G. M. Thomson. — Natural Hisfori/ of Ofof/o Harbour. 225 



Art. XXVII. — The Natural History of Otago Harbour and the Adjacent 

 Sea, together with a Record of the Researches carried on at the Portobello 

 Marine Fish-hatchery : Part I. 



By G. M. Thomson, F.L.S. 



\^Rend before the Ota/jo Institule, 14th November, 1912.] 

 Plate X. 



An examination of the coast-line in and near Otago Harbour shows that 

 at a comparatively recent period there has been a considerable upward 

 movement of the land-surface. It is quite evident that then Otago 

 Peninsula was an island, separated from the mainland by a somewhat 

 deep channel, and itself indented by three or four inlets, of which 

 Tomahawk Lagoon, Hoopers Inlet, and Papanui Inlet are the relics. 

 The last two are still subject to tidal influence, though nearly dry at low 

 water ; Tomahawk Lagoon is now cut o£E from the sea except at very high 

 tides. Partly by elevation of the coast-line, partly by denudation of the 

 adjacent land, and partly by blown sand, these various sheets of water 

 gradually filled up. The main channel ultimately became completely 

 blocked at its south-western end, forming the long narrow inlet now known 

 as Otago Harbour. This is a shallow sheet of water about fifteen miles 

 long, less than four miles broad at the widest point, and full of broad sand- 

 banks which are partly exposed at low tide, and are separated by some- 

 what narrow channels. It is cut into two distinct basins by the projecting 

 peninsulas of Port Chalmers and Portobello, and the two rocky islands. 

 Quarantine Island and Goat Island. These two basins are united by three 

 rock-bound passages, through which a strong tide pours backw^ards and 

 forwards, keeping deep and well-scoured channels. I am informed by Mr. 

 J. Blair Mason, Engineer to the Harbour Board, that the area of the whole 

 harbour is 12,058 acres, and that during spring tides (with a rise of 6 ft.) 

 2,626,232,400 cubic feet of water flow in and out of the harbour each tide, 

 while during neap tides (with a rise of 5 ft.) the volume is 2,100,985,920 

 cubic feet. 



The amount of extraneous nitrogenous matter which finds its way into 

 the harbour is now comparatively small, and is scarcely noticeable. Formerly, 

 when all the drainage of Dunedin went into it, the amount was consider- 

 able, though it could barely be detected on the outflowing tide at the site 

 of the Portobello Hatchery ; but since this source of contamination was 

 cut off it is a negligible quantity. A series of analyses were made for the 

 Otago Institute some eleven or twelve years ago, but, unfortunatelv, I 

 cannot lay my hands on them. 



It is difficult to say how far the operations of the Harbour Board and 

 the movements of steamers have aft'ected the marine life of the harbour — 

 probably much less than is popularly imagined. The results of tow-nettings 

 at various periods of the year and at various times both of day and night, 

 of hand-dredging in the channels, and of shore-collecting between tide- 

 marks, seem to me to show that minute life is as abundant to-day as it was 

 thirty years ago. Wherever this is the case the larger animals are sure 

 to be found to a considerable extent, for food-supply appears to be the 

 most potent factor in the distribution of organisms. 



It was from a consideration of the large volume of constantly renewed 

 sea-water passing and repassing between Quarantine Island and Porto- 

 8— Trans. 



