MATHER ET AL.: TAGGED BLUEFIN TUNA 



Table 7. — Releases, returns, and percentages of return of 

 yellowfin tuna for lATTC Cruise 1055, by time between com- 

 mencement of tagging and release offish (from Bayliff, 1973). 



where X** 



'Inter-American Tropical Tuna Commission. 



for purse-seine-caught yellowfin tuna. Bayliff^ 

 (pers. commun.) believes that bluefin tuna are 

 more hardy than yellowfin tuna and thus expects 

 that Type-I tagging mortality is lower for bluefin 

 than for yellowfin. 



Assuming that sport releases suffered insig- 

 nificant amounts of Type-I tagging mortality 

 and using the Mantel-Haenszel weighting proce- 

 dure, the 1964-68 average Type-I tagging mortal- 

 ity for commercial tagged fish was 21%. During 

 this period, 96% of the releases were commercial 

 tagged fish. The average Type-I tagging mortality 

 for all releases during 1964-68 was 20%. 



The rates of exploitation and mortality were 

 corrected for Type-I tagging mortality as follows: 



Un^'^ 



= Uj^ /n 



where u 



*^^ 



where w'j"' 



estimate of total exploitation 

 corrected for shedding and 

 Type-I tagging mortality. 



Uy = « J /77 



estimate of seasonal 

 exploitation corrected for 

 shedding and Type-I 

 tagging mortality. 



= «r"" (- lns*)/(l 



s*) 



where F*' 



= estimate of F corrected for 

 shedding and Type-I tagging 

 mortality, and 



X** = - In (s*) - F** 



estimate of X corrected for 

 shedding and Type-I 

 tagging mortality. 



The estimates are shown in Table 8. The values 

 ofZ** range from 0.300 to 1.109 (average = 0.678) 

 and are still higher than the expected rate of 

 natural mortality for bluefin. The large difference 

 between values for 1964-65 and the values for 

 1966-68 is worth noting. Food and Agriculture 

 Organization ( 1972) reports that a relatively large 

 number of bluefin tuna tagged in the northwest 

 Atlantic in 1965 were recovered in the Bay of 

 Biscay during the following year. This suggests 

 that a large-scale transatlantic migration oc- 

 curred between the 1965 and 1966 fishing seasons. 

 Thus migration from the fishery is a plausible 

 explanation for a portion of X**. 



Table 8. — Estimates of rates of exploitation and mortality for 

 northwest Atlantic bluefin tuna. The rates have been corrected 

 for tag shedding and a hypothetical value of Type-I tagging 

 mortality. 



«Bayliff, W. H. Inter- Am. Trop. Tuna Comm., P. O. Box 271, La 

 Jolla, CA 92037. 



'Calculated from averages ofF** andX**, 



Total Mortality Estimates - 

 Regression Method 



We next used available effort data for examin- 

 ing the effect of changes in effort on our estimates 

 of mortality. The effort data were obtained from 

 inquiries and logbooks of purse seiners that par- 

 ticipated in the commercial fishery for bluefin in 

 the northwestern Atlantic. The data have not 

 been standardized by vessel class. We question the 

 validity of the data as a measure of fishing effort, 

 i.e. , proportional to F, because a varying portion of 

 the fleet relied heavily on airplane scouting. Con- 

 version of the data to a standard unit of effort is 

 worth a study in itself and we recommend that 

 such a study be carried out. 



In another attempt to estimate total mortality 

 the natural logarithm of the number of returns per 

 unit effort (Table 9), taking the number of boat- 

 days recorded per season as a legitimate measure 

 of effort, was regressed on time as measured to the 

 center of each year following the release. An exact 



911 



