MASS MORTALITY OF CAPTIVE SOUTHERN BLUEFIN TUNA (THUNNUS-MACCOYII) INAPRIL MAY 1996 IN BOSTON-BAY, SOUTH AUSTRALIA - A COMPLEX DIAGNOSTIC PROBLEM/

In April 1996 approximately 75% (1,700 t) of the captive, southern blu efin tuna Thunnus maccoyii in Boston Bay, South Australia died. Most d eaths occurred during the two days, 15 and 16 April. There was a time association with an ocean surge (11-14 April) and strong winds (12-20 knots on 12 and 13 April). Unfortunately, no environmental data such a s levels of dissolved oxygen, suspended solids and toxic algae were av ailable for the critical period 11-17 April inclusive and even some of that collected subsequently was not ideal. Clinical signs reported in the affected fish were that they were obviously distressed, swimming in a haphazard fashion at the surface and, in some instances, exhibiti ng gasping behaviour. On 15 and 16 April dead fish were reported to ha ve large quantities of mucus flowing from their gills. This was not al ways the case with fish dying later in the episode. Most material avai lable for histopathological examination was autolysed to some extent, but a small number of suitable samples revealed swelling of the respir atory epithelial cells and subepithelial oedema of the secondary lamel lae. Possible aetiological factors responsible for the episode, either alone of in concert, were considered to be microalgal toxicosis, hypo xia, smothering by suspended solids and hydrogen sulphide toxicity. Th ese are discussed in relation to the oceanography of Boston Bay and th e unique anatomical and physiological characteristics of tuna.