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OBSERVATIONS ON DISTRIBUTION AND LIFE HISTORY OF 



SKIPJACK TUNA, KATSUWONUS PELAMIS, IN 



AUSTRALIAN WATERS 



Maurice Blackburn^ and D. L. Serventy^ 



ABSTRACT 



Skipjack tuna occur in many areas around Australia, but have been little fished or investigated there 

 because of low commercial demand. Their distribution in Australian coastal waters is not continuous, 

 although suitable temperatures occur in all areas. Abundance in coastal waters is probably highest in 

 the southeast. The southern limit of skipjack tuna range varies seasonally with the 15° C surface 

 isotherm, and that temperature appears to be limiting. Length-frequency polygons for skipjack tuna of 

 southeastern Australia show modes at about 37, 46, 53, and 59 cm fork length in the southern summer. 

 The regression of weight W (grams) upon length L (millimeters) for east coast skipjack tuna is W = 

 0.000000839 L'^^202 Gonads of coastal skipjack tuna are all immature. The euphausiid Nyctiphanes 

 australis is the principal food in east Australian waters south of latitude 34° S. Small fish such as 

 clupeoids are eaten in some of those areas and are the principal food elsewhere. 



The skipjack tuna, Katsuwonus pelamis (Lin- 

 naeus), occurs in tropical and warm temperate 

 waters of all oceans. It supports large fisheries in 

 many areas and is considered to have much poten- 

 tial for further exploitation (Gulland 1971). The 

 scientific literature on the species is large (Klawe 

 and Miyake 1967) and growing. Australian con- 

 tributions to that literature have been few, al- 

 though the organized collection of data on Austra- 

 lian skipjack tuna began in 1938. One reason is 

 that the Australian tuna industry has shown little 

 interest in skipjack tuna. It operates principally 

 upon southern bluefin tuna, Thunnus maccoyii. 



The purpose of this paper is to present unpub- 

 lished biological information on skipjack tuna in 

 Australian waters and relate it to what has been 

 published from the region. Many of the observa- 

 tions were made by us. We do not discuss fishing 

 operations or prospects, except to note briefly here 

 that skipjack tuna have been caught in Australian 

 coastal waters by live-bait fishing, purse seining, 

 mesh netting, and trolling. Most of those catches 

 were incidental to fishing for southern bluefin 

 tuna. In the year 1974-75 the Australian tuna 

 catch was 11,288 t, of which 2,375 t was skipjack 

 tuna and the rest southern bluefin tuna (Anony- 

 mous 1975, 1976). It is our opinion, admittedly 



'741 Washington Way, Friday Harbor, WA 98250. 

 227 Everett Street, Nedlands, Western Australia, 6009, Aus- 

 tralia. 



subjective, that the biomass of skipjack tuna in 

 Australian coastal waters is at least as high as 

 that of southern bluefin tuna. It was much higher 

 than the biomass of southern bluefin tuna off east- 

 ern Tasmania in 1965, according to estimates from 

 aerial surveys (Hynd and Robins 1967). 



One of us originally proposed the name "striped 

 tuna" for this species in Australia, instead of "skip- 

 jack" which is the usual English vernacular 

 elsewhere (Serventy 1941). The intention was to 

 avoid confusion with another pelagic fish which is 

 sometimes called "skipjack" in Australia, namely 

 Pomatomus saltator. "Striped tuna" has not gained 

 acceptance as an English vernacular outside Aus- 

 tralia, however, and both names are now used in 

 Australia. Therefore we now refer to Katsuwonus 

 pelamis as skipjack. 



METHODS 



Our summaries of biological data are based on 

 skipjack tuna taken from 1938 to 1965. All were 

 obtained by hook fishing at the sea surface, except 

 for some Victorian samples which may have been 

 caught near the surface in nets. Thus our biologi- 

 cal observations essentially refer to surface fish. 

 The same applies to other specimens mentioned in 

 literature cited, except those noted to be from 

 Japanese longliners. 



Length was measured from the tip of the snout 

 to the caudal fin fork (FL), sometimes in millime- 



Manuscript accepted June 1980. 



FISHERY BULLETIN: VOL. 79, NO. 1, 1981. 



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