FISHERY BULLETIN: VOL. 79. NO. 1 



SEASONAL DISTRIBUTION 



The range of skipjack tuna in eastern coastal 

 waters is subject to seasonal variation. Table 1 

 shows where specimens have been taken in those 

 waters in each month, in investigations made by 

 or in cooperation with CSIRO, vdth data of all 

 years combined. The southern limit of the range is 



Table l. — Records of captured skipjack tuna (X) and sightings 

 of skipjack tuna (S) in Australian east coast waters by months. 

 Sources are Robins (1952), Hynd (1968), and unpublished data 

 from the Commonwealth Scientific and Industrial Research 

 Organization for the period 1938-78. Waters between lat. 33° 

 and 24° S were completely covered only in May, July, and 

 September, and not covered at all in November and December. 

 Waters between lat. 33° and 44° S were completely covered in 

 all months. 



farthest south in February-March and farthest 

 north in July-August, and this variation is explic- 

 able. All positions of the limit lie in the well- 

 surveyed waters south of Sydney (approximately 

 lat. 34° S), and they generally agree with positions 

 of limiting sea surface isotherms as was shown by 

 Robins (1952). Figure 1 shows the mean February 

 and August positions of the 15° C surface 

 isotherm, the one closest to the lowest tempera- 

 ture at which any skipjack tuna have been caught 

 (14.7° C, by Robins). The isotherm positions agree 

 fairly well with the observed limits of skipjack 

 tuna range in the same months, considering that 

 no observations were made south of lat. 44° S. The 

 lower limiting temperature for skipjack in abun- 

 dance is about 16° C according to Robins (1952). 

 The mean positions of that surface isotherm 

 (about 2° of latitude north of the 15° C surface 

 isotherm in each month) agree almost exactly 



with the skipjack tuna range limits in Table 1. 

 From 1938 to 1942 skipjack tuna were hardly ever 

 found south of lat. 43° S and seldom found south of 

 lat. 42° S. They were fairly numerous between lat. 

 43° and 44° S in 1951, when temperatures were 

 unusually high (Robins 1952). Hynd and Robins 

 (1967) reported aerial sightings of a few schools of 

 presumed skipjack tuna off the southern tip of 

 Tasmania where surface temperatures were prob- 

 ably about 13° C. Neither the species nor the tem- 

 perature was confirmed, however. 



The occurrence of skipjack tuna at tempera- 

 tures down to 15° C is of special interest, because 

 the species has not been found in such cool waters 

 in other parts of the world. It has been recorded at 

 temperatures dowTi to 17° C in the eastern Pacific 

 (Williams 1970) and 18° C near Japan (Uda 1957). 

 Dizon et al. (1977) exposed four captive Hawaiian 

 skipjack tuna to gradually decreasing water 

 temperatures with the followdng results. Three 

 fish stopped feeding at 17° C and died at 16° C; 

 corresponding temperatures for the fourth fish 

 were 15° and 14° C. 



It is difficult to recognize or hypothesize any 

 seasonal change in the northern limit of skipjack 

 tuna range in eastern coastal waters, especially in 

 view of the incomplete vessel coverage north of 

 Sydney (Table 1). The northern limit of any occur- 

 rence (excluding offshore data from Japanese 

 longliners) is between lat. 24° and 25° S, and skip- 

 jack tuna were found there in February and June. 

 If data from sightings are accepted there is evi- 

 dence of skipjack tuna between lat. 24° and 26° S 

 in various months from February to October. It 

 would not be surprising if skipjack tuna occurred 

 in those waters to some extent in all months. Mean 

 monthly surface temperatures are maximal be- 

 tween 27° and 28° C, whereas skipjack tuna can 

 tolerate 30° to 32° C (WiUiams 1970; Dizon et al. 

 1977). 



On the other hand, the northern limit of the 

 range of skipjack tuna in abundance could be 

 south of the limit of total range and could vary 

 with season, as Robins (1952) claimed. He put the 

 northern limit of the main area of occurrence at 

 about lat. 30° S in August-September and lat. 38° 

 S in February, corresponding to the positions of the 

  19° C surface isotherm in those months. Robins 

 considered that temperatures about 19° C were 

 limiting for the main occurrence of skipjack tuna, 

 18° C limiting for occurrence in abundance, and 

 20.5° C limiting for any occurrence, at the warm 

 end of the distribution along the east coast. The 



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