Mass mortality of captive southern bluefin tuna (Thunnus maccoyii) in April/May 1996 in Boston Bay, South Australia: A complex diagnostic problem

In April 1996 approximately 75% (1,700 t) of the captive, southern bluefin tuna Thunnus maccoyii in Boston Bay, South Australia died. Most deaths 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 as levels of dissolved oxygen, suspended solids and toxic algae were available 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, exhibiting gasping behaviour. On 15 and 16 April dead fish were reported to have large quantities of mucus flowing from their gills. This was not always the case with fish dying later in the episode. Most material available for histopathological examination was autolysed to some extent, but a small number of suitable samples revealed swelling of the respiratory epithelial cells and subepithelial oedema of the secondary lamellae. Possible aetiological factors responsible for the episode, either alone of in concert, were considered to be microalgal toxicosis, hypoxia, smothering by suspended solids and hydrogen sulphide toxicity. These are discussed in relation to the oceanography of Boston Bay and the unique anatomical and physiological characteristics of tuna.