Southern bluespotted (‘yank’) flathead

What a targeted yank sampling and monitoring plan looks like

Background
Given reports from some stakeholders of declining southern bluespotted (‘yank’) flathead (Platycephalus speculator) abundance, and some stakeholders believing the fishery could be enhanced through changes to current management, it has been agreed that the VFA, in partnership with recreational fishers, will implement a targeted sampling and monitoring strategy. This has been deemed necessary because bluespotted flathead are underrepresented in current monitoring programs (i.e. creel and angler diary), likely due to behaviour of the species and also the fact that they appear to be rarely targeted, though anecdotal evidence suggests they are being targeted more frequently in recent years.

Need
There has been no directed research into bluespotted flathead in Victoria to date. Two studies have been undertaken on the species previously, one each in Tasmania and Western Australia. Both have found similar results with the species maturing young, growing rapidly and living to at least 12 years of age. Otherwise, there is very little information on the species and even the often-touted maximum size of 90cm and 8kg comes from an unreferenced website meaning there is no certainty as to how big the species can attain. The largest specimen from the thousands that have been measured within VFA monitoring programs has been 73cm, which is consistent with the studies undertaken in Western Australia and Tasmania. However, flathead growth rates vary markedly: for example, sand flathead rarely grow much larger than 35cm in Port Phillip Bay, but will reach >50cm in Bass Strait. It is therefore risky to rely on ageing studies undertaken in other areas, particularly given the relative ease at which local age estimates can be derived.

Proposed sampling regime
Given the current knowledge gaps and uncertainties outlined above, it is of importance that local age estimates are obtained. This is not just to determine how long they live, but to enable the biological productivity of the species to be determined locally. This has several implications for how the species interacts with the Port Phillip Bay recreational fishery, and will help answer the following questions:

1. How fast do yank flathead grow?
2. What age do they reach sexual maturity?
3. Is there evidence of ongoing recruitment?
4. What level of fishing pressure are yank flathead currently under and what level can they sustain?
5. What level of fishing pressure will enable a reasonable proportion to reach large sizes (i.e. enable anglers to have a reasonable possibility of catching a large individual and protect spawning biomass)?
6. What management arrangements would optimise the fishery? The answers to these questions will inform what our management objectives are, including what management arrangements would optimise the fishery.

Methodology
All of the above can be determined by simply ageing a representative proportion of the stock. Noting female and male growth is very different, it is necessary to age a larger number of individuals than would be the case for most other species to ensure a range of samples from both sexes Some stakeholders have indicated that there appears to be localised depletion in some areas of Port Phillip Bay. Given the lack of information available on this species it is difficult to determine whether this is the result of heavy localised fishing pressure, or some other, potentially environmental, factor. This means it would be preferable to stratify the sampling within the proposed study to investigate the age structure, and hence level of fishing pressure, in various regions of the bay. To this end, we propose the following sampling regime, including sample sizes, be obtained from three separate sampling areas of Port Phillip Bay:

This sample size will be more than sufficient to determine the biological productivity of the species in the bay, and the stratified sampling will enable the remaining objectives surrounding fishing pressure to be identified and whether these vary over these three broad regions. However, should this not be possible in all regions, at a bare minimum, it would be possible to generate useful information from a representative length frequency sample from the three regions and convert these to age composition using age-length keys formed from the available ageing data that is 3 generated elsewhere. It is unlikely that growth varies greatly within Port Phillip Bay so this should still provide a relatively reliable indication of regional fishing pressure. It is important to note that given the growth rate is sexually dimorphic, age-length and sex-length keys need to be used so there is some added uncertainty given the number of assumptions required to run through multiple models. Additionally, if VFA, or experienced research anglers, do not attribute sex directly to individuals, there is a possibility that regional variation in sex ratio could bias the outcomes. It is therefore recommended that in order to provide the most useful outcomes the preferred sampling regime is followed as closely as possible.