3. Results
3.1. Comparison between tagging protocols At the end of the 28-day rearing period in a non-pathogen-free environment, fish
Ž .
mortality amounted up to 24.2 Table 1 . The mortality was lower in controls than in Ž
. tagged perch, especially in those tagged with syringe injectors 40 . Tag retention by
sutured fish was 100, whereas it was F 80 in the two other groups. Suturing also favoured healing: it was about 1 week faster than in the other groups. Sutured fish grew
at a slower rate than controls over the first week, whereas other tagged fish were less affected. From the second week onwards, after the stitch was removed, the growth rate
of sutured fish was similar to those in other groups. Sutured fish ended up with the smallest body weight of all groups, but essentially because fish dying in other groups
tended to be the smallest individuals. These preliminary results showed that surgical implantation and suturing was more favourable than other protocols to tag perch in an
environment with pathogens.
3.2. Determination of minimum fish size for pit tagging Ž
Over the first 6 rearing weeks, the mortality of tagged perch was low six of 212 .
Ž
2
. fish , similar to that of controls 28 of 820 fish; x s 0.12; P s 0.7274 and indepen-
Ž .
dent of the initial tag ratio Table 2 . After the fish had been regrouped into nine groups, Ž
y1
. mortality was slightly higher than before 0.95 vs. 0.77‰ d
, but did not differ Ž
significantly between control and tagged fish 47 of 630 and 20 of 206 fish, respectively;
2
. x s 1.07; P s 0.3020 . There was a higher proportion of males among the dead tagged
Ž fish than among the survivors, but the same was observed among controls 15 M:11 F
2
. and 29 M:18 F, respectively; x s 0.02; P s 0.8876 .
Tagging and tag ratio did not affect the development of gonads, which was only Ž
dependent on fish sex ANOVA: P s 0.2697 and P - 0.0001 for initial body weight .
and sex, respectively; Fig. 1a . The mean GSIs of tagged females and males were 0.51 Ž
and 3.10, and were similar to those of controls 0.56 and 3.05, respectively; .
unpaired t-tests, P 0.95 . The proportion of perivisceral fat in tagged fish on day 126 Ž
. was also similar to that in controls 2.97 vs. 3.04; unpaired t-test, P s 0.8948 , but
Ž .
differed significantly with respect to fish size and initial tag ratio ANOVA: P s 0.0007 . Large males and females accumulated more than 4.0–5.0 of fatty reserves, whereas
Ž .
small fish rarely accumulated more than 3.0 Fig. 1b . There was no long term detrimental effect of tagging and transponder presence in the
body cavity on the growth performance of female and male perch, as tagged fish Ž
outweighed slightly control fish at the end of the experiment 24.52 vs. 23.62 g and 23.21 vs. 22.43 g, for females and males, respectively; ANOVA: P s 0.1474 and
. P s 0.2680 for fish sex and treatment, respectively . Fish with tag ratios in the water
Ž higher than 1.25 grew first at a slower rate than controls or larger tagged fish Table
. Ž
. 2 . However, this detrimental effect was restricted to a short period only ca. 11 days ,
then was compensated by much higher growth rates over the following period. During the rest of the study, growth was no longer dependent on the initial tag ratio, even in the
weight classes that still had a tag ratio higher than 1.25 at that time. Tag ratio affected healing rate too, as small fish took significantly longer to heal their incisions than larger
Ž . Ž .
Fig. 1. Variation of gonado-somatic index a and storage of perivisceral fat b by PIT tagged age-0 perch 126 days after tagging, depending on their body weight at tagging. Error bars stand for the standard deviation of
Ž the mean in each weight class. Open bars: tagged females; closed bars: tagged males; dashed bars: control ctl,
. handled but untagged females and males. In each graph, bars sharing at least one common superscript are not
Ž significantly different, whereas the other comparisons differ at P - 0.05 Fischer PLSD comparison of
. means .
Ž .
individuals 15.8 vs. - 12 days, in fish - 2.5 g and 4.5 g, respectively . The covering of incision by regenerated scales also took longer in smaller fish, but
differences vanished after day 33. No perch expelled its transponder over the 126-day study period. X-ray photographs
Ž showed most tags near the abdominal musculature and none higher than midgut Fig.
. Ž
2a . Tags showed no vertical migration but moved slightly anterior as fish grew Fig. .
2b . Few tags were found close to the anterior or posterior edge of the body cavity Ž
. 7.7, 4.8 and 5.3, on days 73, 94 and 115, respectively . On average, the tags
Ž .
deviated from the sagittal axis of the fish by less than 108 Fig. 2c , causing a reduction of coupling of ca. 1. However, angles as high as y188 and q408 were observed, and
Ž .
the 95 confidence intervals of distributions ranged over ca. 268 10 reduction . The Ž
deviation diminished slightly over time, as tags changed their orientation Figs. 2d and .
3 . Changes of orientation of individual tags at 3-week intervals were less than 48 on average, and rarely cumulated over time. Neither the weight of fish at tagging nor the
longitudinal position of the tag influenced its orientation and stability over time
Ž . Ž .
Ž . Fig. 2. Horizontal a , vertical positions b , and orientations c of PIT tags in the body cavity of age-0 perch,
j days after tagging. Negative and positive orientations refer to tags heading ventral and dorsal, respectively. w
Ž .x
The reductions of detection range are calculated as 1ycos angular deviation , assuming that fish crosses Ž .
perpendicular to the plane of the detecting antenna. Changes of orientations over time d are calculated for individual tags.
Fig. 3. Illustration of the variable fate of PIT tags in the body cavity of perch. Plates a and b illustrate the same individual 73 and 115 days after tagging respectively and show a strong deviation of the PIT tag axis over
time. Plates c and d illustrate another individual on the same dates, but with no deviation of PIT tag axis over time.
Ž .
Ž .
P 0.10 . Tags near midgut showed significantly P - 0.0001 stronger deviations and Ž
changes over time than those located near the abdominal body wall mean deviations of .
19.08 vs. 4.08, mean changes of 9.38 vs. 3.48, respectively .
Dissection on day 126 revealed no muscle necrosis, erythema, infection nor damage Ž
. to internal organs. Most tags had become embedded in the adipose tissue 21.7 , or
Ž .
encapsulated into a fibrous capsule inside the adipose tissue 60.3 . Capsules adhered Ž
. Ž
. to the parietal 7.4 or visceral peritoneum 4.8 , but never to the intestine. The
Ž .
remaining tags 5.8 were free in the body cavity of individuals with less adipose Ž
2
tissue than others. The host tissue response to the tag was independent of fish sex x ,
. Ž
2
. P s 0.7930 and was unaffected by the tag ratio x , P s 0.4851 .
4. Discussion
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