MATHER ET AL.: TAGGED BLUEFIN TUNA 



dropped off so rapidly, however, after the first two 

 recapture periods that little reliability can actu- 

 ally be placed on estimates using only data from 

 the tail end of the time series. 



This first examination of the recaptures by time 

 at liberty was followed by a revised analysis of the 

 July releases each year and all releases each year 

 for 1964 through 1968. An attempt to include all 

 tags in the analysis was made by assigning tags 

 without recorded release dates to most likely re- 

 lease dates by means of the accompanying infor- 

 mation, e.g., by cruise number. For the most part 

 this revision produced minor changes in the esti- 

 mates of total survival rates. Raw exploitation 

 rates were estimated from the total number of 

 recaptures per release using all data whether or 

 not any information was available on date of re- 

 capture. 



Estimates of Fishing and Other Losses 



We used the following equations to estimate 

 rates of exploitation, fishing, and other losses: 



where ii 7^ = estimate of total exploitation of 

 tagged bluefin tuna over n years 



R = number of tag returns. 



n 



where s = 



Ui 



survival rate estimated from 



revised data by the Chapman- 



Robson method 

 estimate of single season 



exploitation of tagged 



bluefin tuna. 



F = u (-In s)/(l - s) 



X = ln(s) - F 



These estimates for July releases are shown in 

 Part A of Table 5, and in Part B of the same table, 

 estimates of the same set of parameters for all 

 releases are given with the exception of the single 

 season exploitation rates. We believe that dis- 

 tributing the releases during the entire fishing 

 season, rather than restricting them to the first 

 part of the season, July, makes it impossible to 

 estimate a single season exploitation rate. It will 

 be noted that the actual observed exploitation 

 rates are high, especially in view of the fact that 

 no corrections were made for either immediate 

 tagging mortality, tag shedding, or nonreporting. 

 The total recapture percentages range from 16% 

 to 48%. 



Lenarz et al. (1973) estimated that the rate of 

 immediate tag shedding (1 - p ) for Atlantic 

 bluefin tuna is 0.027 and that the instantaneous 

 rate of tag shedding (L) is 0.310. Their estimates 

 were used to correct our estimates of exploitation 

 and mortality rates for shedding as follows: 



Table 5. — Estimates of total survival, rate of exploitation, fishing mortality rate and total other loss rate of 

 tagged bluefin tuna for July releases and for all releases by year of release. 



909 



