BEARDSLEY: POPULATION DYNAMICS OF ATLANTIC ALBACORE 



(Z) to determine the number of 3-year olds that 

 died (Ds): 



U X Z 

 Ds = -^ . 



Simple back calculation using the annual mor- 

 tality rates gave the original number of 3-year- 

 olds present. I was then able to work forward 

 and backward from this figure to obtain esti- 

 mates at other ages. 



Adjustments were necessary, however, in or- 

 der to obtain reasonable estimates. I used much 

 lower total mortality coefficients for 2-year-olds, 

 for example, than for 3-year-olds. Two-year- 

 olds were not fully recruited, and they constitute 

 only about W;? of the total catch. I also as- 

 sumed total mortality for 1-year-olds was equal 

 to natural mortality since very few 1-year-olds 

 are captured. 



Only one of the total mortality coefficients 

 proved to be completely unreasonable. The esti- 

 mated number of 4-year-olds present was less 

 than the estimated annual catch of 4-year-olds 

 when Z = 1.40, which is obviously an impossible 

 situation. In estimating the other two popula- 

 tions, Z = 0.96 appeared to be more reasonable 

 than Z = 0.50. When the number of 4-year-old 

 albacore that die of natural causes is obtained 

 by subtracting the estimated fishing deaths from 

 the estimated number of total deaths the result 

 corresponds to an M of 0.20 when Z = 0.96, 

 which is close to the assumed natural mortality 

 coefficient of 0.23 (based on Suda's (1966) esti- 

 mate) estimated for ages 1 through 3. When 

 Z = 0.50, the number of natural deaths of 4- 

 year-olds corresponds to an M of 0.32. 



If we use the figures in Table 5 and apply a 

 total mortality coefficient of 0.96 from age 5 to 

 age 6 and 0.79 (fi'om longline data) from age 6 

 through age 10, we can reproduce what theoret- 

 ically occurs in the North Atlantic longline fish- 

 ery in an average year. Table 7 shows that the 

 longline fishery should capture about 718,000 

 albacore, ages 5 through 10, in the North At- 

 lantic each year. The actual average number 

 of albacoi-e captured annually from 1963 through 

 1968 is estimated at 513,000 (data from Wise, 

 1970). This difference is large, but a relatively 



Table 7. — Theoretical yields from ages .5 through 10 

 based on the population in table 5 with natural mortality 

 coefficient 0.23, and total mortality coefficient 0.96 from 

 age 5 to age 6 and 0.79 from age 6 to age 10. 



small adjustment in the number of recruits at 

 age 5 would bring the figui'es closer together. 

 For example, by decreasing the number of re- 

 cruits to 1,100,000 the potential longline catch 

 was calculated as 533,000 fish, much closer to the 

 6-year average of 513,000. 



ACKNOWLEDGMENTS 



Jean-Claude Dao and Philippe Serene, Centre 

 National pour I'ExpIoitation des Oceans, France, 

 supplied length-frequency data from the Bay of 

 Biscay fishery. John A. Gulland, Food and Agri- 

 culture Organization, Rome, Italy, and Ralph P. 

 Silliman, National Marine Fisheries Service, 

 Seattle, Wash., critically reviewed the manu- 

 script, and I am grateful for their suggestions. 



LITERATURE CITED 



Allain, C, and H. Aloncle. 



1968. Migrations du germon en fonction des var- 

 iations thermiques du milieu entre le secteur 

 Portugais et le SO de I'Irlande. Sci. Peche 176, 

 15 p. 



Beardsley, G. L., Jr. 



1969. Proposed migrations of albacore, Thunnus 

 alalungu, in the Atlantic Ocean. Trans. Am. Fish 

 Soc. 98: 589-598. 



Berg, R. A. van den, and J. P. Matthews. 



1969. A preliminary investigation of the tuna re- 

 sources off the South West African coast. South 

 West Afr. Mar. Res. Lab., Invest. Rep. 15, 39 p. 



855 



