The Elastomer Visible Implant system (EVI) is a

relatively new technique for batch marking fish. The aim

of this study was to assess retention rates and the

possible effects of tagging on the growth and mortality

of barbel, Barbus barbus, (81-197mm, fork length) over

approximately 2 months using a syringe injection system

Farooqi, M.A.

Morgan, C.E.

Aquatic Commons

A Preliminary Investigation of Elastomer Visible Implant 
(EVI) tag retention rates and the effect of tagging on the 
growth and survival of barbel (Barbus barbus L.) 
Coho salmon 
NRA/NW/FTR/95/1 February 1995 
A preliminary investigation of Elastomer Visible Implant 
(EVI) tag retention rates and the effect of tagging on the 
growth and survival of barbel (Barbus barbus L.). 
C.E. Morgan and M.A. Farooqi, 
National Rivers Authority 
North West Region 
PO Box 12 
Warrington 
WA4 1HG 
February 1995 
Contents 
Page 
Summary 1 
1. Introduction 2 
2. Method 2 
3. Results 3 
4. Discussion 4 
5. Conclusion 4 
6. Recommendations 4 
7. Acknowledgements 5 
8. References 5 
Figure 1. Tagging sites on barbel. 
Figure 2. Percentage tag retention for each region. 
Figure 3. Mean length of fish with 95% confidence 
limits. 
Table 1. Percentage of tags retained. 
Table 2. Tag quality at each location. 
Table 3. Range, mean length, standard deviation and 
sample size for control and tagged fish. 
Appendix 1. Fish length at each recording interval. 
Summary 
The Elastomer Visible Implant system (EVI) is a 
relatively new technique for batch marking fish. The aim 
of this study was to assess retention rates and the 
possible effects of tagging on the growth and mortality 
of barbel, Barbus barbus, (81-197mm, fork length) over 
approximately 2 months using a syringe injection system. 
69 fish were tagged and 66 were used as a control. 
4 tagging sites were selected for evaluation. The 
following retention rates were obtained; 91.3% for the 
base of the caudal fin, 82.6% for the dorso post-orbital 
tissue (head), 82.6% for the base of the anal fin and 
47.8% for the post-orbital adipose tissue. It was not 
possible to investigate the relationship between tag 
retention and fish length because of the small sample 
size and the narrow length range of the fish. 
The quality of retained tags, expressed as a percentage 
of the total number of tags applied, was subjectively 
classified on a scale ranging from good to very poor. 
Tag quality was highest (i.e. those classed as good) for 
the caudal (53.6%) and head (50.7%) locations. 
Although tag retention was highest for the caudal site, 
it was very difficult to tag in this region, thus the 
head location may be the most practical site for tag 
application with the syringe injection system. 
There was no significant difference between the growth of 
the tagged and control fish. No mortalities were 
recorded during the experiment. 
1 
A preliminary investigation of Elastomer Visible Implant 
(EVI) tag retention rates and the effect of tagging on 
the growth and survival of barbel (Barbus barbus L.) 
1. Introduction 
The ability to be able to differentiate between fish, 
whether it be at an individual level or batch level, is 
often a fundamental requirement of studies concerned with 
obtaining data on which to make informed decisions 
regarding the management of the stock. 
The elastomer visible implant (EVI)-tag system provides a 
relatively benign and easily recognisable technique for 
batch marking fish. The tagging system used in this study 
comes in the form of a starter kit which is manufactured 
and supplied by Northwest Marine Technology. This 
consists of a fluorescent colourant (red in this 
instance) and a hardener, which when mixed can be applied 
using a hypodermic syringe. This mixture sets into a 
solid form within hours of injection. 
Ideally the tag should be placed in clear tissue, 
although semi-transparent and translucent tissue may also 
be suitable. The EVI tag can be easily detected by the 
naked eye, although visibility is enhanced under 
ultraviolet light. 
The objectives of this study were to determine the most 
suitable area for tag placement and to assess tag 
retention rates, growth and mortality of barbel over a 
time period of 57 days. The experiment was carried out at 
the N.R.A.'s rearing facility in Leyland. 
2. Method 
Fqur potential injection sites were identified 
(see Fig. 1.), these were: 
1. Dorso post-orbital tissue (head) 
2. Post-orbital adipose tissue (eye) 
3. At the base of the anal fin (anal) 
4. At the base of the caudal fin (caudal) 
13 5 fish were selected at random, anaesthetised using 
phenoxy-ethanol and the fork length recorded (mm). 69 of 
these fish were then tagged in each of the four sites by ' 
two personnel using a straight 27 G 1/2 needle, the 
remainder were used as a control. The control and tagged 
fish were held in separate rearing tanks under identical 
conditions and fed at regular intervals. 
After a period of approximately one month, the fish from 
both tanks were anaesthetised and their fork length 
recorded. The quality of the fluorescent marks on the 
tagged fish was assessed with the naked eye and 
2 
Figure 1. Tagging sites on barbel (diagram taken from Maitiand 1972) 
classified on a subjective scale ranging from good, 
through poor, very poor to absent. 
This procedure was repeated after a further period of 1 
month. For logistical reasons the duration of this 
experiment was confined to 57 days. 
A t-test was used to compare the mean length of the 
control and tagged fish on each of the three sampling 
occasions, with a null hypothesis (Ho) that the tagged 
and control fish grow at the same rate. 
3. Results 
EVI tag retention rates for the four target areas are 
presented in Table 1. and Fig. 2. The most favourable 
site was found to be at the base of the caudal fin 
(91.3%) while the post-orbital adipose tissue appears to 
be the least suitable site (47.8%). A retention rate of 
82.6% was achieved for the other two tag locations. 
Typically the tag should appear as a small elongated mark 
equivalent to the volume of a coded wire tag i.e. about 
0.05ul, however when applied to the base of the anal and 
caudal fin it tends to take on a more diffuse nature. The 
condition of the retained tags, expressed as a percentage 
of the total number of tags for each location, is 
presented in Table 2. Tags classed as good were in 
virtually the same condition as when implanted. Poor tags 
showed evidence of fragmentation and very poor tags were 
reduced to small fragments. The highest level of tag 
quality was obtained for the base of the caudal fin 
(53.6%) and the dorso post-orbital tissue (50.7%). 
Table 3. shows the size range, mean length and standard 
deviation of the control and tagged fish on each of the 
sampling occasions. The individual length measurements 
are presented in Appendix 1. 
Fig. 3. indicates that the control fish and tagged fish 
grew at a similar rate. The instantaneous growth rate for 
the control fish (3.97 x 10 ~4) and tagged fish (4.74 x 
10 ) over the duration of the experiment were very 
similar. The results of the t-test showed that there was 
no significant difference between the mean length of 
tagged and control fish (P<0.5). No mortalities were 
recorded during the course of the experiment. 
3 
Table 1. Percentage of tags retained. 
Time (days) 
29 
57 
Percentage tag retention 
Head 
97.1(67) 
82.6(67) 
Eye 
82.6 (57) 
47.8 (33) 
Anal 
97.1(67)' 
82.6 (57) 
Caudal 
100.0 (69) 
91.3 (63) 
N.B. Figures in parentheses show the number of tags retained. 
Table 2. Tag quality at each location. 
Head 
Eye 
Anal 
Caudal 
Day 
1 
29 
57 
1 
29 
57 
1 
29 
57 
1 
29 
57 
Percentage of tags 
Good 
100.0 (69) 
73.9 (51) 
50.7 (35) 
100.0 (69) 
69.6 (48) 
29.0 (20) 
100.0 (69) 
52.2 (36) 
36.2 (25) 
100.0 (69) 
71.0 (49) 
53.6 (37) 
Poor 
0.0 (0) 
21.7 (15) 
24.6 (17) 
0.0 (0) 
10.1 (7) 
10.1 (7) 
0.0 (0) 
40.6 (28) 
31.9 (22) 
0.0 (0) 
27.5 (19) 
26.1 (18) 
V.poor 
0.0 (0) 
1.4 (1) 
7.2 (5) 
0.0 (0) 
2.9 (2) 
23.2 (16) 
0.0 (0) 
4.3 (3) 
14.5 (10) 
0.0 (0) 
1.4 (1) 
11.6 (8) 
Absent 
0.0 (0) 
2.9 (2) 
17.4 (12) 
0.0 (0) 
17.4 (12) 
55.2 (36) 
0.0 (0) 
2.9 (2) 
17.4 (12) 
0.0 (0) 
0.0 (0) 
8.7 (6) 
N.B. Figures in parentheses show the number of tags retained. 
Figure 2. Percentage tag retention for each region. 
Table 3. Range, mean length, standard deviation and sample size for control and 
tagged fish. 
Range(mm) 
Mean length(mm) 
S.D. 
Number 
10/10/94 
79-232 
124.53 
28.99 
66 
Control 
07/11/94 
81-236 
126.09 
28.90 
66 
05/12/94 
81-236 
127.33 
29.33 
66 
10/10/94 
82-195 
122.41 
24.46 
69 
Tagged 
07/11/94 
81-195 
124.23 
24.69 
69 
05/12/94 
84-197 
125.70 
24.29 
69 
Figure 3. Mean length of fish with 95% confidence limits. 
The tag site at the base of the caudal fin achieved the 
best retention rate (91.3%). Tag retention rates for the 
base of the anal fin and the dorso post-orbital tissue 
were the same (82.6%) however, tag quality was much 
higher for the dorso post-orbital tissue. In addition, 
tag placement in the base of the caudal fin and anal fin ' 
locations was more difficult and hence more time 
consuming. 
Due to the small sample size (n=69) and the narrow length 
range of the tagged fish (84% of the sample having length 
class mid points between 110 and 140 mm) it was not 
possible to determine the relationship between fish 
length and tag retention rates. 
A large amount of pressure was required to release a tag 
from the hypodermic syringe. This may have resulted in 
less precise tag insertion and hence lower retention 
rates. It may also have effected tag quality. Northwest 
Marine Technology provide a compressed air tag injection 
system which is likely to be more effective than the 
syringe injector. 
5. Conclusion 
Relatively high tag retention rates (82.6 - 91.3%) were 
obtained for 3 of the 4 target areas studied. The post-
orbital adipose tissue was the least successful site with 
a retention rate of 47.8%. 
There was no difference between the growth rate and 
mortality rate of the tagged and control fish. 
6. Recommendations 
1. On the basis of this study, taking into account tag 
retention rate, tag quality and ease of application 
it is recommended that the dorso post-orbital tissue 
be targetted as the most suitable location for tags 
on barbel, of size between 81-197mm. 
2. Tag retention rates were based on the presence of all 
or part of the tag. The persistance of fragmented 
tags will require a more long term study. 
4 
4. Discussion 
7. Acknowledgements 
The authors would like to thank Miran Aprahamian and 
Simon Nicholson for their help in producing this paper. 
Thanks to Rod Taylor and John Stone for supplying and 
maintaining the fish at Leyland Hatchery. Thanks also to 
Northwest Marine Technology for permission to use the 
photographs on the front cover. 
8. References 
Maitland, P. S. (1972). Key to British Freshwater 
Fishes. Freshwater Biological Association Scientific 
Publication No. 27. 
5 
Appendix 1. Fish length at each recording interval. 
Appendix 1 continued 


A preliminary investigation of Elastomer Visible Implant (EVI)tag retention rates and the effect of tagging on the growth and survival of barbel (Barbus barbus L.).

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A preliminary investigation of Elastomer Visible Implant (EVI)tag retention rates and the effect of tagging on the growth and survival of barbel (Barbus barbus L.).

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