Science meets fishing
Sustainability: how your snapper stocks rate MELBOURNE
Ross Winstanley
During 2019, teams of fisheries scientists around Australia conducted the fifth in a series of assessments of major Australian fish stocks. Among the 148 species covered was snapper (Pagrus auratus), examined as 12 separate ‘stocks’, distinguished either by genetic or geographical separation, or by the jurisdiction primarily responsible for fisheries management. These assessments examined whether each snapper stock, and the level of harvest from each stock, is sustainable. Six snapper stocks and fisheries were assessed to be “Sustainable”, four as “Depleted”, one as “Recovering” and one as “Undefined”. EAST COAST STOCK Recent genetic studies have distinguished snapper occurring from the central Queensland coast to Eden belonging to a single stock, separate from those occurring further south. Designated biologically as the East Coast Stock, for practical purposes this stock is assessed as separate ‘management units’ under the Queensland and NSW state jurisdictions. Queensland ‘stock’ Combined, the commercial and recreational (including charter) catches from the East Coast stock off Queensland total about one third of the catches occurring off NSW. Despite this, the part of the snapper stock occurring off Queensland is classified as “depleted”. This means that this part of the East Coast stock has been reduced to the point where recruitment or spawning success is “impaired”, that is the most recent management controls have not yet been shown to be “adequate to recover the stock”.
With highly variable annual spawning success underpinning sustainability, all states limit catches of large mature snapper. The 2018 assessment indicated that the spawning stock is less than 20% of that expected from an unfished stock, with declining numbers of large fish over the previous 12-year period. More recent studies show that, while commercial fishing p res s u re d e c lin ed over the past decade, the recreational catch increased by 49% by number and 72% by weight between 2013/14 and 2019/20, despite declining catch rates. Since September 2019, snapper may only be taken by line fishing, and several new control measures now apply: a one-month closure, a commercial Total Allowable Catch of 42 tonnes, and a recreational possession limit of 4/person. The snapper off Queensland are “considered to be recruitment impaired” and there is no evidence that the stock is recovering. New South Wales ‘stock’ Most of the catch from the East Coast Stock is taken off NSW where this part of the stock has been classified as “sustainable”. This means that management measures
currently applying off NSW are sufficient to prevent overfishing and to allow, on average, s pa w ning s uc c e s s adequate to maintain the current stock size. Assessments based largely on commercial fisheries data indicate that this stock is “unlikely to be depleted” and that recruitment is unlikely to be impaired. In fact, from about 2008, the sizes and ages of commercially landed snapper suggest that the stock is rebuilding. EASTERN VICTORIA STOCK Snapper occurring between Wi l s o n s Promontory and Eden are recognised as genetically distinct from the East Coast Stock. While they are genetically closer to snapper west of Wilsons Promontory, tagging and other biological studies indicate separation from the Western Victorian stock. The lack of adequate commercial and recreational fisheries data continues to hinder assessment of the state of this stock, which is ranked as “undefined”. The lack of fisheries and biological data also continues to limit our understanding of the
relationship between snapper off eastern Victoria and those occurring off Tasmania. WESTERN VICTORIAN STOCK The Western Victorian snapper stock is classified as “sustainable”. This single biological stock of snapper occurs west of Wilsons Promontory, including Western Port and Port Phillip bays, and extends westwards towards the Murray River mouth in South Australia. As a result, for convenience, they are managed as separate management units by Victoria and South Australia, under collaborative arrangements. This western stock relies heavily on recruitment from Port Phillip Bay, the main spawning and nursery area. With annual spawning success being highly variable, the stock – and catches – can be dominated by strong year-classes occurring at intervals of 5-8 years. After the large stock of adult snapper spawned in 2004 and 2005 had been depleted through fishing and natural mortality by the mid-2010s, the exceptional 2018 recruitment event
is expected to rebuild the stock of adult fish and to boost the fishery for the next 5-10 years, peaking in 2024. With commercial catches from Port Phillip Bay capped at 88 tonnes from 2022, limited commercial fishing in Victorian coastal waters, and SA’s commercial and recreational catches capped, the main openended source of fishing pressure will come from Victoria’s recreational fishers. There is no recent estimate of the recreational catch in Victoria. SOUTH AUSTRALIAN STOCKS As well as the extension of the Victorian Western stock, there are two stocks wholly located in South Australian waters: the Gulf St Vincent stock and the Spencer Gulf/West Coast stock. Gulf St Vincent stock The snapper stock in Gulf St Vincent is classified as “depleted”. This means that the stock has been reduced to the point where recruitment or spawning success is “impaired”, that is the most recent management controls have not yet been shown “adequate to recover the stock” through improved spawning success. Historically, the northern region of Spencer Gulf supported the bulk of this stock, and was the nursery area for the stock throughout the gulf. SA’s highest ever snapper catches came from this gulf during the period from 2006 to 2015. The 2020 assessment found that this stock had deteriorated since 2015, despite years of increasingly tight restrictions which were intended to reduce commercial fishing pressure and boost recruitment. This included the closure of all snapper fishing in the gulf, in November 2019. Government surveys indicated that, between 2014 and 2020, the
Fig.1. Genetic ‘markers’ for individual East Coast (red) and Eastern Victorian (green) snapper. It shows mixedstock features near Eden, and the close similarity between Eastern and Western Victorian stocks. 40
SEPTEMBER 2021
spawning stock declined from 2780 to 811 tonnes. In addition, modelling showed that the “fishable biomass” or stock had increased through the 1990s, peaked in 2011, then declined by 90% by 2020. Poor recruitment while catches remained high, culminated in estimated egg production falling to 2% of that of an unfished stock. Despite the assessment as “depleted”, there is evidence that, under the current controls, this stock can be expected to recover. Spencer Gulf/West Coast stock Like the Gulf St Vincent stock, based on a substantial body of information and research, the Spencer Gulf/West Coast snapper stock is also classified as “depleted”. While the take of snapper from these waters was closed in November 2019, and the removal of fishing pressure should favour stock recovery, no “measurable improvements” have yet been detected. Like the Gulf St Vincent situation, the norther Spencer Gulf is the main recruitment and nursery area for this stock. Commercial fisheries data show that, after 20 years of cyclical fluctuations, the stock has declined in size, particularly since 2012, with all indicators reaching their lowest levels by 2019. The lack of any strong year classes since 1999 reflects 20 years of poor recruitment. Modelling suggests that the available stock declined from 5350 tonnes in 2005, to 468 tonnes in 2020. Through much of the 2000s, modelling suggests that continued high catch levels from this dwindling stock occurred while snapper egg production fell. In 2019, estimated egg production was 2% of that expected from an unfished stock. WESTERN AUSTRALIAN STOCKS Snapper are divided
