WANKOWSKI: ESTIMATED GROWTH OF SURFACE-SCHOOLING TUNAS 



Over the period of the study, 9.97'^ of the total 

 catch consisted of yellowfin tuna, 89.83% skipjack 

 tuna, and the remaining 0.20% other tunas and 

 tunalike species. However, it should be noted that 

 the proportion of yellowfin tuna in the catch is 

 quite variable (Warikowski 1980). The fishery 

 since its inception in 1970 is described by Wari- 

 kowski (1980) while the 1978 and 1979 seasons are 

 described in detail in Anonymous. "* ^ 



All vessels catch bait each day and the return to 

 the baiting grounds is almost always accompanied 

 by unloading the day's tuna catch onto one of the 

 five motherships from which the fleets operate. 

 From March 1978, length-frequency data were col- 

 lected every day from each fishing area during 

 unloading. The fork lengths (FL) of a sample of 10 

 skipjack and 5 yellowfin tunas from each vessel 

 were measured to the nearest centimeter. This 

 sample size was chosen since it represented an 

 acceptable compromise between statistical and 

 logistic requirements. The measurement of more 

 than 15 fish/vessel proved impossible during busy 

 periods of unloading, and it was considered more 

 important to collect data from as large a number of 

 vessels as possible than to increase sample size. 

 Yellow^n tuna were measured only when suffi- 

 cient numbers were present in individual catches 

 to enable easy sampling, since the catches are 

 rarely sorted by species. 



During 1977, samples were measured on an ad 

 hoc basis by Papua New Guinea Fisheries Division 

 personnel during the course of their normal duties 

 on board fleets operating in the Manus and eastern 

 Bismarck Sea areas. The New Hanover fishery 

 was sampled on a daily basis, but each sample was 

 obtained from a small proportion of the total catch 

 transhipped to a shore-based processing plant. It 

 was therefore not possible in 1977 to differentiate 

 between fish caught north or south of New 

 Hanover. 



During the ^Vi yr of study, 106,933 skipjack tuna 

 and 47,405 yellowfin tuna were measured. 



DATA ANALYSIS 



The length-frequency data were analyzed by 

 area on a monthly basis. Individual monthly his- 

 tograms were plotted by 1.0 cm FL interval for 



^Anonymous. 1979. Fisheries research annual report for 



1978. Dep. Primary Ind., Port Moresby, Papua New Guinea, 

 98 p. 



^Anonymous. 1980. Fisheries research annual report for 



1979. Dep. Primary Ind., Port Moresby, Papua New Guinea, 

 103 p. 



each of the four areas designated in Figure 1 and 

 for the New Hanover area as a whole (areas 2 and 

 3, Figure 1). The separate data for north and south 

 of New Hanover (areas 2 and 3, respectively) were 

 obtained from fleet C only, while those for the New 

 Hanover area as a whole were obtained only from 

 fleet B. Monthly length -frequency distributions 

 were therefore available separately for the 

 Manus, New Hanover, and eastern Bismarck Sea 

 areas. 



Polymodal distributions were divided into suc- 

 cessive unimodal distributions, using the method 

 of "successive maxima" (Daget and Le Guen 1975). 

 This method does not require the assumption of 

 normality of distributions, but merely their sym- 

 metry in relation to the modal value. For samples 

 with only one prominent mode and for the uni- 

 modal distributions resulting from the above anal- 

 ysis, the midpoint of the fork length interval of 

 maximum frequency was taken as the modal 

 length. Examples of length-frequency distribu- 

 tions and the resultant modal fork lengths are 

 shown in Figure 2. Two conditions were attached 

 to mode selection. The first was that model fork 

 lengths were considered separate only if the mid- 

 points of adjacent length intervals of maximum 

 frequency were themselves separated by intervals 

 of 3.0 cm or more. The second was that isolated 

 peaks of only one 1.0 cm interval were not taken to 

 represent modes (e.g.. Figure 2: 1979, month 5, 61 

 cm). 



Modal lengths were derived from all monthly 

 samples where n >30 fish (Figures 3, 4). However 

 mean size of these monthly samples was 1,215 

 skipjack tuna and 578 yellowfin tuna. Only 19 (of 

 88) monthly skipjack tuna samples contained 

 <400 fish and 1 <99; similarly, 21 (of 82) monthly 

 yellowfin tuna samples contained <200 fish and 2 

 <49. 



Because of the apparent large-scale migration 

 between the relatively closely associated fishing 

 areas, it was not possible to consider the results 

 independently for each area. A serial succession of 

 increasing modal lengths with time was desig- 

 nated a single group of fish distinguishable from 

 other groups on the basis of size and progression 

 with age. The progression which appeared to be 

 most logical was used without taking into account 

 the relative strength of each mode, major and 

 minor modes being treated equally. As can be seen 

 from Figures 3 and 4, the data does not naturally 

 fall into conventional year-class or cohort struc- 

 ture. This absence of structural form, other than 



519 



