the summer typically are 45 and 70 cm. Modes 

 also occur in intermediate positions. The 35-cm. 

 group does not always appear in the catches; the 

 cannery which receives the bulk o£ the Hawaii 

 catch has established a policy which discourages 

 fishermen from taking fish of this size. The 

 same length-frequency pattern persists for sev- 

 eral months in many years; for example. May 

 through July of 1954 showed relatively small 

 proportionsof fish smaller than 60 cm., whereas 

 summer samples for 1952 contained relatively 

 large proportions of fish under 60 cm. The con- 

 sistency of the frequency distributions for some 

 adjacent months is a further suggestion that they 

 are representative of the population as it is sam- 

 pled by the fishery. 



Another interestingfeature of the distributions 

 is the occasional occurrence of a modal group 

 in the April-September samples between the 45- 

 and 70-cm. groups. Definite examples of this 

 group are found, for instance, in the August 1959 

 distributions. This intermediate group also is 

 evident in a less pronounced degree in other 

 summer samples. The occurrence of this group 

 probably is not due to chance because the samples 

 are large and the group is evident in some cases 

 for at least 2 months. 



Modal sizes. --The modal sizes for the domi- 

 nant modal groups were determined from length- 

 frequency distributions (fig. 4). The determina- 

 tion of which modal groups were dominant was 

 subjective but based on a conservative criterion 

 because only prominent modes well separated 

 from one another were selected to the exclusion 

 of minor irregularities in the frequency profile. 

 These selected modal points were plotted in 

 figure 5 for each month and year for which data 

 were available. It should be emphasized that 

 other modal groups are evident and probably 

 real, but they were not considered dominant and 

 are therefore not included in figure 5. 



Several characteristics of figure 5 are evident. 

 Most months contain two modes, other months 

 three or four modes. Certain years (e.g., 1950, 

 1959) have typically three modes for many 

 months. The intermediate mode (about 60 cm.) 

 is most common during the nonsummer months. 



The positions of the modes occur at about 

 45 cm., 60 cm., and 70 cm. Modal lengths for 

 the 45-cm. group, which is presumed to be the 

 fastest growing of the various modal groups, do 

 not necessarily increase with time; in fact there 

 are some months when the modal length stays 

 the same or even decreases. There is a tendency 

 for modal lengths to follow a U-shaped curve 

 with a minimum in length occurring during the 



summer. The lengths of the 70-cm. group seem 

 to have no consistent pattern except that there 

 has been a trend in some years for these fish to 

 increase in size. It is evident that the month-to- 

 month change of modal sizes in many instances 

 cannot be representative of growth; these data 

 suggest passage of successive groups through the 

 fishery. The varying rate of change in modal 

 length suggests that this passage is not uniform 

 with respect to time. 



7T 



•>. ' 



1950 1951 1952 1953 1954 1959 



1961 1962 1963 



Figure 5. --Size distribution of dominant modes 

 in the length- frequency samples of skipjack 

 taken during various months from the Hawaiian 

 fishery. The shaded portion shows the sum- 

 mer peak fishing period. 



Size distribution . — The proportions of skipjack 

 in the length frequencies (fig. 4) over 60 and 

 70 cm. are plotted in figure 6. The proportion 

 of skipjack over 60 cm. is shown invariably to 

 exceed the 50th percentile during the summer. 

 The total yearly catch is weighted toward sum- 

 mer catches, since a large proportion of the 

 total landings is taken during the summer. Skip- 

 jack larger than 60 cm. are often less available 

 in the presummer and postsummer periods. 

 Occasionally fish larger than 60 cm. reach a 

 peak in availability in December, January, or 

 February; this peak occurs most often near 

 February. 



Proportions of fish larger than 70 cm. also 

 are plotted in figure 6, and they likewise show a 

 peak for most summer months. In some years, 

 such as 1960, the proportion of fish over 60 cm. 

 is a fairly close parallel to the proportion of fish 

 over 70 cm. In other years, such as 1951, 1953, 

 1954, and 1959, there are relatively large devia- 

 tions between the proportions of fish over 60 and 

 70 cm.; in these years the commercial catch 

 was relatively large, indicating that the peak 



