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Estimating tag-shedding rates for skipjack tuna, 
Katsuwonus pelamis, off the Maldives 
M. Shiham Adam 
Renewable Resources Assessment Group 
T. H. Huxley School of Environment, Earth Sciences and Engineering 
Imperial College of Science Technology and Medicine, RSM Building 
Prince Consort Road 
London SW7 2BP, United Kingdom 
Present address Pelagic Fisheries Research Program 
Joint Institute of Marine and Atmospheric Research 
University of Hawaii at Manoa 
Honolulu, Hawaii 96822 
E-mail address: msadam@soest hawaii.edu 
Geoffrey P. Kirkwood 
Renewable Resources Assessment Group 
T. H, Huxley School of Environment, Earth Sciences and Engineering 
Imperial College of Science, Technology and Medicine 
RSM Building, Prince Consort Road 
London SW7 2BP, United Kingdom 
One source of uncertainty in fishery 
assessments based on tag release and 
recapture data arises from tag shed- 
ding — the loss of tags from fish from the 
time of tagging until tag recovery. Inde- 
pendent estimation of tag-shedding 
rates from double tagging experiments 
is an integral part of well-designed 
tagging experiments. Failure to allow 
for tag shedding can result in biases 
in estimates of important parameters 
derived from tag-recapture data, such 
as fishery-induced and natural mortal- 
ity, and migration rates. A wide variety 
of methods have been proposed for esti- 
mating shedding rates using data from 
double tagging experiments, the vari- 
ety in part resulting from the nature 
of the data available from the experi- 
ments. The earlier literature has been 
well integrated and reviewed by Weth- 
erall (1982). More recently, Xiao (1996) 
has developed a general model that 
unifies the estimation of tag-shedding 
rates from double tagging experiments 
with exact and pooled times at liberty. 
Some recent examples of application 
of these methods include Xiao et al. 
(1999) for school gummy shark and 
Fabrizio et al. (1999) for lake trout. 
Tag shedding is of two types (Wether- 
all, 1982 ). Type-I shedding is a one-time 
event and occurs immediately after tag- 
ging, usually as a result of suboptimal 
placement of tags in the fish. Effective- 
ly, it reduces the number of tags ini- 
tially put out to sea. Type-II shedding 
is the loss of a tag or both tags over a 
period of time after the fish has been 
tagged and released back into the sea. 
For long-lived species, it may not occur 
at a constant rate because some tags 
are likely to have been applied more ef- 
fectively than others, and some may be- 
come firmly embedded (with growth of 
muscle tissues), such that they are very 
unlikely to be detached from the fish 
(Kirkwood, 1981). 
The Ministry of Fisheries and Agri- 
culture, Maldives, carried out two tag- 
ging experiments: one in 1990-91 (Ye- 
saki and Waheed, 1992) and the other 
between 1993 and 1995 (Anderson et 
al., 1996). The latter tagging experi- 
ment included a double tagging experi- 
ment in which 504 skipjack tuna, Kat- 
suwonus pelamis , were double tagged, 
by using the same type of tags and 
techniques of tagging as used in the 
single tagging experiment (Anderson 
et al., 1996). As of end 1996,atotal of53 
of these fish had been recovered. These 
data are considered to be of imme- 
diate importance for designing large- 
scale experiments in the Indian Ocean, 
strategic tagging simulation studies 
(e.g. Bertignac, 1996), and for compar- 
ing the estimated rates with those ob- 
tained from similar tuna double tag- 
ging experiments conducted elsewhere. 
Our note reports estimates of tag-shed- 
ding rates carried out in the Maldives. 
Materials and methods 
Tagging methods 
Tagging was conducted on board local 
vessels fishing for skipjack tuna with 
pole-and-line gear using livebait. Plas- 
tic dart tags (100 mm x 1.5 mm diam- 
eter), manufactured by Hallprint™, 
Australia, were used throughout the 
experiment. The captured fish were 
gently placed on deck, quickly slipped 
onto a wet wooden measuring board, 
and held in place by biologists wear- 
ing cotton gloves. The first tag was 
inserted dorsally on the left-hand side, 
at an acute angle adjacent to the 
second dorsal fin so that the barb was 
caught under the fin-ray extension or 
the neural spine. The second tag was 
inserted about 1-2 cm posterior to the 
first on the right-hand side in the same 
manner. Consecutively numbered pairs 
of tags were used; even numbers were 
inserted on the left-hand side and odd 
numbers on the right. Where possible, 
fish were returned to (he water, facing 
the vessel’s bow, in a slightly head- 
down fashion. Tagging times (from 
hooking to release into the sea) ranged 
from 14 to 18 seconds. More details of 
the tagging program can be found in 
Yesaki and Waheed ( 1992) and Ander- 
son et al. ( 1996). 
Parameter estimation 
The method of parameter estimation 
used here is the maximum likelihood 
approach introduced by Kirkwood and 
Walker (1984) and later extended by 
Hampton and Kirkwood (1990) and 
Hampton (1997). This method was de- 
veloped for use with sets of data where 
recaptured fish are few, and with few 
fish that have shed a tag, provided that 
Manuscript accepted 21 July 2000. 
Fish. Bull. 99:193-196 (2001). 
