WANKOWSKI: ESTIMATED GROWTH OF SURFACE-SCHOOLING TUNAS 



modal progression, occurs whether the data are 

 analyzed using all the modes as shown, or using 

 the major mode of each monthly distribution only. 

 The von Bertalanffy growth function (Ber- 

 talanffy 1960) was used to describe growth. This 

 function is usually expressed as 



Lt = Ly:[l - exp - kit - to)] 



where Lt = length at age t, L-^_ = asymptotic 

 length, k = coefficient describing the rate of 

 growth, and ^o = theoretical age where Lt is zero. 

 The Fabens' (1965) least squares procedure can be 

 used to fit length data to the von Bertalanffy func- 

 tion, which then takes the form: 



Lt+it = Lt + iLy: - Lt) (1 - exp - k). 



Although originally devised to fit tag return data, 

 this procedure is equally applicable to length ob- 

 servations of untagged fish, and is especially use- 

 ful since length at known age is not required. This 

 procedure has been used by Rothschild (1967) and 

 Joseph and Calkins (1969) to estimate growth of 

 skipjack tuna from tagging data, and Le Guen and 

 Sakagawa (1973) to estimate growth of yellowfin 

 tuna from modal progression data. 



In the present study, Fabens' (1965) procedure 

 was used with monthly modal length data for in- 

 dividual groups offish of unknown age. Estimates 

 ofLx in centimeters and of A on a monthly basis 

 were derived. Length-at-age information from 

 published sources was used to fix the derived 

 growrth functions on a known time scale. 



RESULTS 



Recruitment and Exploited Size Range 



Recruitment in this report refers to first entry 

 into the fishery. Size at recruitment, as inferred 

 from length-frequency samples, and the size range 

 of both skipjack and yellowfin tunas exploited by 

 the fishery varied throughout the duration of the 

 study. The smallest size of fish of either species 

 occurring in monthly samples was between 30 and 

 46 cm FL (Table 1), with the exception of a few 

 yellowfin tuna <30 cm FL in July 1979. Skipjack 

 tuna <46 cm and yellowfin tuna <54 cm FL were 

 absent in February 1979. The maximum lengths of 

 skipjack and yellowfin tunas were 74 and 96 cm 

 FL, respectively (Table 1, Figure 5). However, few 

 samples contained fish >69 and >85 cm FL, re- 



TabLE 1. — Fork length range (centimeters) of skipjack and 

 yellowfin tunas from all areas of the Papua New Guinea region 

 combined, and corresponding catch per unit effort (CPUEl 

 (metric tons per boat fishing day) and effort (number of fishing 

 days). The fishing effort remained fairly constant from April to 

 November of each year. 



spectively. There were no consistent differences in 

 size at recruitment, or in the size range exploited, 

 among the four fishing areas. 



Size at recruitment for yellowfin tuna was 

 smaller than has been reported in the past for 

 the eastern Pacific (Hennemuth 1961; Davidoff 

 1963; Anon3anous®'^) or eastern Atlantic (Le Guen 

 and Sakagawa 1973), where recruits are report- 

 edly 40-60 cm and 60 cm FL, respectively, but sim- 

 ilar to that recently reported for the eastern Pa- 

 cific (Anonymous footnote 7) and for the western 

 Indian Ocean (Marcille and Stequert 1976a). A 

 similarity between skipjack tuna size at first re- 

 cruitment in the western Pacific and western In- 

 dian Oceans is also apparent from a comparison of 

 the results of the present study with those of Mar- 

 cille and Stequert (1976b). 



Suzuki (1971), Kikawa and Warashina (1972), 

 Le Guen and Sakagawa (1973), Marcille and 

 Stequert (1976b), and others have noted the ex- 

 treme size-specificity of pole-and-line catches. In 



"Anonymous. 1975. Annual report of the Inter-American 

 Tropical Tuna Commission, 1974, [In Engl, and 

 Span.] Inter-Am. Trop. Tuna Comm., La Jolla, Calif, 169 p. 



'Anonymous. 1980. Annual report of the Inter-American 

 Tropical Tuna Commission, 1979. [In Engl, and 

 Span.] Inter-Am. Trop. Tuna Comm., La Jolla, Calif., 227 p. 



521 



