392 



Fishery Bulletin 90(2), 1992 



floating fish ration (Texas Farm Products, 

 Nacogdoches, TX), 0.45kg/day, 5 days/week, 

 as a supplement to natural forage available in 

 the pond. Fish were harvested on 11 October 

 1988, 464 days post-tagging. Microtag reten- 

 tion was determined with the FSD, fish were 

 measured (x 473mmTL), and 10 of 31 sur- 

 viving fish were selected at random and pre- 

 served in 50% formalin for X-ray analysis of 

 tag retention. X-ray negatives of the preserved 

 fish were visually inspected to confirm the 

 presence or absence of tags as determined by 

 the FSD. 



Tag retention was determined for each in- 

 terval, and overall or cumulative tag retention 

 was determined at the end of each interval. As 

 mortality was not considered tag loss in this 

 study, cumulative tag retention reflects only 

 the percentage of tag losses from shedding and 

 nondetection of tags. A problem encountered 

 in the course of this program was the calcula- 

 tion of tag retention rates when fish which had 

 shed tags were not removed from the group 

 at the end of the interval (2-23 days, 24-114 

 days, and 286-464 days). The percent decrease in 

 cumulative tag retention was selected as an estimate 

 of the percentage of fish losing tags in these intervals. 

 Conversely, when fish that had lost tags were removed 

 from the group, determination of tag retention for that 

 interval (days 115-285) was simple (no. fish with tags/ 

 total no. fish examined), but cumulative tag retention 

 had to be calculated. Tag retention for the interval in 

 question was multiplied by cumulative tag retention 

 from the previous interval to determine cumulative tag 

 retention for the interval. The relationship used in 

 these calculations was 



50 100 150 200 250 300 350 400 450 500 



Day 



Figure 1 



Observed values (X) of cumulative tag retention for microtagged red 

 drum Sciaenops oceUatiis fmgerlings through 464 days post-tagging. 



TR; = 



CTRj 

 CTRi. 



X 100, 



where TR; is percent tag retention for interval i, CTRj 

 is percent cumulative tag retention for interval i, and 

 CTRj. 1 is percent cumulative tag retention for the in- 

 terval prior to interval i. Percent tag retention and per- 

 cent cumulative tag retention for 1-23 and 24-114 day 

 intervals for fish from individual ponds were used to 

 calculate weighted means reported in Table 1. The 

 weighting factor used was the number offish harvested 

 from each pond. 



Results and discussion 



Tag retention for surviving fish at 115-464 days post- 

 tagging was 83.9%. Tag retention was 93.9% at 115- 



285 days post-tagging, and 89.3% at 286-464 days 

 post-tagging (Table 1). Cumulative retention of coded 

 wire microtags for red drum was 38.0% at 464 days 

 post-tagging (Table 1, Fig. 1). Lack of replication at 

 all intervals prevented statistical comparison of tag 

 retention for different intervals. However, tag reten- 

 tion values of 96.6% for 24-114 days, 93.9% for 115- 

 285 days, and 89.3% for 286-464 days post-tagging in- 

 dicate cumulative tag retention decreased in the inter- 

 val 24-464 days post-tagging, although at a slower rate 

 than for the period 0-23 days (Table 1). Numerous 

 authors (Gibbard and Colura 1980, Klar and Parker 

 1986, Fletcher et al. 1987, Williamson 1987, Bum- 

 guardner et al. 1990, and Dunning et al. 1990) have 

 reported that the majority of coded-wire tag losses 

 occur within a relatively short period (14-90 days) post- 

 tagging. Our results agree with this generalization, but 

 indicate tag losses may continue at a much reduced rate 

 for extended periods after tagging. While our results 

 are based on a small unreplicated sample {n 31 fish at 

 study end), we believe they indicate long-term tag loss 

 may be important when estimating the contribution of 

 hatchery fish to a population. Accounting for this con- 

 tinued tag loss would prevent underestimation of the 

 proportion of tagged fish occurring in the population 

 (Heimbach et al. 1990). 



Although Bumguardner et al. (1990) reported the 

 FSD failed to detect tags present in 9% of live fish 114 

 days after tagging as determined by examination of 

 X-ray negatives (n 186), no difference in tag detection 

 between the FSD and X-ray negatives was found in this 



