FISHERY BULLETIN: VOL. 79, NO. 1 



Table 2.— Length range, sex ratio, and female gonad indices 

 (see text) for three groups of Australian east coast skipjack 

 tuna. 



offshore tropical waters. Ueyanagi (1970) showed 

 that skipjack tuna larvae occur between 

 November and February in the Coral Sea east of 

 tropical Queensland, north and east of New 

 Guinea, and in ocean waters west of northwest 

 Australia. From May to August the larvae are 

 scarcer in Coral Sea and New Guinea waters than 

 in the preceding period, and possibly so off north- 

 west Australia. Gonad indices are higher in Coral 

 Sea and New Guinea waters in the southern sum- 

 mer than in winter (Naganuma 1979). Thus the 

 skipjack tuna spawning season in waters near 

 Australia is probably the southern summer, and 

 the principal spawning areas seem to be in offshore 

 waters northeast and northwest of the continent. 



FOOD 



Observations were made on stomach contents of 

 660 skipjack tuna from east coast waters and 30 

 from west and northwest coast waters (Table 3). 

 Euphausiids were mostly Nyctiphanes australis 

 although Thysanoessa gregaria was occasionally 

 observed. Also included with euphausiids were 

 several stomachs which contained a red liquid. 

 This liquid was often found together with 

 euphausiids, never with any other food, and was 

 certainly a product of the digestion of euphausiids. 



The main point of interest in Table 3 is the 

 proportion of stomachs with euphausiids. Evi- 

 dently euphausiids are almost the sole food of skip- 

 jack tuna in Tasmania and the principal food in 

 southern New South Wales, but a small component 

 of diet in the other sampled areas. Small pelagic 

 fish are a large food item in all areas except Tas- 

 mania. Cephalopods are a minor item in all areas. 

 Table 3 does not include data on east coast 

 skipjack tuna from Robins (1952) because they are 

 non quantitative, but his findings were similar, as 

 follows. Euphausiids were the principal food in Tks- 

 mania and New South Wales waters south of Syd- 

 ney. North of Sydney the principal food was fish, 

 especially the young of pilchard, Sardinops 

 neopilchardus , and anchovy, Engraulis australis. 

 Park and Williams^ found the following in 

 stomachs of skipjack tuna taken near Sydney: fish 

 larvae, mainly pilchard; A^. australis; brachyuran 

 and decapod larvae; copepods; and squid. 



These changes in diet by area appear to reflect 

 the kinds of small nekton and large zooplankton 

 that are available to skipjack tuna in coastal 

 waters. Nyctiphanes australis is the principal 

 coastal euphausiid in the southeast Australian 

 region. Its range along the east coast is from lat. 

 31° S to the southern end of Tasmania (Sheard 

 1953). It is abundant off Victoria and Tasmania 

 (including all of Bass Strait) and also off southern 

 New South Wales, but not common in waters north 

 of Sydney (Blackburn 1980). The species is un- 

 recorded off Western Australia, although it occurs 

 in South Australian waters. Off eastern Tas- 



'Park, J. S., and K. Williams. 1977. A study of the relation- 

 ship between the composition of food organisms of skipjack tuna 

 Katsuwonus pelamis and the abundance and species composition 

 of the plankton in the waters adjacent to Cronulla, New South 

 Wales, Australia. Unpubl. manuscr Commonwealth Scien- 

 tific and Industrial Research Organization, Division of Fisheries 

 and Oceanography, Cronulla, 2230, Aust. 



Table 3.— Foods of skipjack tuna collected in Australian coastal waters, by numbers of stomachs in which they occurred. Nil means 



empty stomachs. 



^Sardinops neopilchardus . 



'Scomber australasicus . 



^Yellow liquid, except for two stomachs from Tasmania which contained salps. 



'Including three stomachs which also contained hyperiid amphipods. 



^Bellows fish (Macrorhamphosidae). 



^Euphausiids plus fish or squid. Fish included Scomberesox forsteri , Trachums sp. and Macrorhamphosidae. 



'Euphausiids plus squid. 



'Flying fish (Exocoetidae), juvenile Gonorhynchus greyi, Harengula sp. and anchovy (probably Engraulis australis). 



'Fish plus crustaceans or cephalopods or pteropods. Fish included Myctophidae. Cephalopods included squid and paper nautilus. 



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