ified to give good tag penetration. To compen- 

 sate for the effect of tagging mortality, which 

 may be greater in the field than in laboratory 

 experiments, we planned to tag sufficient num- 

 bers of juveniles so that we would have an 

 adequate number of recoveries on which to test 

 our hypotheses concerning movement and popu- 

 lation structure. 



EXPERIMENTAL TAG RECOVERY 



Tags can be recovered at processing plants 

 by two methods. Tagged fish can be located by 

 passing the catch through an electronic detec- 

 tor system, or tags can be recovered with mag- 

 nets from the processed fish scrap. The detec- 

 tor system gives more useful recovery data, but 

 construction, maintenance, and operation are 

 too costly for operation at all the plants (Park- 

 er, 1972). Permanent magnets were used for 

 tag recovery at all plants. These included mag- 

 nets already installed in the plants to collect 

 tramp metal and additional magnets installed 

 specifically for tag recovery. 



In 1965 experiments were conducted to cal- 

 culate the recovery rate of adult tags at differ- 

 ent processing plants. The recovery efficiency at 

 five plants ranged from 55 to 90% (Table 11). 

 Differences in rates of tag recovery resulted 

 from a combination of factors such as type 

 and location of magnets and plant conveyor 

 systems. 



In 1968 and 1969, experiments were conduct- 

 ed to determine the recovery rate of juvenile 

 tags at different processing plants. Two sizes 

 of juvenile tags and the adult tag were tested to 

 determine whether recovery efficiency varied 



Table 11. — Estimated tag~recovery efficiency rates 

 at five menhaden reduction plants. 



Table 12. — A comparison of tag-recovery efficiency rates for three 

 sizes of tags • 



Plant 



Number of 

 test tags 



Percentage of test 

 tags recovered 

 within 4 months 



A 





200 





56 



E 





400 





89 



C 





96 





90 



D 





196 





75 



E 





198 





55 



Plant 





Number and 

 tags used 

 experiment 



type 

 in each 



Percentage 



recovered 



experiment 



of tags 

 from each 





1/3 



1/2 



Adult 



1/3 



1/2 



Adult 



A 





200 



- 



200 



37 



— 



36 



B 





100 



- 



100 



45 



— 



80 



C 





100 



- 



100 



33 



- 



59 



D 





- 



100 



99 



- 



58 



69 



E 





- 



88 



100 



- 



39 



68 



F 





105 



105 



100 



49 



53 



77 



G 





200 



200 



200 



92 



94 



94 



Totals 

 1/3 vs. 



adult 



705 





700 



55 





68 



1/2 vs. 



adult 





493 



499 





68 



80 



greatly with tag size. The recovery efficiency of 

 the small tags, which were one-third or one-half 

 the mass of the adult tag, was about 70% of the 

 recovery rate of adult tags at most plants (Table 

 12). Plant G had about equal returns for the 

 three types of tags, but it is a small -capacity 

 plant. Since the one-third- and one-half-sized 

 tags had about the same recovery rates, we 

 used the smaller tag in the laboratory tagging 

 experiments because it seemed less likely to 

 damage the fish internally. 



CONCLUSIONS 



1. In summer, tagging mortality usually oc- 

 curs within 3 days and tag shedding within 

 2 weeks; but both, as well as incision heal- 

 ing, are prolonged at the lower water tem- 

 peratures encountered in the fall. 



2. Treatment of tags with antibiotics and dis- 

 infectants does not decrease the tag loss. 



3. Injection of adult tags posteriorly by the 

 scalpel-forceps method from above the tip 

 of the pectoral fin causes more total tag 

 loss than does the regular anterior method. 



4. The tagging-gun method is faster and safer 

 than the scalpel-forceps method and does 

 not increase the total tag loss. 



5. The total tag loss varies among taggers. 



6. Menhaden as small as 114 mm can be suc- 

 cessfully tagged using tagging guns and 

 adult tags. 



10 



