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. International Council for the Exploration of the Sea. C.M. 1995/B Ref.G. B:19 16 p
NOT TO BE CITED WITHOUT PRIOR REFERENCE TO THE AUTHOR
International Council for the
Exploration of the Sea
CM 1995/B Ref.G
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
by
Vilhjálmur Thorsteinsson
Marine Research Institute,
Skúlagata 4, Reykjavík Iceland.
ABSTRACT:
A tagging experiment is described where Data Storage Tags (DSTs) are used to investigate
behavior, such as vertical and horizontal migration between spawning grounds and feeding
grounds of sexually mature cod. In April 1995, 2200 mature cod on the main spawning grounds
of the Icelandic cod (Selvogsbanki and Eyrarbakkabugur) were tagged with conventional tags
(Floy tag, FD-89sl T-bar Anchor Tags) and 81 of them also tagged with DSTs. In order to find
if difference in behavior exists between cod spawning at various parts of this large area, several
tagging localities were chosen both deep and shallow and conventional and DS tags allocated to
each. This activity was preceded with a tagging experiment in 1994 in the same area, using
only conventional tags to survey the probabilities of recaptures in the area. Results indicated
that post spawning vertical migrations generally increased in depth, with a rate of descent
within the maximum descent rate given by Harden Jones and Scholes (1984). Visual
comparisons of plots of depth or temperature on time grouped according to tagging locality
seem to reveal some difference in behavior although more data and a multivariate approach is
necessary for further analysis. The distributions of conventional tag recaptures seem to be
repeated annually for each spawning locality. The recaptures of DSTs fall within the
distributions of recaptures of conventional tags tagged at same locality. The experiment could
be considered a double tagging experiment because of the fish with DSTs also being tagged
with a conventional tag. The results indicate that such an experiment could be useful for
estimates of tag retention and could also give information on non-reporting of conventional
tags.
Keywords:
Cod (Gadus morhua), tagging, Data Storage Tags, vertical migration, double tagging.
INTRODUCTION
The objectives of this project were:
A) Investigations of behavior, such as vertical and
horizontal migration between spawning areas and feeding areas of sexually mature cod in
relation to the spawning-grounds using tagging experiments with DSTs and conventional
tags.
B) Investigations of tag loss or non-return of tags in tagging experiments with cod, using
double tagging experiments with conventional tags and DSTs (Data-Storage-Tags).
An effort was made in these experiments to tag the cod on spawning grounds or as close
them as possible. The reason for relating the tagging to spawning grounds is that if
variations in behavior are stock related, there should be less variations in behavior within a
spawning locality than between. It should be important therefore, if one is to study the
behavior of mature cod, to relate the tagging to spawning grounds.
The use of tags with electronic equipment to study fish behavior has by now a long
history. A recent bibliographic survey of underwater telemetry, spanning the period
from 1956 to 1990 (Baras 1991) came up with more than 1100 references.
There are however at present very few references found that deal with tags which store
measurement data in memory (archival tags, data storage tags or DSTs). The interest of
researchers has lately been directed the development of the technology for the DSTs
(Metacalfe et al. 1992).
The Marine Research Institute in Iceland and a small Icelandic enterprise called Stjörnu
Oddi sf, have been cooperating since 1993 in developing a Data Storage Tag for fisheries
research. Stjörnu Oddi sf has developed the Data Storage Tag and the MRI, has been
developing the methodology for tagging experiments with cod in Icelandic waters using
DSTs.
The tagging with conventional tags along with the DSTs have an important function. The
conventional tags, because applied at the same time and locality as the DSTs, should
increase the information on the migration behavior of the cod tagged in the same spawning
locality. Because of high cost of DSTs it is useful when planing tagging experiments with
DSTs, to look at historical data of tagging results in the same area (Jónsson 1986) or to run
an tagging experiment with conventional tags prior to the DST tagging.
There are many uncertainties concerning tagging experiments, some of them are tagging
mortality, tag shedding and non-reporting. All these factors will vary according to species,
tags used and conditions in each area. It would therefore be useful if possible to design the
experiment in such a way that at least some of these uncertainties can be estimated. It is
hoped that the double tagging can give information on tag retention, similarly that
comparisons of percentages of recaptured double tagged fish versus percentages of
recaptures of conventional tags may give information on non-reporting of recaptured tags.
It must however be emphasized that these ideas need a further experimental work.
MATERIAL AND METHODS
Equipment
The tags used in the study:
A) Conventional tag, Floy tag, FD-89sl T-bar Anchor.
B) Electronic Data Storage Tag (DST100), produced by Star-Oddi 1995.
The time interval for measurement is adjustable for each tag at the time of closure.
The DST is started by removing a trigger magnet which is attached to the DST with a tape.
The first measurements of the tag are adjusted to 1 hour after the removal of the trigger
magnet. For the possibilities of 13 month recording most of the tags were adjusted to wake
up at 8 or 9 hours interval and record depth and temperature.
For the identification of a fish with a DST inside, a spaghetti tag is attached to one end of
the DST and this spaghetti tag is left hanging through the body wall for visual
identification of a fish with a DST.
The specifications of the DST:
Dimensions: 17 mm (diameter), 56 mm (length).
Weight 12g (in air), 0.5g (in fresh-water).
Depth range: 0 - 400 m,
Resolution 2 m
Accuracy: +- 4.0 m
Temperature range: 0.0 - 15.0 ºC
Accuracy: +- 0.1 ºC.
The capacity of memory chip: 2 Kbyte
The capacity of battery: 80 mAmps.
Sutures used for closing cuts in abdominal wall: synthetic absorbable sutures, Coded Vicryl
nr. 9321, ETHICON. LTD.UK.
Treatment of fish
After capture the fish were put into a tank with continuously circulating sea-water. Only
the cod that appeared to be in good condition was selected for the tagging. When double
tagging, the conventional tag ( T-bar anchor) was applied in a traditional way near the first
dorsal fin, but the DST was applied surgically, into the peritoneal cavity, by cutting through
the body wall on the left side.
At first sedatives were used to pacify the fish but the conditions at sea (sub zero
temperatures and considerable moment of ship) made these procedures very difficult to
carry out and resulted in high mortality rate. Therefore the use of anesthetics was stopped.
The fish was taken from the tank, total length measured and tagged with conventional tag.
A slit just large enough to slip the DST inside, was cut with a scalp through the body wall
on the left side into the peritoneal cavity. The DST was started by removing the trigger
magnet and placed in the peritoneal cavity with the indicator through the body wall to the
exterior, the cut was then closed with a synthetic absorbable suture. After this the fish was
released and notice taken if it was swimming normally. If it didn't swim downwards, it
was taken back inside with a dip-net. The procedure, from taking the fish from the tank
until it was released back into the sea, was timed on several occasions. The average time
was c.a. 3 minutes and the best results less than 2.5 minutes.
The fishing gear
The fishing gears used were gill-nets, trawl and Danish seine. The choice of fishing gear
has to be made in relation to the conditions of the sea-floor. The rocky mounts described in
"4.0 The Area" are not suitable for active fishing gear like trawl or for Danish seine and
these fishing gears have to be applied as close to the rocks as possible but fish with much
less success than the gill-nets. The gill-nets however can be set right on top of the rocks
and are the most successful fishing gear for spawning cod in this area. Fish obtained from
this gear must be chosen very carefully for the tagging because of possible injury by the
netting.
Analysis
The data from the DSTs were analyzed graphically. For each individual DST the plot of
depth and temperature on time were observed for changes in depth and temperature. For
depth the descending or ascending parts were analyzed calculating the changes in depth as
m/hours and the rates compared to expected values (Harden Jones & Scholes 1984,). Plots
of depth on time, and similar plots of temperature on time, for the DSTs grouped
according to tagging locality were compared visually.
For the calculation of the maximum rate of descent for cod, maintaining the neutral
buoyancy at depth, the formula for the expected (Arnold and Walker 1992) is:
y = 0.066T + 0.25,
where T is temperature and y is the rate of descent in meters/hours.
For the calculation of the maximum rate of ascent for cod, maintaining the neutral
buoyancy at depth, the formulas is:
t = 254 ln (P1/P2) + 15,
where P1 and P2 are the higher and lower levels of pressure (in Bars) and t is time of
adjustment to depth in minutes.
The further analyzes for the investigation of habitat choice of cod from different spawning
ground would depend on various data such as:
Date and location of tagging and recapturing
Time series of measurement of various data of environmental factors in the interval of
release and recapture (depth, temperature).
This data would need multivariate analyzes for similarities or dissimilarities and annual
repetition of the tagging experiments to find out if structure in behavior related to spawning
grounds is maintained over time.
Analysis of double tagging data.
For the calculation of retention rates of tags (Gulland 1963, 1983, p113).
For non- reporting of tags the comparison of the percentages of recaptures from total
releases of DSTs versus the same percentage for conventional tags.
THE AREA
The area where this experiment was carried out is the Selvogsbanki -Eyrarbakkabugur area
off the South-West Coast of Iceland, which is considered to be the main spawning grounds
of the Icelandic cod stock. Figure 1, shows the area and the tagging localities marked with
capital letters fro A to I and 6 recaptures of DSTs marked with the number of DST (213,
318, 319, 325, 334, 336) and table 1 lists the tagging localities with local name, position,
depth range and nature of substrate.
Figure 1. The distribution of tagging localities where DSTs were released in 1995 (A-
I) and localities of 6 recaptured DSTs marked with their id-numbers (213, 318, 319,
325, 334,336).
Table 1, Tagging localities of the experiment.
Legend: Local-id = an alphabetic identification code for the localities where tagging with both conventional and
DSTs occurred in 1995; Name of locality shows the local name of area; the coordinates indicate the mid-point of each
area of activity; Depth range indicates the extremes of depth in each locality.
The area Selvogsbanki -Eyrarbakkabugur reaches from the shore and out to approximately
150 m depth and has a bottom topography of lava-fields interspersed and surrounded by
sand or gravel. The sub-marine lava fields or mounts vary in sizes from a fraction of a
nautical square mile to approximately 100 nautical square miles and are generally very
rocky and elevated from the sea-floor.
The actual spawning aggregations are generally concentrated over or near these rocky sea-
mounts. This is demonstrated by lack of success of trawling for cod on the sand or gravel
areas around the lava fields versus the success of fishing with gill-nets that are laid across
the lava-fields. The concentrations of spawning cod in this area are therefore patchy in
distribution. In the experiment the area was divided into a near-shore area which is within
10 nautical miles of the shore and off-shore area which is between 10 and 30 nautical mile
off the shore of the mainland. Tagging localities are labeled from A to F (near-shore) and
G to I (off-shore).
RESULTS
Results of temperature and depth readings from recaptured DSTs
Figures 2 - 8 show the readings of temperature and depth for 7 recaptured DSTs.
It must be kept in mind that the first measurements in the DST occurred 1 hour after a
trigger magnet had been removed, which was done when the DST is put into the fish. The
time interval between measurements of temperature and depth was 8 hours and in some
cases 9 hours. The accuracy of the measurements is also important especially in the
analyzes of behavior, the depth range as each reading has an uncertainty of +- 4 m.
The behavior shown by the choice of depth in figures 2 to 8 as descents, ascents or
stationary í depth, must be greatly influenced by the limitations in the physiology of the cod
that concerns the adaptations of pressure in the swim-bladder (Arnold and Walker 1992).
Rates of descent and ascent of cod (Harden Jones and Scholes 1985) are physiologically
limited by secretion of gas on the way down by the anterior-ventral gland and on the way
up by the posterio-dorsal oval for re-absorption of gas. The maximum rate of descent if the
fish remains neutrally buoyant is therefore strongly dependent on physics Boyles law of gas
(Arnold and Walker 1992).
Generally the DSTs showed a very similar pattern, where the fish went first down to the
bottom depth at the locality of release, where they stayed for relatively short time, after
which they rose close to the surface and then increase depth, some gradually, others in
rapid dives. The initial dive, probably a fleeing response, may not be recorded with all the
fish because the first recording was after 1 hour and the next after 8 (or 9) hours.
Figure 2 - 3, Time series of temperature and depth from the DSTs released at locality
A (Eyrarbakki) 19/4/95. DST nr 336 was recaptured after 48 days at liberty, DST
nr. 334 was recaptued after 72 days at liberty.
The cod with the DST Nr. 336 (Fig. 2) was 77 cm, 6 years old male and had spent 48 days
in the sea at liberty. The cod with the DST Nr. 334 (Fig. 3) was 88 cm, male (age not
known) and had spent 72 days with the tag among its guts. These fish when tagged were
taken from the same set of gill-nets, in the same location, at the same time.
The first readings of depth of Nr. 336 are all near the surface and are abnormal if
compared to the readings of other DSTs.
Nr. 334 showed an initial plunge to the maximum depth in the tagging locality and a
ascend to a level of 4 to 12 m depth.
Both of these DSTs (Nrs. 336 and 334) showed rapid descents to the depth range of 80 -
100 m and then remained at this depth for several weeks. The DST Nr. 334 showed how
ever a gradual descent of 18.6 m for 56 hours from 20th of April to 22 of April which is
0.3 m/hour, well within the predicted the maximum descent rate at the temperature range
of 4.5 - 4.6 ?C which is 0.6 m/hour.
Figures 4 - 5. time series of temperature and depth, for the recaptures of DSTs. Nrs.
213 and 319, which were released at locality B (Knarrarós) 19/4/95.
The cod carrying the DSTs 213 and 319 (Figs. 4 and 5), 318 and 325 (Figs. 6 and 7) were
all captured with the same set of gill-nets at the same time and location and released within
30 minutes. The cod carrying DST 213 (Fig. 4), was 112 cm, male, 11 years old and had
13 days at liberty. The cod with Nr. 319 (Fig. 5), was 99 cm, male, 9 years old and had 16
days at liberty.
Both these DSTs descend with the rate of 0.3 m/hour after the initial plunge and
ascendance to depth levels of less than 15 m.
In the DST 213, there were several dives where the descent would be at a faster rate than
0.3 m/hour ( 20/4, 22/4, 23/4, 25/4), but each time the cod returned to a level which was
linear with the slope of a line c.a. 0.3 m/hour.
DST Nr. 319 showed an initial dive to the maximum depth at the location of release (56-
64m) but this reaction could be missed in Nr. 213 because of long time intervals between
measurements.
Figures 6 - 7. Time series of temperature and depth, for the recaptures of DSTs. Nrs.
318 and 325 which were released at locality B (Knarrarós) 19/4/95.
The cod carrying the DST 318 (Fig. 6), was 61 cm, male and had 61 days at liberty. The
cod with the DST 325 (Fig. 7), was 83 cm, male and had 63 days at liberty.
The time-series of these DSTs show both descending and ascending trends. The DST 318
came close to the surface after the initial dive to the maximum depth of the release area and
then descended rather rapidly with a rate of 3 m/hour to 85 m depth, where it stayed for
more than 9 hours, but then ascended to a depth of 38 m where it stayed for nearly 3 days.
The change in depth from 4 m to 38 m in 54 hours gives 0.6 m/hour (Arnold 1992), but
this depth is almost exactly the level of calculated neutral buoyancy at the temperature of
5.5 C which would give the maximum descendant rate of 0.6 m/hour from the shallowest
position. This fish made another descending from 11th to 14th of April from 45 m to 85 m
depth again with the rate of descent close to a predicted 0.6 m/hour. The same fish ( DST
318) made an ascent during the period of 29th to 31st of May but the rate of ascent is well
under the maximum rate allowed by gas absorption.
Figure 8. Time series of temperature and depth from the DST Nr. 328, of the
releases 20/4/95 in locality E. Recaptured after 8 days at freedom.
The cod carrying the DST Nr. 328 (figure 8), was 106 cm, 11 years old, male, released in
locality E, which is a shallow water area and was recaptured after 8 days at liberty.
This fish took a plunge after the release and was down at the maximum depth in tagging
locality after 1 hour. Nine hours later it has ascended to less than 6 m depth (2m +- 4 m)
and stays there for at least 9 hours but then showed a gradual descend rate over the 8 days
of freedom. The rate of descent for the period from 21st to 24th of April is 0.3 m/hour.
From 24th to 28th of April the fish had the same rate of descent as before (0.3 m/hour) if
considering the readings at the upper level of the of the depth range in successive
descending and ascending trace. The return of the fish to the level of descendance of 0.3
m/hour might indicate that the fish was not able to secrete gas faster than this into the
swimbladder.
The largest fish (106 -112 cm) were among those which could descend with the rate of
0.3m/hour, but the smallest fish (DST 318) seemed to be able to descend with the rate of
0.6m/hour.
Figure 9. The comparison of depth readings from two DSTs tagged and released in same
location and at the same time at locality A (Eyrarbakki), 19/4/95.
The two DSTs in figure 9, showed a similar behavior from the time of tagging until May
21st. After this period the behavior concerning depth choice deviates.
The difference in choice of depth between these two fish, during the first period is less than
30 m, but vertical migration patterns are similar.
Figure 10. The comparison of depth readings from four DSTs tagged and released in
same location and at the same time, in locality B (Knarrarós), 19/4/95.
Comparisons of temperature readings from DSTs of two tagging
sites
Figur 11. Comparisons of temperature readings from two DSTs (334 and 336),
released in locality A (Eyrarbakki) 19/4/95.
The results from the temperature readings of DSTs 334 and 336 show relatively little
temperature variations between the two of them or within 1?C.
Figure 12. Comparisons of temperature readings of four DSTs (213, 319, 318 and
325), released in locality B (Knarrarós), 19/4/95.
The results from the temperature readings of DSTs 213, 319, 318 and 325, show relatively
little temperature variations between all of them for the first month, or within 1?C.
Results of the double tagging experiment in 1995 (preliminary).
The locations of tagging stations (A - I) where shown in figure 1. On the same figure the
locations of 6 recaptued DSTs are markt with the id-number.
Table 2 below shows the preliminary results of the tagging experiment or releases of
tagged fish with conventional tags and DSTs and the returns of these tags.
Table 2 (preliminary) results from double tagging experiments in spring 1995.
Legend: Near- or off-shore = Near-shore are spawning grounds within 10 nautical miles from the shore
Off-shore = spawning grounds between 10 and 30 miles of the shore; Local-id = an alphabetic identification code for
the localities where tagging with both conventional and DSTs occurred ( see table Nr 1 for more information); Gr.Nr. = a
number code which identifies the particular tagging; Depth (m) = the average depth in meters of the fishing gear; Gear
= fishing gear; Tag -rel. conv. = the total number of fish released with conventional tags; Tag rel. DSTs = the number of
fish which have a Data Storage Tag in addition to the conventional tag (double tagged); DST % conv. = numbers of
fish tagged with Data Storage Tags expressed as percentage of total number tagged with conventional tags; Rec.-conv. =
recaptures of conventional tags; Rec.-DSTs = recaptures of Data Storage Tags; % rec.conv. = numbers of recaptured
conventional tags expressed as percentages of total number tagged; %rec. DSTs = numbers of recaptured DSTs as
percentage of total number (tagged with DSTs); (-) conv. = numbers of double tagged fish which have lost the
conventional tag; (-) DST = numbers of double tagged fish which have lost the DST.
In the group of double tagged fish, the ratio of double tagged cod found with both tags and
the cod found with the conventional tag missing should give us the retention rate of the
conventional tags.
Comparison of the group of fish which was double tagged to the group which was tagged
with only one conventional tag could also give us information on non-reporting of tag
recaptures. Tagging mortality should be higher in the group that is double tagged than in
the group which is only tagged with a single tag. If the recaptures of DSTs are relatively
higher than the recaptures of conventional tags plus the estimated loss of tags in the sea,
this could be due to non-reporting or that the conventional tags are less conspicuous to the
fishermen or fish factory workers.
Only 1 double tagged fish was found with a missing conventional tag but with the DST
intact. No fish was found which had lost DST but with the conventional tag intact.
If the tag loss only for the conventional tags it is 12.5%. Comparison of the recaptures of
conventional tags and DST, in the near shore area, show that the recoveries of conventional
tags from gill-net caught fish, were 2%, but the relative numbers of recaptured DSTs were
18%, of the cod released in the same area. This difference in recapture rate between the
double tagged fish and the fish with single conventional tag, may indicate a very serious
non- reporting problem.
Conditions of recaptured fish was noted when possible from descriptions from the
fishermen. Two fish were received intact. The observed conditions near wound was
satisfactory with one of the fish but the other had infections or abrasions but the cause
was not identified. The method had been tried in 1993 -1994 on cod kept for up to 1 year
in captivity with no observed mortality or abnormal infections (Thorsteinsson
unpublished), but these results give reason to revise the surgical method applied.
Tagging-experiment with conventional tags in 1994
To have information (a priory) on the likely return rates of tags in the area, the results of
tagging experiment in this area from 1994 are used (Thorsteinsson unpublished). Table 3,
shows the return rates for conventional tags in the same areas as in the tagging experiment
with DSTs + conventional tags in April 1995.
| Local-id |
gear |
Depth (m) |
Tag rel. |
Rec. |
%rec. |
| near A-B trawl |
trawl |
45 |
450 |
60 |
13,3% |
| B |
gill-nets |
50 |
212 |
46 |
21,7% |
| A |
gill-nets |
50 |
144 |
25 |
17,4% |
| C |
gill-nets |
55 |
196 |
48 |
24,5% |
| D |
gill-nets |
55 |
100 |
14 |
14,0% |
| E |
gill-nets |
55 |
197 |
41 |
20,8% |
| near B |
Dan-seine |
40 |
22 |
6 |
27,3% |
| I |
trawl |
140 |
416 |
45 |
10,8% |
| G |
gill-nets |
100 |
21 |
3 |
14,3% |
| H |
gill-nets |
100 |
415 |
21 |
5,1% |
| Total |
|
|
2173 |
309 |
14,2% |
Table 3, Tagging in the Selvogsbanki- Eyrarbakkabugur area, with conventional tags in
april 1994 and recaptures until June 1995.
The table shows the data from tagging experiments with conventional tags only, to show
the probable return rate of tagging with conventional tags in the area of Selovogsbanki-
Eyrarbakkabugur.
Gear = fishing gear used for obtaining the fish for tagging (Dan.-seine is Danish seine); Depth (m) = average depth the
fishing gear was operating at; Tag rel. = releases of fish tagged with conventional tags; Rec. = recaptures in numbers; %
rec. = recaptures in percentages.
The results show in percentages the recaptures according to depth, locality, fishing gear and
off-shore near-shore categories, during more than 12 months at liberty for fish tagged 1994,
in the same areas as the double tagging experiment in 1995 with the DSTs.
The recapture rate for fish tagged in the near shore area is different depending on fishing
gear used to obtain cod for the tagging. In the near-shore area the recaptures are 13% for
fish caught with trawl, 21% for fish caught by gill-nets and 27 % for fish caught with
Danish seine.
Taggings in the Off-shore part of the area show relatively less recaptures than do the
taggings in the near shore area from 5 to 14%.
The results from the tagging in 1994 indicate that the fish tagged in the near-shore areas
have probability of recapture between 13% and 27%, over the first year at liberty. Fish
caught and tagged in the Off-shore area have probability between 5% and 14%. The
distribution of recaptures of the tagging in 1994, over 13 month period indicate that the
off-shore recaptures are mainly found in deep waters or more than 100 m depth, but the
near-shore tagging are more commonly found closer the shore and in less than 100 m depth
(Thorsteinsson, unpublished report).
6.0 DISCUSSION
The results from DST readings of temperature and depth, indicate that vertical migration
of cod is very dependent on swimbladder physiology (Harden Jones & Scholes 1984,
Arnold & Greer Walker, 1992).
Some fish seem to be able to descend at the rate which is described as the maximum rate of
descent for the temperatures measured at the same instant, but others seemed not to be able
to descend that fast. There are also indcations that rate at which the cod descends could be
related to size of the fish. In the future with more recaptures of DSTs, behavioral
difference in vertical migraton could be related to size of fish as well as different
localities.
Although there has not been time to apply multivariate methods to the DST data, visual
observation of figures 9 to 12 which compare separately, time series of depth and
temperature for two spawning localities A and B, reveal some differences in behavior
between tagging stations.
The results from the readings of the DSTs show very promising results for the
investigations of migration and a lot can be learned from this pre-experiment for a larger
study of this kind.
The results indicate that there is a possibility of different behavior at tagging localities
which are only few miles appart. Therefore when catching cod for tagging in the study of
differential behavior within or between spawning grounds, the activity of the fishing gear
should preferably have a limited range. The range distribution of gill-nets should be kept
within a mile, the diameter of the circle of activity for Danish seine should be less than
mile and when using trawl very short hauls should be taken or 30 minutes at the maximum.
It appears that the releases of DSTs in the experiment vere too few in relation to the fish
tagged with only conventional tag. It is necessary to have sufficient releases of DSTs at
each locality of tagging so number of stations should be reduced in the shallow area and the
double tagging experiments should aim at least 10% DSTs per total number of
conventional tags in the experiment.
The system of getting the recaptured tags back must be revised.
The surgical method should be improved.
AcknowledgmentsI would like to thank Mr. Sigmar Guðjónsson the manager of the Stjörnu-Oddi who
developed and manufactured the DSTs used in the experiments and tailored them to our
particular needs. I am also grateful to the captains and the crew of the RV Árni
Friðriksson and similarly the captain and the crew of the gill-net fishing boat MB Friðrik
Sigurðsson AR17, from Thorlákshöfn and the captain and crew of Audbjörg SH 197, from
Ólafsvík. Without the keen interest and cooperation of the Icelandic fishermen this study
and other tagging projects would not be possible. Finally I am grateful to the staff of the
Marine Research Institute for the help and cooperation I have received.
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