FISHERY BULLETIN: VOL. 70, NO. 3 



Table 1. — Annual landings of skipjack in Hawaii, 1952- 

 1970, with time of initial warming and mean July sa- 

 linities of sea-surface water. 



1 Time of warming and July salinities are based on observations at 

 Koko Head, Oahu, except 1952 to 1955 when they are based on obser- 

 vations irregularly made in the vicinity of Oahu. 



initial warming occurring before the end of Feb- 

 ruary precedes favorable fishing conditions. 

 Weak or late warm advection is reflected by ini- 

 tial warming in March and precedes unfavorable 

 fishing conditions. 



The empirical associations are summarized in 

 Table 1, giving the time of initial warming 

 (temperature index), the mean July salinity, 

 and the annual landings of skipjack in Hawaii. 

 The indices are based on measurement, regularly 

 made at Koko Head, Oahu, since November 1955. 

 Indices between 1952 and 1955 are estimates 

 based on sea-surface temperature and salinity 

 observations irregularly made in the vicinity of 

 Oahu. Prior to 1952 there were insufliicient data 

 to make estimates. 



When the associations between availability 

 of skipjack and time of initial warming were 

 first established, monthly mean temperatures 

 were used. Now Koko Head temperatures are 

 expressed by harmonic functions of time (Seckel 

 and Yong, 1971). These functions have been 

 used to determine objectively the time of initial 

 warming given in Table 1. Formerly, a salinity 

 of 35%c was used to indicate the boundary be- 

 tween North Pacific Central and California Cur- 

 rent Extension waters. Here, in keeping with 



the definitions given by Seckel (1968) , a s'alinity 

 of 34.8/f f has been used to indicate this boundary. 



The temperature and salinity indices can each 

 be assigned a rating of favorable (F) or unfa- 

 vorable (U) for fishing. There are therefore 

 three categories according to which the landings 

 are classified and shown in Figure 2a: Both in- 

 dices favorable (FF), one unfavorable (FU ar 

 UF), and both unfavorable (UU). 



The 1968 and 1969 landings did not fall within 

 the FF range of previous years. Examination 

 of environmental conditions revealed that during 

 these years minimum salinities were the lowest 

 recorded since the sampling series began at Koko 

 Head, Oahu (Seckel and Yong, 1971). A sharp 

 drop in values took place in late winter 1968 

 and the salinity, remained low until late summer 

 of 1969 when values rose to a normal level. 

 Minimum salinities occurred in June 1968 and 

 May 1969 and averaged 34.49^/r and 34.59%f, re- 

 spectively. This information, as well as that to 

 be introduced in the next section, indicates that 

 favorable "skipjack water" is confined to the 

 high-salinity portion of the California Current 

 Extension. For favorable fishing conditions, 

 therefore, the additional constraint is introduced 

 that mean monthly salinities during spring must 

 be above 34.6'/ (. With this new constraint, land- 

 ings for 1968 and 1969 fall into the FU category 

 (Figure 2b). 



Of the 19 years for which skipjack landings 

 in Hawaii can be associated with environmental 

 conditions, 6 years fall in the FF category, 7 in 

 the UU category, and 6 in the FU category when 

 the revised salinity index is used. Figure 2b il- 

 lustrates that there is a clear separation between 

 the landings in the FF category on the one hand 

 and the FU and UU categories on the other. 

 The average annual landings are 5,800 tons, and 

 3,700 tons and 3,600 tons in the FF, FU, and UU 

 categories, respectively. The lowest value in the 

 FF category (4,900 tons) is higher than the 

 highest value in the FU category (4,400 tons) 

 or in the UU category (4,300 tons). The 19- 

 year average of annual landings was 4,300 tons. 



Annual landings, therefore, can be classified 

 into two groups. One group comprises the 

 landings in the FU and UU categories that av- 

 erage 3,600 tons and range from 2,710 to 4,400 



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