BEARDSLEY: POPULATION DYNAMICS OF ATLANTIC ALBACORE 



66' ?? 76 80 84 88 v: 96 100 104 108 112 



69 73 77 81 85 89 93 97 101 105 109 11*3 



FORK LENGTH (CM) 



Figure 7. — Length-frequency distribution of 598 male 

 and female albacore measured at canneries in Puerto 

 Rico from December 1969 to September 1970. All were 

 caught in the Atlantic by longline gear. 



1970. Males constituted 61% of the samples 

 and were more abundant at the larger sizes 

 (Figure 7). Few females appear to attain a 

 length of much over 100 cm. This dominance 

 of males both in numbers and at the larger 

 sizes has been reported for Pacific albacore 

 by Otsu and Uchida (1959a, 1959b) and Otsu 

 and Hansen (1962) and for Atlantic albacore 

 by de Jaeger (1963) and Talbot and Penrith 

 (1963). 



MORTALITY 



There have been no mortality estimates for 

 Atlantic albacore that I am aware of. Suda 

 (1963) stated that the total instantaneous mor- 

 tality coefficient of North Pacific albacore is 

 probably around 0.4. He later estimated that 

 the natural mortality coefficient for the same 

 stock is about 0.2 (Suda, 1966). I estimated 

 the total instantaneous mortality coefficient (Z) 

 for Atlantic albacore using Bay of Biscay length 

 frequencies (Figure 2) and longline length fre- 

 quencies (Figure 3). All mortality rates in 

 the following discussion are instantaneous rates 

 unless specifically designated as annual. 



BAY OF BISCAY 



If we assume that the 1967, 1968, 1969, and 

 1970 samples from the Bay of Biscay are reason- 

 ably representative of the total catch which is 



in turn an accurate estimator of the true rel- 

 ative abundance of the different age groups in 

 the fishery, we can calculate a total mortality 

 coefficient from the decline in abundance of a 

 given year class from one year to the next, be- 

 ginning with the first year it is fully recruited 

 (age 3). This method requires weighting the 

 frequencies by catch per unit of eflfort (CPUE) . 

 Accurate estimates of CPUE for the entire Bay 

 of Biscay fishery are not available, although re- 

 cent research suggests that a slight decline in 

 CPUE has occurred at selected ports in France 

 in the late 1960's (Jean-Claude Dao, personal 

 communication). I have assumed a constant 

 CPUE over the 4 years in question for the pur- 

 pose of this analysis since complete figures are 

 not available. 



I have also assumed equal i-ecruitment for ages 

 3 and 4 though this is not likely. Any mortality 

 estimates based on the relative abundance of ages 

 3 and 4 in the fishery will be affected by relative 

 differences in recruitment. Some 3-year-old al- 

 bacore and many 4-year-olds are recruited to the 

 winter longline fishery in the North Atlantic; 

 however, for the 4 years, 1965-68, the average 

 number of 4-year-old albacore caught in the 

 North Atlantic winter fishery was only about 

 40,000. This is a relatively insignificant number 

 when compared with the total number of 4-year- 

 olds available in the Bay of Biscay fishery. It 

 is not known if all the survivors return to the 

 Bay of Biscay the following summer although it 

 is probable that most of them do. If 4-year-old 

 albacore do not all return to the Bay of Biscay 

 from the winter longline grounds then mortal- 

 ities will be slightly overestimated. 



Mortality coefficients were calculated from the 

 decline in abundance from age 3 to age 4 only, 

 and the frequency polygons were divided in the 

 following manner. Where there was an obvious 

 null between age classes, for example at 70 to 71 

 cm in the 1967 plot, the number of fish in that 

 length group were evenly divided, half were as- 

 signed to the age above and half to the age below. 

 Where there was no obvious null, as between ages 

 2 and 3 in the 1969 plot and between ages 4 and 5 

 in almost all the plots, the dividing point was 

 placed at a length approximately half way be- 

 tween the assigned lengths at age obtained from 



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