AN ENERGETICS MODEL FOR THE EXPLOITED YELLOWFIN TUNA, 



THUNNUS ALB AC ARES, POPULATION IN 

 THE EASTERN PACIFIC OCEAN 



Gary D. Sharp and Robert C. Francis^ 



ABSTRACT 



An energetics model (ENSIM) for the exploited yellowfin tuna, Thunnus albacares, population in the 

 eastern Pacific Ocean is developed. Hydrodynamic properties and respiration-swimming work theory 

 are combined to describe the energy expenditure due to swimming as a function of length for tunas. 

 Growth and maintenance energetics are estimated and incorporated into a simplistic three process 

 model. This model is interfaced with a population simulator (TUNP0P) and minimal energy 

 requirements for the exploited yellowfin tuna population are derived for the simulated fishing years 

 1964-72. A theoretical unexploited population simulation is made, and the energy requirements by 

 this population are compared with primary productivity rates and minimum micronekton (forage) 

 standing stock availability. No obvious food limitation is indicated for yellowfin tunas greater than 

 40 cm, particularly since the exploited population is at a level of, at most, 50% of the unexploited 

 biomass estimates. Population limitation processes are examined and indications that the recruit- 

 ment rates are independent of exploited biomass are discussed. 



The intent of studies of the population dynamics 

 of exploited populations is the determination of 

 the numbers, biomass, age structure, and poten- 

 tial yield from a population in order that rational 

 management decisions can be made about the 

 manner and rate of exploitation in order to insure 

 efficient utilization of the resource. The validity 

 of the resulting estimates of numbers, biomass, 

 and potential yield is of concern to all those 

 involved with the resource. Underestimations 

 generally result in conservative efforts which are 

 "safe" but not necessarily efficient. Overestima- 

 tions can result in reduced profit margins or, in 

 the extreme case, decimation of the resource. 



Since the implementation of the program for 

 conservation of yellowfin tuna, Thunnus alba- 

 cares, in the eastern tropical Pacific in 1966, a 

 series of complex changes in the fishery have 

 occurred which make production model results 

 less and less comparable between years (Inter- 

 American Tropical Tuna Commission Annual Re- 

 ports). Attempts to account for multiple changes 

 in the effort variables and corresponding but 

 independent changes in the exploited population 

 have resulted in serious interpretation problems 

 as to the relative status of the exploited stock. 



'Inter-American Tropical Tuna Commission, c/o Scripps In- 

 stitution of Oceanography, La Jolla, CA 92037. 



The economic and temporal problems inherent 

 in the collection and analysis of biological data 

 and the difficulties in representation of the 

 biological processes in a useful mathematical 

 manner has served to hinder utilization in the 

 management procedures of what sparse physi- 

 ological and ecological information is available. 



In this report, an energy budget model is de- 

 veloped for the exploited yellowfin tuna popula- 

 tion in the eastern Pacific Ocean within the 

 Inter- American Tropical Tuna Commission's Yel- 

 lowfin Regulatory Area (CYRA). The model will 

 be used to assess the energy flow through the 

 exploited yellowfin tuna population and also to 

 compare the estimated utilization of energy by 

 yellowfin tuna with the estimated primary pro- 

 ductivity in the CYRA. Comparisons will be 

 made using simulations of the population under 

 both exploited and unexploited conditions. 



The energy budget estimates are interfaced 

 with an age dependent population simulation 

 model (TUNP0P) (Francis 1974) resulting in a 

 model of the energy utilization by semiannual 

 recruitment cohorts. This model is referred to as 

 ENSIM. The model incorporates the population 

 parameter estimates and variables of TUNP0P 

 and the empirical and estimated size dependent 

 relationships for the major energy consuming 

 processes, resulting in estimates of energy utili- 

 zation rates. The development of the empirical 



Manuscript accepted June 1975. 



FISHERY BULLETIN: VOL. 74, NO. 1, 1976. 



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