unique in that three individuals conducted the 

 tagging operation, whereas only one individual 

 tagged and released the remainder of the fish from 

 the other years. Therefore, we analyzed the data 

 from the time intervals 1971-73 and 1975-77 sepa- 

 rately from the 1974 data. For the following 

 reasons we decided to examine only two recapture 

 periods, k = (1, 2): 1) 1977 has only two recapture 

 periods possible (Table 1); 2) the number of single 

 as well as double returns for ^ =3 for both 1971-73 

 and 1975-77 constitutes very small sample sizes 

 (Table 1); and 3) L appears to have been changing 

 over k = (1, 2) to A; = (2, 3) (Table 7). 



Our analysis showed (Table 8) that there was a 

 significant difference in return and nonreturn 

 rates between time intervals for each type of dart 

 tag. Also a significant difference in shedding rates 

 was found between the plastic and metal type tags 

 during the time interval 1971-73. A small sample 

 size may account for the lack of a significant dif- 

 ference in shedding rates for each type of tag dur- 

 ing the time interval of 1975-77. There also was no 

 significant difference found in the shedding rates 

 between each tag type during 1974. As previously 

 mentioned, fish were released under different cir- 

 cumstances during 1974. During the 1971-73 time 

 interval, plastic tags were found to be superior. We 

 again found that the metal tag improved (Table 8), 

 i.e., L decreased, between 1971-73 and 1975-77. 



Table 8. — Estimates of immediate (1 - p) and annual instan- 

 taneous (L) tag shedding rates for northwestern Atlantic bluefin 

 tuna double-tagging study for 1971-73, 1974, and 1975-77 based 

 on a * = 2-yr return period. (A contingency table, 7x2, was 

 constructed containing the number of double and single returns 

 for each of the three recapture periods plus the number of non- 

 returns for each tag type and each time interval.) Results of 

 chi-square tests (df = 4) for differences in double and single tag 

 returns and total nonreturns between time intervals and tag 

 types over a 2-yr recapture f)eriod are given. (Results for the 

 period 1971-72 from Lenarz et al. (1973) are showTi for compari- 

 son.) 



Also, L -values for the plastic tag decreased, but 

 results yielded a negative value for the 1975-77 

 time interval, which is theoretically impossible 

 and is due to variability in the data. 



From our analysis, we cannot conclusively show 

 that one of the two types of dart tags is better for 

 bluefin tuna tagging. Both tag types appeared to 

 improve between 1971-73 and 1975-77. Plastic 

 tags were significantly better than metal in the 

 first period and nonsignificantly better in the 

 second. 



We have showTi that tag shedding rates vary 

 from 1 yr to another. There are some possible 

 reasons for the observed variability. One reason 

 may be changes in tag design or quality. To our 

 knowledge there was no intentional effort made by 

 the manufacturers to change the design of the 

 metal or plastic dart tags used in this study. A 

 different type of glue, however, was used during 

 1972 through 1977 for the plastic dart tags. Before 

 using the plastic dart tags, we tested them by 

 pulling on the barb. On several occasions, we dis- 

 covered that the barbs were not adequately se- 

 cured. We reglued these tags before using them. We 

 also examined the metal dart tags prior to their 

 use. In general, they appeared to be trouble free. 

 Several orders of both types of tags were used 

 during the course of this study. We were unable to 

 correlate changes in the shedding rates with the 

 specific batch of tags that were used. The shelf life 

 of the plastic used in the tags may be another 

 factor. In some instances we used tags which were 

 manufactured several years before their actual 

 use. Since there were no changes in the tagging 

 method, this reason was discounted. Tagging oc- 

 curred throughout the purse seine fishing season 

 during 1971, 1973, and 1974, and at the end of the 

 season during 1972, 1975, 1976, and 1977. We do 

 not see why this would have more of an effect on 

 one type of tag than on the other. 



Summary and Conclusions 



Return data for double-tagged northwestern At- 

 lantic bluefin tuna were used to estimate the 

 shedding rates of plastic and metal dart tags. No 

 significant difference was found between the re- 

 turn rates of the plastic and metal tags when the 

 data were tested for all years combined, but plastic 

 tags appeared to have lower shedding rates than 

 metal tags in most cases. We believe that the com- 

 bining of the data of all years together (1971-77) 

 probably yields a reasonable approximation to the 



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