NOTE Adam and Kirkwood: Estimating tag-shedding rates for Katsuwonus pe/amis 
195 
The type-II shedding rate estimate obtained from our 
experiment is similar to those obtained by Hampton and 
Kirkwood (1990) for southern bluefin tuna tagged and re- 
leased during the 1960s and 1970s. However, recent esti- 
mates of type-II shedding rates obtained for skipjack tuna 
from the tropical western Pacific were much lower than 
those estimated in our study, as were estimates for south- 
ern bluefin tuna tagged in 1984 (Table 2). These differ- 
ences in shedding estimates may be due to differences in 
the tagging method used and the use of measuring boards 
instead of cradles. Estimates of A obtained for fish tagged 
on measuring boards ranged from 0.17 to 0.22/yr. For fish 
tagged in cradles, the corresponding range of estimates of 
A was 0.05-0.09/yr (Table 2). 
In the South Pacific Commission (SPC) double tagging 
experiments of 1989-92 (Hampton, 1997) and in the 
CSIRO experiment of 1984 (Hampton and Kirkwood, 
1990), specially designed vinyl cradles were used to posi- 
tion the fish dorsal side up, making it easier to insert the 
tags. It is also likely that such cradles may have helped 
to keep fish calm, perhaps because the eyes of the fish 
were covered. In the Maldives experiment, the CSIRO ex- 
periments of 1963-77 and the Western Australian Depart- 
ment of Fisheries’ experiment of 1970-78 (Hampton and 
Kirkwood, 1990), measuring boards were used. In many 
instances in the Maldivian experiment, the fish, particu- 
larly large individuals, struggled quite a lot while being 
tagged. This may have caused additional trauma to the 
fish and at the same time may have increased the likeli- 
hood of the tag being placed suboptimally, thereby increas- 
ing the apparent tag-shedding rate. Some fish handlers of 
the Maldivian tagging program observed that fish strug- 
gled less if they were gently laid on the board and the eye 
was quickly covered with one hand. This technique, how- 
ever, was not applied universally. 
Hampton and Kirkwood (1990) suggested that differenc- 
es in shedding rates observed in the southern bluefin tuna 
experiments might also be due to differences in the quality 
Table II 
Numbers of recoveries of originally double tagged fish 
retaining one (indicating which tag, left or right) or two 
tags on recapture, and times at liberty. 
Days at 
liberty 
1 tag 
2 tags 
Days at 
liberty 
1 tag 
2 tags 
6 
1 
61 
1 
7 
1 
63 
1 
8 
1 
66 
1 
9 
1 
71 
1 
10 
2 
73 
1 
11 
1 (L) 
1 
78 
1 
13 
1 
90 
1 
14 
3 
91 
2 
15 
1 
98 
1 
16 
2 
142 
1 (R) 
17 
1 
146 
1 
18 
1 (L) 
154 
1 
19 
1 
191 
1 
20 
2 
210 
1 
21 
1 
230 
1 
23 
1 
240 
1 
36 
1 
258 
1 ( L ) 
50 
1 
279 
1(R) 
52 
1 
335 
1 
59 
2 
341 
1 
60 
1 (R) 
433 
1(L) 
of the tags used. Tags used in the earlier experiments ( 1960s 
and 1970s) were inferior, because the streamers were prone 
to detach. Tags used in later experiments had their heads 
heat-fused to the shafts and thus were almost impossible to 
detach under normal conditions (Hampton, 1997). The tags 
Table 2 
Comparison of tag shedding rates estimated by using the maximum likelihood method from different double tagging experiments, 
with data on exact dates of recovery for skipjack (SKJ) and southern bluefin tuna (SBT). CSIRO = Commonwealth Scientific and 
Industrial Research Organization, Tasmania, Australia; WA Dept. Fish. = Western Australia Department of Fisheries; SPC = South 
Pacific Commission. P/L = pole and line. 
Parameter estimates ±SE 
Species 
a 
A (per yr) 
Reference 
Experiment 
Tagging method 
SKJ 
0.97 ±0.03 
0.22 ±0.13 
Our study 
Maldives 1993-95 
P/L board 
SBT 
0.93 ±0.02 
0.29 ±0.05 
Hampton and Kirkwood, 1990 
CSIRO 1963-70 
P/L board 
SBT 
0.97 ±0.006 
0.17 ±0.01 
Hampton and Kirkwood, 1990 
CSIRO 1963-70 
P/L board 
SBT 
0.96 ±.0.01 
0.17 ±0.04 
Hampton and Kirkwood, 1990 
CSIRO 1977 
P/L board 
SBT 
0.90 ±0.02 
0.19 ±0.05 
Hampton and Kirkwood, 1990 
WADept. Fish. 1970-78 
P/L board 
SKJ 
0.97 ± — 
0.09 ± — 
Hampton, 1997' 
SPC 1989-92 
P/L cradle 
SBT 
0.97 ±.007 
0.05 ±0.008 
Hampton and Kirkwood, 1990 
CSIRO 1984 
P/L cradle 
' Estimates obtained by using pooled data. 
