Have you ever seen black areas in fish fillet (inside the fish)? 
This is called melanisation.

This suggests that factors other than
pollution may be involved.

We do not know if other fish species are also affected, what the geographical extent of the melanisation is, what causes this condition and what factors increase the risks. 

Where muscle melanisation has been reported in wild fish in the Northern Hemisphere it has been caused either by parasites or presence of copper. In contrast, preliminary results suggested that neither parasites nor copper are involved in melanisation of Tasmanian flathead muscle.

A recent survey of melanisation in marine fish in Tasmania attracted almost 450 respondents and established that the vast majority of the fishers had observed melanisation in at least some of the sand flathead they had caught. The survey showed that while the occurrence of melanised flathead was widespread around the Tasmanian coast, the phenomenon was most common in flathead from Tamar River in the north and the Derwent Estuary, D’Entrecasteaux Channel, Norfolk-Frederick Henry Bays and the Tasman Peninsula in the south of the state.

Recently there have been concerns regarding the health of fish in the Tamar estuary, particularly in relation to the consumption of seafood. Over the past few years, “blackening of muscle” has appeared in sand flathead, the most-fished recreational species in northern Tasmania 

If you've ever seen this in Sand Flathead or any other fish that you have caught around Tasmania