MATHER ET AL.: TAGGED BLUEFIN TUNA 



Table 10. — Estimates of total survival rates of tagged bluefin 

 tuna from regression analysis of return per unit-of-effort data. 



'Recaptures in release year not included in 1966 regression analysis. 



^Regression estimates using average unweighted percentage returns 

 for 1964-68 are: Z = 1.67 and s = 0.19. This estimate happens to be exactly 

 the same as the estimate of Z = 1.67 = (average time at liberty) ' where 

 average time at liberty is 0.60 yr. 



^Survival rate computed from average Z-value. 



sizes are more vulnerable than others, a Peterson 

 tagging experiment is apt to overemphasize the 

 vulnerable ones both in respect to tags put out and 

 recaptures made; hence the estimate of rate of 

 exploitation is too high and the population esti- 

 mate is too low." Future analysis stratifying the 

 data by age at release would help to answer some 

 questions that arise because of the migratory be- 

 havior of bluefin. We suggest cohort analysis in 

 the fashion of Bayliff (1971) as a productive 

 method of analysis. 



ACKNOWLEDGMENTS 



DISCUSSION 



Exactly what these "survivals" are measuring 

 is of prime interest. They are properly thought of 

 as the results of disappearance rates which are 

 composed of mortalities {F, M, and G), tag shed- 

 ding, and changing migratory patterns with age. 

 Apparently few of the older fish entered the sur- 

 face fishery in the western north Atlantic during 

 the period of the study and the decreasing propor- 

 tion of the population that entered the fishery is a 

 primary factor lowering the apparent survival. In 

 1966 when the fish tagged were significantly 

 younger than in the other years — having an aver- 

 age age of 1.4 yr as opposed to average ages of at 

 least a year older in other tagging years — the 

 greatest return of tags occurred during the year 

 after release. For these younger bluefin, a higher 

 proportion returned to the fishing area the year 

 after they were released than for the older tagged 

 fish. 



The exploitation rate on the tagged fish was 

 exceedingly high when they were in the fishery. 

 The high fishing mortality rates in Table 8 may 

 underestimate the true rates for tagged bluefin; 

 most of the various sources of error that may bias 

 these estimates act to decrease the estimate of the 

 fishing rate relative to its true value. However, it 

 is known that bluefin available to the northwest 

 Atlantic fisheries are not a closed population. 

 Tagging studies have revealed transatlantic mi- 

 grations to and from the fishery. Migration could 

 also occur from the middle Atlantic. Thus it must 

 still be determined whether the high exploitation 

 rate applied to the population or just to the portion 

 of the population that entered the fishery. Ricker 

 (1958:35) describes the effects of tagging fish that 

 are more vulnerable to fishing than other mem- 

 bers of the population. "Again, if fish of certain 



We are greatly indebted to W. H. Bayliff of the 

 Inter-American Tropical Tuna Commission, La 

 Jolla, for reviewing an early version of this paper 

 and providing many useful suggestions. During 

 the course of the study, discussions with W. W. 

 Fox, Jr., T. D. Smith, and J. R. Zweifel of the 

 Southwest Fisheries Center, National Marine 

 Fisheries Service, NOAA, La Jolla, proved quite 

 fruitful. We thank D. Kramer also of the South- 

 west Fisheries Center for his technical editing of 

 the paper. We greatly appreciate the extensive 

 efforts of M. R. Bartlett and J. M. Mason, Jr. of the 

 Woods Hole Oceanographic Institution, Woods 

 Hole, Mass., towards the tagging of bluefin tuna 

 and collection of associated data. 



The principal financial support of the WHOI 

 Cooperative Game Fish Tagging Program 

 since 1956 has been from the National Science 

 Foundation (Grants G-861, G-2102, G-8339, 

 G-6172, G-19601, GB-3464, and GH-82), 

 the Bureau of Commercial Fisheries (now 

 National Marine Fisheries Service) (Contracts 

 14-17-0007-272, -547, -870, -975, and -1110), and 

 the Office of Sea Grant, National Oceanic and 

 Atmospheric Administration, U.S. Department of 

 Commerce (Grant GH-82). Important additional 

 support has been received from the Sport Fishing 

 Institute; the Charles W. Brown, Jr., Memorial 

 Foundation; the Tournament of Champions 

 (through Mrs. R. C. Kunkel and E. D. Martin); A. 

 Minis, Jr.; the Joseph A. Teti, Jr., Foundation; the 

 Port Aransas Rod and Reel Club; P. A. B. Widener; 

 the Jersey Cape Fishing Tournament; the As- 

 sociates of the Woods Hole Oceanographic Institu- 

 tion; and many other sportsmen's organizations 

 and individual sportsmen. 



The National Marine Fisheries Service and its 

 predecessor, the Bureau of Commercial Fisheries, 

 the Inter-American Tropical Tuna Commission, 



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