BLACKBURN AND SERVENTY: DISTRIBUTION AND LIFE HISTORY OF SKIPJACK TUNA 



mania, where skipjack tuna occur only in summer 

 and autumn, N. australis is probably the only 

 abundant food organism available to them. Pil- 

 chards are not common in that area (Blackburn 

 1950b). Mackerel, Scomber australasicus, have 

 been recorded from Tasmania, but were not found 

 there by us and are probably rare. Anchovies are 

 common in Tasmania, but their occurrence in 

 summer and autumn is mostly in inlets which 

 skipjack tuna do not enter (Blackburn 1950a). 

 Jack mackerel, Trachurus declivis, are also abun- 

 dant, but mostly occur closer inshore than skip- 

 jack tuna (Hynd and Robins 1967). 



DISCUSSION 



The need for further information of various 

 kinds on Australian skipjack tuna has been 

 shown. It would be particularly interesting to 

 know if the apparent discontinuities in distribu- 

 tion are real, and if so, what causes them. The 

 probable determinants of skipjack tuna distribu- 

 tion in surface waters are temperature, food, and 

 turbidity (Blackburn 1965, 1969). Dissolved oxy- 

 gen concentration can be an additional limiting 

 environmental property in the vertical plane, 

 since skipjack tuna are stressed at concentrations 

 below about 2.8 ml/1 (Dizon 1977; Sharp 1978). 

 Concentrations at 100 m in waters near Australia 

 are higher than 3.0 ml/1, however, except in an 

 area off the west coast of West Irian (Reid et al. 

 1978). Temperatures required by skipjack tuna 

 larvae may be higher than those preferred by 

 adults, causing spawning adults to seek warmer 

 waters than those not spawning (Blackburn and 

 Williams 1975). It has been shown that all surface 

 waters around Australia are warm enough (>15° 

 C) for adult skipjack tuna in the warm season, and 

 most waters warm enough at all seasons. The ab- 

 sence or rarity of skipjack tuna in some of those 

 areas probably indicates that suitable food or- 

 ganisms are scarce, that the waters are too turbid 

 for the fish to find food, or that the vertical dis- 

 tribution of temperature is not such as to force the 

 fish to the surface. 



ACKNOWLEDGMENTS 



Many valued colleagues helped with the field 

 work of this study They included J. G. Clark, D. 

 Connolly, R. J. Downie, A. Flett, S. Fowler, and G. 

 E Whitley, all now deceased. We are grateful to 

 others for personal communications mentioned in 



the text. Kevin Williams provided some of the rec- 

 ords in Table 1. Ian Munro was helpful with 

 nomenclature and records of some food organisms. 

 The regression of weight on length was calculated 

 by Dennis Reid. We received useful comments from 

 J. S. Hynd, G. I. Murphy, K. F Williams, and 

 anonymous reviewers. The first author acknowl- 

 edges support received from CSIRO as a visting 

 scientist in the Division of Fisheries and Oceanog- 

 raphy, which facilitated this work. 



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