SHARP and FRANCIS: ENERGETICS MODEL FOR YELLOWFIN TUNA POPULATION 



relationships and the resulting formulations are 

 presented so as to encourage research in the area 

 so that improvements on this crude model can be 

 made in the future. 



THE MODEL 

 Population Dynamics 



In an attempt to produce a new, more detailed 

 method for evaluating the population or stock 

 status it was decided that the development of 

 TUNP0P, a biologically oriented population 

 simulator, would be appropriate. The only avail- 

 able population data which are collected on a 

 routine basis from within the fishery are length- 

 frequency information from commercial catches. 

 These data are collected according to criteria 

 which require that the several time-area strata 

 be sampled regularly and multiply, whenever 

 possible (Hennemuth 1961). Data from the period 

 1963-72 have been analyzed and processed in the 

 following manner. 



The 12 existing sampling areas in the CYRA 

 were reassembled into three major areas: N — 

 North of lat. 10°N except east of long. 95°W; 

 5 — North of lat. 5°N to the boundary of area N; 

 S — all the CYRA south of the boundary of area 5 

 (see Figure 1). The areas N and S tend to have 

 separable length-frequency distributions during 

 any given time interval. Area 5 tends to have 

 unique components as compared to N and S, but 

 also contributions from both the other areas can 

 be observed in the data from area 5. (This phe- 

 nomenon is tjqjically nonseasonal or noncyclic 

 with respect to the fishing year and is probably 

 related to population and environmental pres- 

 sures within the separate areas.) In all three 

 areas, recruitment components of a semestral 

 nature are evidenced. The apparent relative 

 abundance of these components within the areas 

 changes seasonally and also between years (Table 

 1). Analysis of this phenomenon has made the 

 separation of the semestral cohorts seem the first 

 logical step when the available genetic, mor- 

 phometric, and length-frequency data are con- 

 sidered. 



The catch data associated with each length- 

 frequency sample were obtained. The individual 

 sample sets were then given relative values pro- 

 portional to the contributions of the catches (in 

 weight) from which they were drawn. From this 

 basic processing of all the length-frequency data, 



Figure l. — The study area CYRA (Commission Yellowfin 

 Regulatory Area) used in the simulations is enclosed in the 

 dark outline. Three subareas were used in the preliminary 

 population dynamics work in estimating cohort strength from 

 the length-frequency and catch and effort data appropriate to 

 these areas. N = North of lat. 10°N except inside of long. 

 95°W; 5 = North of lat. 5°N to boundary of N; S = all CYRA 

 South of boundary of subarea 5. 



estimates of the catch composition with respect to 

 size-age for each fishing area were made and a 

 growth curve was determined for each of two 

 semestral cohorts. The two curves were essen- 

 tially identical and warrant no further discussion 

 here other than to say that from 40 to 145 cm fork 

 length it is possible to give relative monthly ages 

 to all individuals, given a length and correspond- 

 ing date of capture. The labeling problem was 

 handled such that any fish that was 40 cm from 1 

 January to 30 June is labeled S^ and correspond- 

 ingly 40-cm recruits from 1 July to 31 December 

 are labeled 83. The cohorts are identified in 

 relation to their recruitment year when they are 

 40 cm, not their spawned year. For example, a 

 40-cm fish caught in February 1969 is attributed 

 to the cohort labeled S^, 1969; and a 40-cm fish 

 caught in October 1968 is attributed to the 

 semester cohort labeled Sg, 1968. The two 

 semestral groups can be treated as independent 

 units in the population and provide a biological 

 basis in assessment of population size with re- 

 spect to size-age classes within the fishing year. 

 The annual growth increment in the most often 

 encountered cohort classes (40-140 cm) in the 

 fishery appears to be about 32 cm/yr; therefore, 



37 



