SHARP and FRANCIS: ENERGETICS MODEL FOR YELLOWFIN TUNA POPULATION 



10 



RE 

 (AS OF 



20 30 40 50 



LATIVE MONTHS OF AGE 

 JANUARY OF INDICATED YEAR) 



basis for determining some of the relative energy 

 expenditures in the population simulation study. 



The energy utilization which is simulated in 

 ENSIM is that attributable to 1) maintenance of 

 metabolic stasis, 2) growth, and 3) swimming. 

 Each is calculated independently and summed 

 with the others to give an estimate of the total 

 minimum energy utilized on a daily basis. No 

 attempt has been made to evaluate energy ex- 

 penditures due to gonad maturation or migratory 

 behavior beyond the daily forage or flight be- 

 havior levels because of the inherent void in our 

 knowledge of these processes in tunas. 



Metabolic maintenance of stasis energy re- 

 quirements (£',„) are difficult to assess under 

 optimum conditions and are typically derived 

 from extrapolation of O2 consumption versus 

 activity relationships to a zero activity level. The 

 magnitude varies greatly between species and in 

 general is a tenuous function of size and physio- 

 logical state. It is essentially impossible to di- 

 rectly measure the stasis energy requirements 

 of tunas due to their continuous swimming be- 

 havior. Estimates of E,,, should not include the 

 energy expenditures due to even minimum swim- 

 ming activity if it is to be useful in the deter- 

 mination of energy expenditures due solely to 

 swimming work. 



The respiration rate attributable to tissue stasis 

 can be estimated from the metabolic weight 

 (Wn^gt) of fish of length / from the equation: 



£„, = 24 ^ W^et (modified from Winberg 1960) 



where W^^^ = iMf)''-^ 



and 



Mf = 1.858 X 10-2 (/)3.o2i (grams) (Chatwin 1959) 



and where k is estimated to equal 1 cal/g h from 

 data and estimates for other highly active fishes 

 (Fry 1957; Winberg 1960). Therefore 



E^ = 4.46 X 10-1 (/)302i cal/day. 



Figure 2.— The numbers offish caught in the fishing years 1966, 

 1968, 1970, as a function of their recruitment month, and age, 

 relative to the fish of the year are graphically represented. Se- 

 mestral (A, B) and annual cohort labeling is as indicated. Note the 

 central tendency of the peaks within the semestral limits in the 

 years 1966 and 1968. In these years the fishing "season" was quite 

 long (>6 mo) as compared to 1970 (<3 mo), which combined with 

 cyclic migratory behavior and subsequent availability of cohorts 

 probably results in the drastic change from multimodality to the 

 amorphous distribution seen in the 1970 data. 



39 



