Hampton: Estimates of tag-reporting and tag-shedding rates for tuna in the tropical Pacific Ocean 
73 
all, classification of the model by tagger did not re- 
sult in a significant improvement in fit (P=0.253, 
Table 2). It is therefore appropriate to use the pa- 
rameter estimates for the pooled model in models 
such as that defined by Equation 1. 
Tag reporting 
Return rates of seeded tags by species, unloading 
location, and time period Tag seeding was carried 
out on 111 observer cruises between May 1990 and 
September 1994. During these cruises, 532 tuna were 
tagged and placed in fish wells. Of these, 342 (64%) 
were later recovered during unloading or processing 
of catches in canneries. The species breakdown of 
seeded tag releases and returns is given in Table 3. 
The numbers of returns by species did not differ sig- 
nificantly from those expected from the return rate 
pooled across species (P=0.648). On this basis, re- 
porting of tags can be assumed to be independent of 
species. 
Most tag-seeding cruises (77) were undertaken on 
United States purse seiners because this fleet had 
the highest observer coverage during the period of 
the experiment. Tag-seeding cruises were also un- 
dertaken on purse seiners from Japan (18), Taiwan 
(8), Korea (4), Federated States of Micronesia (3), and 
Table 2 
Statistical tests of the pooled tag-shedding model versus 
species-specific and tagger-specific tag-shedding models. 
No. of 
Model 
parameters 
Q 
Z 2 
df 
P 
Pooled 
2 
201.498 
4.570 
4 
0.334 
Species-specific 
6 
199.213 
Pooled 
2 
201.498 
19.310 
16 
0.253 
Tagger-specific 
18 
191.843 
Solomon Islands (1). It was expected that the tag- 
reporting rate would vary by fleet, not because of 
variable cooperation by fishing vessel crews, but be- 
cause different fleets tend to unload their catches in 
different ports. As tag detection took place during 
unloading of catches and at later stages of process- 
ing, it was suspected that variation in the effective- 
ness of tag detection and reporting at unloading ports 
would result in large differences in tag-reporting 
rates. The individual tag-seeding cruises were there- 
fore classified by unloading location. In several in- 
stances, a vessel’s catch was transshipped to two or 
more ports. In these cases, the seeded tags were classi- 
fied individually according to the destination of fish in 
the wells into which the seeded tags had been placed. 
The numbers of seeded tag releases and returns, 
by unloading location, are given in Table 4. The re- 
turn rates vary considerably among unloading loca- 
tions; for example, the 95% confidence intervals on 
the return rates for the two unloading locations with 
the largest numbers of seeded tags, American Sa- 
moa and Thailand, do not overlap and are in fact 
widely separated. Not surprisingly, the observed 
numbers of returns by unloading location differed 
significantly from those expected from the return rate 
pooled across unloading locations (P<0.001). 
For American Samoa, there were sufficient seeded 
tags to test the hypothesis of constant return rate of 
seeded tags over time. The return rate was low in 
1990 but was consistently high for 1991 through 1994 
(Table 5). The differences in return rates among years 
were statistically significant (P=0.005), but this was 
due entirely to the lower than expected (on the basis 
of the return rate pooled across years) number of 
seeded tags returned in 1990. The differences among 
years 1991 through 1994 were not statistically sig- 
nificant (P=0.706). Other locations (Japan and Thai- 
land) also had highly variable reporting rates across 
years, but the numbers of seeded tags for these loca- 
tions were too few to support a statistical treatment 
of the data. 
Table 3 
Numbers of seeded tag releases and returns, by species. The 95% confidence intervals were calculated assuming a binomial 
distribution. 
Tuna species 
Number seeded 
Number returned 
Return rate 
95% confidence interval 
Skipjack 
333 
222 
0.667 
0.613-0.717 
Yeilowfin 
158 
94 
0.595 
0.514-0.672 
Bigeye 
35 
23 
0.657 
0.478-0.809 
Unknown 
6 
3 
0.500 
0.118-0.882 
Total 
532 
342 
0.643 
0.600-0.684 
