BLACKBURN AND SERVENTY: DISTRIBUTION AND LIFE HISTORY OF SKIPJACK TUNA 

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Figure l. — Australia and vicinity, showing areas of skipjack investigations and fisheries, edge of continental shelf (dotted line), 

 mean February and August positions of the 15° C surface isotherm, and the following localities mentioned in the text: 1 — Lady Elliott 

 Island, 2 — Sydney, 3 — Lakes Entrance, 4 — Fumeaux Group, 5 — Tamar River, 6 — Storm Bay, 7 — Portland, 8 — Kangaroo Island, 

 9 — Albany, 10 — Cape Leeuwin, 11 — Fremantle, 12 — Shark Bay, 13 — Onslow, 14 — Port Hedland, 15 — Broome. 



ganism Nyctiphanes australis, mentioned later, is 

 abundant. Skipjack tuna can tolerate tempera- 

 tures down to 15° C as shown below, and all waters 

 in the area of no skipjack records have mean sur- 

 face temperatures over 15° C in some months in 

 most years. Aerial sightings of presumed skipjack 

 tuna have been made near Portland and off west- 

 ern Tasmania (Williams footnote 5). Nevertheless, 

 we think the gap is real, at least in central and 

 western Bass Strait. We and our colleagues have 

 done much trolling in those areas in various sea- 

 sons and years without catching any skipjack 

 tuna. Southern bluefin tuna are likewise absent or 

 very rare in central and western Bass Strait, al- 



though quite plentiful east and west of that region 

 (Serventy 1956). Hynd and Robins (1967) showed 

 that surface temperatures in the western ap- 

 proaches to Bass Strait are under 15° C in parts of 

 the summer because of upwelling, and perhaps as 

 cold as that all summer in some years. This might 

 restrict the distribution of skipjack tuna in waters 

 east of Kangaroo Island. Hynd and Robins (1967) 

 discussed the possibility of a similar effect upon 

 southern bluefin tuna. Another possibility is that 

 Bass Strait is too turbid for skipjack tuna, since it 

 is shallow, receives several rivers, and is well 

 mixed by waves and tides. Very little is known 

 about effects of turbidity on tunas, however. 



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