Fisher survey of muscle melanisation in Sand Flathead (Platycephalus bassensis)

Flathead are an important component of Tasmania’s recreational and commercial fisheries, with flathead accounting for more than 60% of the total recreational finfish catch by numbers taken. While several species of flathead are taken in Tasmanian waters, Sand Flathead (Platycephalus bassensis) alone represents more than 90% all flathead caught by recreational fishers.

Reports of black pigmentation in the usually white fillets of Sand Flathead have been common for many years and is due to a phenomenon known as melanisation. Most commonly melanisation occurs when the naturally occurring pigment melanin becomes concentrated in small areas of the body.

In an effort to connect the practical knowledge of the Tasmanian recreational fishing community with the current scientific understanding of muscle melanisation in Sand Flathead, the Institute for Marine and Antarctic Studies conducted an on-line questionnairebased survey of recreational fishers during 2018. A total of 437 completed surveys were received, of which 95% indicated that respondents had observed areas of blackened muscle in their catches, with the vast majority (99%) having observed the phenomenon in Sand Flathead. A small number of respondents (8%) also reported having observed melanisation in other Tasmanian fish species, including Tiger Flathead, Jackass Morwong, and Bastard Trumpeter, although this has yet to be confirmed.

Based on locations where fishers had observed muscle melanisation in Sand Flathead, a total of 225 unique locations across Tasmania were identified, suggesting that the phenomenon is widespread. However, 37 locations attracted multiple reports and therefore may represent melanisation “hotspots”. These locations confirm observations that the phenomenon is particularly prevalent within the Tamar River and south-eastern Tasmania, including the Derwent Estuary, D’Entrecasteaux Channel, Norfolk-Frederick Henry Bays and the Tasman Peninsula. Furthermore, four locations of specific interest were identified as having a particularly high proportion of affected Sand Flathead, with “extensive” (large) areas of melanisation in some fillets and/or an apparent increase in the incidence of melanisation in recent years. These locations included: Bell Bay (Tamar), Carnarvon Bay (Tasman Peninsula), Simpsons Bay (D’Entrecasteaux Channel) and Southport.

Although the cause and origin of muscle melanisation in Sand Flathead remains elusive at this stage, a variety of avenues for future research have been established, including continued investigation into heavy metals, type of melanin present, site specificity and environmental histories and conditions in melanisation hotspots.