FISHERY BULLETIN: VOL. 74, NO. 1 



Because many tags that entered a plant be- 

 came lodged in machinery or passed over mag- 

 nets w^ithout being captured, the total number of 

 tags that entered a plant could only be estimated. 

 The estimates were based on the actual number 

 of tags recovered and the collective efficiency of 

 the magnets that recovered them. The efficiency 

 for each plant w^as estimated by adding 100 

 tagged fish to catches at regular intervals and 

 then determining the number of these test tags 

 that were recovered during the fishing season on 

 both primary and secondary magnets. The effi- 

 ciency for each plant, expressed as a percentage, 

 was the ratio of the number of test tags recov- 

 ered each fishing season to the number applied. 



The number of tests varied from year to year 

 and plant to plant. In 1969 the number at each 

 plant ranged from 1 to 8 (100-800 tags); in 1970, 

 2 to 20 (200-2,000 tags); in 1971, 2 to 16 (200- 

 1,600 tags); in 1972, 3 to 17 (300-1,700 tags); in 

 1973, 3 to 16 (300-1,600 tags). 



The percentage of tags recovered from each 

 series of 100 test tags varied from 10 to 907c . The 

 mean seasonal efficiency varied from 13^^ for the 

 least efficient plant to 73% for the most efficient. 

 It also varied from year to year for each plant. 



For this study, the estimated total number of 

 field tags entering a plant was based on the 

 actual number of field tags recovered on both pri- 

 mary and secondary magnets. The total number 

 of field tags entering a plant each month was 

 estimated by dividing the actual number of tags 

 recovered by the mean annual plant efficiency. 

 Tags recovered in spring before fishing began 

 were added to recoveries from the previous year. 



Tags remaining in various parts of a plant for 

 up to 2 yr before being recovered caused errors in 

 the recovery data. Nearly 1% of the test tags were 

 recovered in the second or third year (Table 1), 

 but the percentages varied from plant to plant. 

 Test tags introduced late in the season were re- 

 covered in subsequent years in greater numbers 

 than tags introduced early in the season. When a 

 field tag that actually had entered a plant in a 

 previous season was recovered, it would in effect 



Table l. — Number and percentage of test tags recovered 

 during the year applied and after 1 and 2 yr 



be counted twice and expanded by the efficiency 

 factor two or more times. For example, if 100 tags 

 entered a plant whose efficiency was 0.50, the 

 number recovered would be 50. If 1 of the 50 un- 

 recovered tags were to be recovered the following 

 year and the recovery efficiency of the plant had 

 dropped to 0.25, the estimated number recovered 

 would be 4 ( 1/0.25 = 4). The estimated number of 

 tags recovered would be 104 instead of 100, an 

 error of about 4%, and 4 tags would be assigned to 

 the wrong year. 



SPRING RELEASES AND 

 RECOVERIES 



We tagged 26,995 fish in 1969, 17,775 in 1970, 

 and 22,800 in 1971. Of the number offish tagged, 

 the estimated percentages recovered through 

 1973 were 30.2, 51.5, and 32.5%, for 1969, 1970, 

 and 1971, respectively. Of the total number of 

 tags recovered, the largest percentages were in 

 the first year: 70.9% (1969); 84.3% (1970); 84.6% 

 (1971). Returns in the second or following year, 

 accounted for most of the remainder; 26.7% 

 (1969); 15.0% (1970); 14.3% (1971). Returns after 

 the second year ranged from 0.7 to 2.4% (Ta- 

 bles 2-4). 



The actual numbers of field tags recovered 

 after the second year probably were much 

 smaller than the numbers reported. The percent- 



TABLE 2. — Numbers of adult Gulf menhaden tagged in the 

 spring of 1969 and the estimated number recaptured in subse- 

 quent fishing seasons, by area. 



Release 



No. of Year 



fisfi of 



Area of recovery 



114 



