of 88 (47 %) during 22 months. Of these about 30 tags were used in this analysis, but only about five presented, since they had long time span and showed interesting features.

The temperature recordings showed, that all cod stayed exclusively in warm water (> 5 oC) as indicated by five selected tags (Fig. 1). The ambient temperature of the fish decreased almost linearly from october to march, when it rose again. Althoug one fish (DST036) stayed in a deeper, and somewhat warmer water layer for about three months (Fig. 1 and 2), the maximum difference between fish was about 1 oC, and usually much less.

The fish stayed at a relatively constant depth during the whole winter as shown by three selected tags in Figure 2 in addition to Figure 3 (before about 1st of January 1998). Although not shown in Figure 2, most cod behaved in a similar way as DSTs 046 and 091. A simultaneous change in depth about 27. February indicates onset of spawning migration.

Clear vertical migration was seldom observed, but two very clear excamples are shown in Figure 3 and enlarged in Figure 4. In some cases the cod moved up during the night and down (assumingly to the bottom) at day as DST0073 shows. In other cases it was opposite as DST0086 shows. The spectacular thing is, that this vertical migration started at almost excactly the same time 30.-31. December 1997. Interestingly DST046 is showing the same tendencies in Figure 2. The vertilcal migrations almost disappeared 5.-15. January, but continued then until about 7. February 1998 for both individuals.


A tag return of 47 % shows, that the catching and tagging procedures were satisfactory.

Contrary to Icelandic cod, Faroe Plateau cod stayed exclusively in warm (> 4 oC), shallow water. One reason to this difference could be the small size (55-70 cm) of the tagged cod, since large (> 80 cm) cod actually is present in deep, cold Faroese waters at about 400 meters, as observed in the Greenland halibut fishery. The decrease in temperature during the winter is considered to reflect seasonal changes in temperature and not migration. The similarities between ambient temperature of dividuals indicate, that all cod stayed on the Faroe Plateau the whole period.

The onsed of spawning or spawing migration in late February is in good accordance to survey results of maturing stages. However, the simultaneous start of this migration indicates some powerful cues, probably other migrating fish, but one has to bear in mind, that some fish did not change depth during the whole spawing season.

Faroe Plateau cod mainly stayed near the bottom, as indicated by very moderate changes in depth with time. Diel activity patters, demonstrated for cod off northern Norway (Løkkeborg and Fernö 1999), could be present since horisontal movements are not recorded by the tag.

Two individuals, however, showed clear vertical migration, for about one month each. It is unknown why this happened at the same time of the year. According to the Coastal Guard, no seismic shooting or other disturbing activities were conducted on the Faroe Plateau prior, or during that period. It is also unlikely that the fish should use tidal currents as transport medium (as cod in the North Sea, Arnold et al. 1994), since the migrations did not have a 6 hour cycle (using both directions) or 12 hour cycle (using one direction). The pause in vertical migrations 5.-15. January 1998 was not due to dramatic changes in tidal current speeds. One possibility is feeding in upper layers, althoug it is a bit unclear what type of food it should be (no, or few fish larvae are present at that time of the year).

In order to investigate the the effect of fish size on vertical behaviour, it is planned to tag large Faroe Plateau cod caught from deep (> 400 m) water.


Nordic Council of Ministers for financing the Data Storage Tags. Sigmar Guðbjørnsson for technical advises. Vilhjálmur Thorsteinsson and Olav Rune Godø for biological advises.


Arnold G.P., M. Greer-Walker, L.S. Emerson, and B.H. Holford 1994. Movements of cod (Gadus morhua L.) in relation to the tidal streams in the southern North Sea. ICES J. mar. Sci., 51: 207-232.

Baras E. 1991. A Bibliography on Underwater Telemetry, 1956-1990. Canadian Technical Report of Fisheries and Aquatic Sciences 1819.

Godø O.R. 1995. Transplantation-tagging-experiments in preliminary studies of migration of cod off Norway. ICES J. mar. Sci., 52: 955-962.

ICES 1997. Report of the study group on ocean salmon tagging experiments with data logging tags. ICES CM 1997/M:3.

ICES 1998. Report of the study group on ocean salmon tagging experiments with data logging tags. ICES CM 1998/G:17.

Løkkeborg S. and Fernö A. 1999. Diel activity pattern and food search behaviour in cod, Gadus morhua. Environmental Biology of Fishes 54: 345-353.

Nord Fiskeri 1998 (North Fishery 1998). Elektronisk merking forteller mer om fiskens bevegelser (electronic tagging tells more about fish migrations). Newsletter from Nordic Council of Ministers, December 1998.

Thorsteinsson V. 1995. Tagging experiments using conventional tags and electronic Data Storage Tags for the observations of migration, homing and habitat choice in the icelandic spawning stock of cod. ICES CM 1995/B:19.


Figure 1. Temperature profile of selected tags.


Figure 2. Depth profile of selected tags showing possible onset of spawning.



Figure 3. Depth profile of selected tags showing vertical migration.

Figure 4. Enlarged part of Figure 3 showing regular diurnal migration.

Vertical bars at the 100 m line indicate length of night.