Biology of the southern bluespotted flathead in Victoria

The Victorian Fisheries Authority recently conducted a report on the biology of southern bluespotted flathead in Victoria. Read the full report here.

Executive Summary

As the first detailed investigation into the biology of southern bluespotted flathead (SBF) in Victoria, the present study considered age and growth, maturation, various mortality components (Total mortality – Z; natural mortality – M; and fishing mortality – F), spatial distribution, trends in abundance, the catch composition of the Port Phillip Bay recreational fleet (both size and numbers), and undertook modelling to investigate how various management interventions would influence recreational landings, and hence overall F and the population dynamics of the stock.

The results suggest that SBF are distributed throughout Port Phillip Bay, however large individuals appear to be more abundant in shallow waters, whereas SBF in deeper regions tend to be smaller, often sub-adults and juveniles. As few very large individuals were sampled, the relationship between very large, and potentially very old, SBF and particular habitats (e.g. estuarine habitats) was unable to be confirmed. Nevertheless, there is considerable evidence that this is the case with most reports of very large SBF being from Swan Bay and the Grammar School Lagoon in Port Phillip Bay, and estuaries such as Shallow and Anderson Inlets. More work is required to understand whether these individuals are particularly long-lived or whether they grow to larger sizes as a result of the environs they inhabit: both hypotheses are plausible given the biological plasticity of Platycephalids.

Similar to previous studies on SBF undertaken in Tasmania and Western Australia, SBF in Port Phillip Bay grow rapidly, with females and males maturing at 28cm and 26cm respectively, generally at less than two years of age. The oldest individual observed weighed 4.1kg and measured 81cm, with the bulk of the population being between 3 and seven years of age, and 30 to 50cm total length.

Using the observed age structure in Port Phillip Bay, Z was estimated to be 0.37 (95% CI: 0.28, 0.47), which is relatively low considering the productivity of the species. Given their biology, M was unsurprisingly estimated to be fairly high at 0.30 (95% CI: 0.22, 0.41) and the resulting estimate of F was therefore quite low at 0.07 (95% CI: 0.03, 0.11), implying that the stock is currently exploited at a relatively low level.

Based on relative abundance derived from fishery independent surveys, SBF abundance appears to have been reduced during much of the 2000s increasing between around 2010–2020 as a result of increased recruitment.  The period of low abundance in the 2000s can be linked to a period of low recruitment, quite possibly arising from the millennium drought, which also negatively affected sand flathead recruitment. Recruitment has been lower since around 2018, which, given the rapid growth rate, has already seen larger cohorts begin to reduce in abundance, but levels remain well above those observed in the 2000s. The reason behind the reduced recruitment in recent years cannot be linked to climatic conditions and the data required to further investigate causation are currently lacking. It should be noted, however, that the above trends in abundance, derived from the snapper prerecruit surveys, are only likely to accurately reflect the abundance of juvenile, sub-adult and relatively young mature cohorts, and the fact that the relatively low mortality estimates are derived from the adult component of the stock, suggests that even under a lower recruitment regime the stock is still being lightly exploited.

A variety of alternative bag and size limit scenarios were investigated to determine how the different options would impact recreational fishers’ landings, and hence F. Given that few anglers catch large numbers of SBF, the greatest reduction in F would occur if the LML was increased substantially e.g., to 35–40cm total length, especially if incorporated with a narrow upper slot-limit e.g., 50cm total length with few (e.g. 0–2), SBF able to be retained above the slot. All scenarios retaining the current LML, or large upper slot-limit sizes e.g., 60cm maximum limit, had little impact on landings.

Given the Port Phillip Bay SBF stock is currently lightly exploited, because few anglers target SBF and therefore few anglers catch large numbers of SBF, there appears to be little value in altering current management arrangements, both in terms of the sustainability of the species and potential benefits to the fishery by reducing F. Moreover, the buyout of commercial net fishing in Port Phillip Bay is expected to reduce F, with some 25,000–37,500 fewer individuals being captured by this sector as they are rarely captured and/or retained by the remaining longline commercial fishery.

Read the full report here.