FISHERY BULLETIN: VOL. 74, NO. 1 



regulation occurred. The indication is that since 

 approximately 1969, the biomass and exploita- 

 tion levels on the semestral cohorts have some- 

 how paralleled a somewhat uniform energy utili- 

 zation by the two cohorts, whereas from 1966 

 until 1969 a larger semester A biomass was 

 under exploitation compared to the semester 

 B cohort. The large discrepancies in biomass 

 caught as compared to gross growth in the early 

 data (1964-65) compared to the recent data 

 (1969-72) may be an indicator of the relative 

 health of the stocks under exploitation in recent 

 years in contrast to the preregulatory years. 



SPECULATIONS 



The utility of simulation studies lies in the 

 process of linking together observations, using 

 generalized principles where possible, to gen- 

 erate testable hypotheses which ultimately lead 

 to resolution of cause and effect relationships. 

 As examples, from the results of the simulation 

 model ENSIM, hypotheses were conceived con- 

 cerning the relative importance of forage or- 

 ganisms, primary productivity and the size of the 

 animals with respect to recruitment limitations. 



Food as a Population Regulator 



The availability of food is classically attributed 

 the role of limiting population size. We do not 

 intend to assail this premise, but intend only to 

 show that the most probable source of limitations 

 is at very early ages in tunas (<40 cm), and not 

 on the late juvenile or adult population. 



Forage for tunas is generally considered to be 

 in the micronekton size range (1-10 cm). It 

 probably extends upwards to 30 cm or more in 

 length for larger sizes of tunas (Magnuson and 

 Heitz 1971; Perrin et al. 1973). Tunas eat largely 

 crustaceans, fishes, and cephalopods in most 

 regions (Alverson 1963; Magnuson and Heitz 

 1971; Perrin et al. 1973). These organisms are 

 poorly sampled by micronekton sampling devices. 



The EASTROPAC cruises sampled from our 

 study area over the year 1967 and early 1968. 

 Productivity, micronekton, and most physical 

 and chemical properties which are linked to 

 biological productivity were sampled. EASTRO- 

 PAC data (Blackburn et al. 1970) indicate that 

 the average minimum micronekton night haul 

 contained 5 ml of micronekton per 10^ m^ of 



water sampled. The samples represent a 200-m 

 water column. 



The surface area of the CYRA is estimated to 

 be 5,012,643 sq nautical miles or 1.696 x lO^^ m2. 

 The minimum available forage is therefore 



(1.696 X 1013 m2) (200 m) 



5 ml forage 



103 m^ 



) 



= 1.696 X 10^3 cc. 



If 1 cm^ forage has approximately 1 g or 1.25 

 kcal caloric equivalency, then one should expect 

 that there is a minimum forage availability of 1.25 

 kcal/m^ or assuming 80% utilization efficiency 

 of these calories by predators (Winberg 1960), 

 1.0 kcal/m^ are present for metabolic utilization. 



Owen and Zeitzschell (1970) in their analysis 

 of EASTROPAC data also show that the primary 

 productivity averages 169 mg carbon m'^ day^ 

 over long. 119°- 112° W, 219 mg carbon mr^ day^ 

 at long. 105°W, and 282 mg carbon m'^ day^ 

 along long. 98°W. They also indicate coastal 

 effects as being the probable cause of the east- 

 ward increase in productivity. The average pro- 

 ductivity over the entire study area was 205 mg 

 carbon m'^ day^. 



The energetic equivalent value for 1 mg carbon 

 fixation is 11.4 cal (Piatt and Erwin 1973), so that 

 the average caloric productivity is 2,340 cal/m^ 

 day (or 2.34 kcal/m^ day). 



We have seen that the minimum estimate of 

 the micronekton standing stocks caloric value is 

 1,250 cal/m^, indicating that the probable daily 

 turnover rate is less than 125 cal/m^ so that 

 maintenance of this stock is not unreasonable if 

 the primary production is 2,340 cal/m^ day. 



The yellowfin tuna population simulation pro- 

 cedure based on average Murphy recruitment 

 estimates of the 1966-71 S^ and Sg cohorts indi- 

 cates that an unfished population (exhibiting a 

 stable age structure) would have the biomass of 

 600,000 metric tons (6.0 x lO^^ g). Assuming 

 that the yellowrfin tuna (YF) are distributed pro- 

 portionally over the forage: 



6.0 X 1011 g YF 



1.696 X 1013 ni 



= 3.54 X 10-2 g YF/m2 



= 35.4 mg YF/m2; 



35.4 mg YF/m2 x 1.2 cal/mg YF = 42.5 cal/m^. 



46 



