BLACKBURN AND SERVENTY: DISTRIBUTION AND LIFE HISTORY OF SKIPJACK TUNA 



data in Table 1 are insufficient to support or deny 

 Robins' conclusions as far as the actual skipjack 

 tuna distribution is concerned. We note however 

 that skipjack tuna are often abundant in other 

 parts of the world at temperatures much above 

 those mentioned (Blackburn 1965; Williams 1970). 

 They have been found plentifully at 24° C off New 

 South Wales (Williams footnote 5). Of course, 

 abundance may reflect other conditions, as well as 

 the distributions of temperature most suitable for 

 adults. The occurrence of skipjack tuna in coastal 

 waters north of Sydney, and its possible connec- 

 tions with offshore distributions to the east and 

 northeast, should be further investigated. 



Our knowledge of seasonal distribution in other 

 coastal waters of Australia is incomplete. Off 

 Western Australia skipjack tuna have been taken 

 as follows, including records by Robins (1975): 

 Broome to Port Hedland, July, August, and Oc- 

 tober; Port Hedland to Shark Bay, January, 

 April-June, August, September, and November; 

 Shark Bay to Cape Leeuwin, February, March, 

 and June-August; off Albany, May-July. Our rec- 

 ords for the area south of Timor are for September 

 and October. Australian and Soviet records be- 

 tween Albany and Kangaroo Island are all for the 

 period December-May. All the areas just men- 

 tioned and others in northern Australia where no 

 skipjack tuna have been found are warm enough 

 for some skipjack tuna to occur all year (i.e., over 

 15° C at the sea surface. Figure 1). 



In the Japanese longline fishing area east of 

 Queensland there may be some seasonal change in 

 abundance of skipjack (Matsumoto 1975), but the 

 pattern is not clear. In the similar area west of 

 Western Australia the abundance appears to be 

 low at all seasons. In surface waters of Papua New 

 Guinea, according to Lewis and Smith (1977), 

 there is no obvious seasonal change. 



LENGTH AND WEIGHT 



Length measurements of about 4,500 east coast 

 skipjack tuna were made in CSIRO investigations 

 to the end of 1965. The observed range was 30-65 

 cm FL north of Sydney, 35-66 cm FL in mainland 

 waters south of Sydney, and 35-66 cm FL off Tas- 

 mania. We have no length data for Great Aus- 

 tralian Bight or South Australian fish except 

 those of Shuntov (1969), which were 48-52 cm FL. 

 Larger skipjack tuna to about 80 cm have been 

 taken off New South Wales and South Australia in 

 recent years (Williams footnote 5). Robins (1975) 



measured about 300 skipjack tuna from Western 

 Australia, which were 29-78 cm FL. Our earlier 

 measurements from the same area fall in that 

 range. For Papua New Guinea a range of 35-62 cm 

 FL was reported by Kearney et al. (1972). All these 

 skipjack tuna were taken very near the sea sur- 

 face. Barkley et al. (1978) hypothesized that large 

 skipjack tuna require lower temperatures than 

 small skipjack tuna and are therefore more abun- 

 dant in the upper thermocline than at the sea 

 surface, in the tropical Pacific. 



Figure 2 summarizes most of the east coast data 

 in length-frequency polygons for various periods. 

 Most of these sets of measurements are rather 

 small in number, even when combined for certain 

 months and years as in some of the polygons. Data 

 for the Southern Hemisphere winter (polygons A, 

 H, and M) show modes at about 34, 44, and 51 cm. 

 Polygons for the southern summer (C, E, F, G, and 

 J) have modes at about 37, 46, 53, and 59 cm. The 

 first three modes for the summer are close to the 

 three for the winter and are shown as I, II, and III, 

 respectively, for each season. Modes at similar 

 sizes in other polygons are labelled in the same 

 way. This labelling does not imply that the modes 

 represent successive age-groups a year apart, or 

 that the absolute age is knov^n for any mode, be- 

 cause such conclusions could not be drawn with 

 confidence from these scattered data. If the modes 

 do represent successive age-groups, the mean 

 grov^h rate of east coast skipjack tuna must be 

 about 6-10 cm/yr for fish between 35 and 60 cm. 

 Most published estimates of skipjack tuna growth 

 rate in that range of length are higher, as dis- 

 cussed by Shomura (1966), Joseph and Calkins 

 (1969), and Chi and Yang (1973) for Hawaii, Japan, 

 the eastern Pacific, and Taiwan. The range of an- 

 nual growth increment in those studies was 11-27 

 cm, with many values near 15 cm, and some of 

 those estimates were obtained from tagging. On 

 the other hand, Batts (1972) estimated 8-9 cm/yr 

 for skipjack tuna >40 cm from North Carolina, 

 from annuli in cross sections of dorsal spines. 

 Kearney^ referred to an estimate of 7 cm/yr for 

 Papua New Guinea skipjack tuna, based on tag- 

 ging, but gave no details. 



Skipjack tuna of modal group I were obtained 

 only from 1938 to 1941 in east coast waters. They 

 may have been particularly abundant then, or 

 there may have been some difference in trolling 



^Kearney, R. E. 1978. Some hypotheses on skipjack {Kat- 

 suwonus pelamis) in the Pacific Ocean. South Pac. Comm., 

 Noumea, New Caledonia, Occas. Pap. 7, 23 p. 



89 



