The decline of sand flathead stocks in Port Phillip Bay: magnitude, causes and future prospects
Recreational Fishing Grants Program Research Report
Sand flathead (Platycephalus bassensis) was once both a significant commercial fishery and the largest recreational fishery in Port Phillip Bay (PPB). Since 2000 stocks have declined substantially in PPB. The cause/s of this decline are unknown; and attempts to rebuild stocks are unlikely to be successful without identifying and addressing the cause/s as part of any management response. This report draws on a range of data sources to summarize what is known about the current status of the fishery in PPB, the probable causes of the decline and the prospects for recovery. Finally we review the adequacy of the current management settings for sand flathead in PPB as part of ongoing efforts to assist the recovery of this stock.
Sand flathead stocks in Port Phillip Bay declined by 80–90% between 2000 and 2010, but had recovered to 30% of 1990s levels, and 50% of 1980s levels, by 2012/13. There was no evidence that fishing pressure exacerbated this decline. Stock exploitation remained stable between 2000/01 and 2006/07 at 15–30% of the stock biomass, despite the decline, due to a three-fold reduction in the total fisheries catch over this period (from 338 tonnes in 2000/01 to 115 tonnes in 2006/07).
A comprehensive review of the available evidence overwhelmingly supports the conclusion that declining recruitment (i.e. the introduction of new 'baby' fish to a population) from the mid-1990s onwards led to the decline of sand flathead stocks from 2000. In comparison, there is little evidence that sand flathead growth (and hence mortality) was affected by either the introduction of the exotic seastar Asterias amurensis in the late 1990s or the drought from 1997–2009.
The decline in sand flathead recruitment coincided with a period of prolonged drought in Victoria from 1997–2009; characterised by substantially lower rainfall and river flows. Sand flathead recruitment in Port Phillip Bay from 1988–2013 was significantly correlated with Yarra River flows during November and December when the majority of sand flathead larvae occur in the water column. The relationship between flow and recruitment was positive up to 3000 ML/day, but negative for flows in excess of 3000 ML/day. This means that recruitment was lowest in years when flows were either very low or very high and highest in years with intermediate flows between 1000 and 3000 ML/day. Almost all low flow years during the drought corresponded with low recruitment.
This analysis suggests that sand flathead recruitment in Port Phillip Bay is heavily influenced by climatic conditions and this conclusion is consistent with our understanding of the forces that drive cycles of productivity for other fisheries in Port Phillip Bay (e.g. snapper and King George whiting). However, the magnitude of this decline is unprecedented since catch and effort records began in 1978, and there is little evidence of declines of a similar scale amongst commercial catches recorded since 1914. We directly link the magnitude of this decline to the prolonged, severity of the most recent drought in Victoria.
The future prospects for the recreational sand flathead fishery in Port Phillip Bay are mixed. In the short to medium term, prospects are a more positive. Sand flathead stocks have transitioned from steady decline to slow recovery. The drought is over and the future outlook for Victoria's climate in the short-term (based on the Bureau of Meteorology's POAMA climate model) is for average rainfall. This should lead to enhanced river flows, particularly in spring, and if the relationship between river flows and recruitment holds, overall better recruitment.
Over the longer term, the prospects for this fishery are less positive. This is because south-eastern Australia's future climate is expected to become drier on average as a consequence of global warming. Projected decreases in rainfall and run-off, coupled with increasing frequency and intensity of El Nino events are expected to result in higher incidence of drought in south-eastern Australia. If the relationship between river flows and recruitment holds for sand flathead in PPB, then a drier climate is likely to result in less optimal conditions for sand flathead recruitment over the longer-term and overall lower stock biomass.
This future outlook poses a number of challenges for fisheries managers over both the short and longer term. In contrast to other major fisheries in PPB, the sand flathead fishery is dominated by a single sector: the recreational fishery. The recreational fishery accounts for >95% of the total catch and this potentially simplifies the overall management of this stock in comparison to other multi-sector fisheries in PPB. In the short term the management focus should be on how best to assist recovery. Given the fishery is now showing evidence of slow recovery, we propose a monitor and review approach. This approach would be based on monitoring of commercial and recreational CPUE and sand flathead recruitment surveyed as part of the snapper pre-recruit survey operated by The Victorian Fisheries Authority. This review should be used as the basis to consider further management options.
The management options for this species are limited and comprise essentially changes to size and bag limits. Increasing the minimum size limit is likely to increase the sex bias of catches (females are larger than males and estimated to be currently caught at twice the rate of males). Although the effect of this is unknown, this is considered to be an undesirable outcome for the sustainability of the stock. Consequently, reductions to current bag limits are likely to be the only effective tool available to managers to assist the recovery of this fishery in PPB. Bag-limit scenario modelling demonstrates that significant cuts to current bag limit settings for flathead are required to have any meaningful impact on reductions to total catch. A reduction in the maximum bag-limit from its current level of 20 to 5 flathead would be required to reduce the total recreational catch by 16% and the exploitation of the stock by between 2.1–4.4%.
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