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Silver Trevally inhabits estuarine and coastal waters throughout southern temperate Australia. Of the seven separate Australian stocks, five (in WA, SA, VIC, TAS and the Commonwealth) are sustainable. The NSW stock is depleted and the QLD stock is undefined.  

Silver Trevallies comprises a complex of species that inhabits estuarine and coastal waters (depths of 10–230 m) throughout southern temperate Australia, from southern Queensland, south through New South Wales, Tasmania, Victoria, South Australia and southern and central Western Australia [Smith-Vaniz and Jelks 2006, Bearham et al. 2020].

The biological stock structure of Silver Trevallies is uncertain. Fisheries are based on a species complex that varies by region, with Pseudocaranx georgianus present in all jurisdictions except Queensland, Pseudocaranx wrighti present in all jurisdictions except Queensland and New South Wales, Pseudocaranx dinjerra only present in Western Australia, and Pseudocaranx sp. ‘dentex’ only present in Queensland [Smith-Vaniz and Jelks 2006, Gomon et al. 2008, Bearham et al. 2020]. Investigations of population connectivity and post-settlement movement are also limited. Despite fast swimming ability, tag-recapture studies in Western Australia, New South Wales and New Zealand indicate restricted post-settlement movement of P. georgianus, potentially leading to ecological stock structuring over moderate (hundreds of kilometres) spatial scales [James 1980, Fairclough et al. 2011, Fowler et al. 2018].

Here, assessment of stock status is presented at the jurisdictional level—Commonwealth, Western Australia, Queensland, New South Wales, Victoria, Tasmania and South Australia.

New South Wales

Morphological and genetic evidence indicates that the Silver Trevallies stock in New South Wales is single-species, consisting only of Pseudocaranx georgianus [Smith-Vaniz and Jelks 2006, Bearham et al. 2020]. The stock supported historical commercial catches in excess of 1 000 t per year during the 1980s, but the commercial catch has declined steadily since that time to 41.9 t in 2019; the lowest level since the 1980s peak. Interpreting this decline is complicated by changes in the historical reporting of catch between the state and Commonwealth jurisdictions, as well as management changes within New South Wales that have affected the spatial distribution of effort and fishery reporting through time. Within the state, reduction in the area available to commercial fisheries for Silver Trevallies, through the implementation of recreational fishing havens and marine parks (particularly the Batemans Marine Park), has likely reduced catch and potentially influenced catch rates, thereby creating difficulties in defining useful reference points to assess current stock status. A minimum legal length (MLL) of 300 mm total length (TL) was also introduced in late 2007, further impacting the quantity of landed catch and potentially confounding the interpretation of trends in fishery-dependent indicators through time.

Trends in standardised catch rates (kg per day, hereafter ‘catch rates’) in New South Wales vary, depending on the area, fishing method, and time-period examined [Fowler and Chick 2019]. Catch rates including all areas declined during 1998–2009 for both major fishing methods - fish trawling and fish trapping. Catch rates during the most recent period (2010–2019) either declined or remained steady, depending on fishing method and area examined. Catch rates during 2019 were among the lowest observed since the start of time-series in 1998. 

Two data-limited assessment models were applied to historical trawl catch data from New South Wales during 1955–2019 - a boosted regression tree (BRT) model [Zhou et al. 2017a] and an Optimised Catch-Only Model [OCOM, Zhou et al. 2017b, reported in Fowler and Chick 2019]. Estimates of biomass relative to biomass at maximum sustainable yield (B/B_msy) from the BRT modelling decreased throughout the period, with a substantial decline in 2016 to their lowest levels in 2019 (< 0.1). A value below 1 indicates biomass is insufficient to achieve maximum sustainable yield. However, estimates were highly uncertain throughout the period. B/B_msy estimates from OCOM remained stable and above one between 1955 and the mid-1970s, then decreased substantially during the 1980s and 1990s, remaining low (<0.5) and stable from 2000 to 2019. Estimates of fishing mortality relative to fishing mortality at maximum sustainable yield (F/F_msy) from OCOM remained below one until the early 1980s, after which they increased substantially and remained above one until the late 2010s. A value above 1 indicates fishing mortality is too high to achieve maximum sustainable yield. Trends in B/B_msy and F/F_msy from OCOM were similar across a range of natural mortality values examined (0.05-0.15).

Observer studies and monitoring of landed catches in New South Wales have shown that the length of Silver Trevallies captured by the Ocean Trawl Fishery (OTF) declined substantially between the periods 1987–90, 1993–95 and 1997–99 [Liggins 1996, Rowling and Raines 2000]. The proportion of larger-sized Silver Trevallies landed has continued to decline since 2007, when the MLL was introduced [Stewart et al. 2015].

A Length Based – Spawning Potential Ratio (LB-SPR) model was applied to commercial length data sampled from fish markets in New South Wales during 12 years between 2004 and 2018 [Fowler and Chick 2019]. SPR represents an estimate of current spawning potential of the stock relative to that in an unfished population. Estimates of fishing mortality relative to natural mortality (F/M) from the model were highly variable throughout the period (range: 1.6 to 7.4), with values in the last three years ranging between 3.0 and 5.4. Estimated SPR was consistently low and stable across years, ranging between 0.14 and 0.26. 

The only age-based assessment of the Silver Trevallies stock indicated that total mortality increased substantially between 1987–90 and 1997–99 [Rowling and Raines 2000]. This analysis estimated that fishing mortality was greater than natural mortality by the 1997–99 period and that the fishery exhibited age class truncation. Given the ongoing length truncation observed in the fishery, it is likely that the total mortality rate and degree of age class truncation have persisted. Due to the MLL in New South Wales waters, discarding in the OTF is substantial and may exceed 50 per cent at times, based on number of individuals [NSW DPI, unpublished data]. Discard mortality in the OTF therefore remains a concern to the status of the stock. Some protection to the Silver Trevallies stock is afforded by marine parks in eastern Australia, but total fishing mortality is still likely higher than natural mortality. 

While acknowledging difficulties in interpreting the change, estimates of retained landings by resident recreational fishers in New South Wales have decreased. The most recent estimate of the recreational harvest of Silver Trevallies in NSW was approximately 15 300 fish, with an estimated weight of approximately 8 t during 2017–18 (Murphy et al. 2020). Estimates were based on a survey of Recreational Fishing Licence (RFL) Households. RFL households were comprised of at least one member who possessed a long-term (1 and 3 years duration) fishing licence and included other fishers resident within their households. A similar survey of RFL households was done in 2013–14 and provides a comparison with data from the 2017–18 survey. The estimated catch of Silver Trevally in 2013–14 was approximately 49 000 fish weighing approximately 27 t (West et al. 2015).

The lack of a suitable reference period from which to evaluate changes in biomass (or a proxy) creates uncertainty around stock status and will continue to hinder detection of biomass reductions to levels that might impair recruitment in New South Wales. Uncertainty about stock status is further driven by limited understanding of stock structure, with at least some component of the New South Wales stock shared with the adjacent Commonwealth jurisdiction, which has classified the resource as sustainable. However, the substantial declines in commercial and recreational catch and CPUE in New South Wales since the late-1990s, combined with model estimates indicating biomass is at or below 20 per cent unfished biomass, and long-term truncation of length structure indicate that biomass is likely depleted and recruitment is likely impaired. The above evidence indicates that current fishing mortality levels are expected to prevent the stock recovering from a recruitment impaired state. 

On the basis of the evidence provided above, Silver Trevallies in New South Wales is classified as a depleted stock.

Silver Trevallies biology [Rowling and Raines 2000]

Distribution of reported commercial catch of Silver Trevallies. No catch distribution data are provided for Queensland as Silver Trevallies are not distinguished from other trevally species in relevant Queensland commercial fishery logbooks.

Commonwealth – Commercial (Management Methods/Catch) Data provided for the Commonwealth align with the Commonwealth Southern and Eastern Scalefish and Shark Fishery for the 2018-19 financial year.

Commonwealth – Recreational The Commonwealth does not manage recreational fishing in Commonwealth waters. Recreational fishing in Commonwealth waters is managed by the state or territory immediately adjacent to those waters, under its management regulations.  

Commonwealth – Indigenous The Australian government does not manage non-commercial Indigenous fishing in Commonwealth waters, with the exception of Torres Strait. In general, non-commercial Indigenous fishing in Commonwealth waters is managed by the state or territory immediately adjacent to those waters.

Western Australia – Recreational (management methods) In Western Australia, a licence is required to recreationally fish from a powered vessel.

Western Australia – Recreational (Catch) Shore based catches are unknown, thus landings would be underestimated.

Queensland – Indigenous (management methods) for more information see https://www.daf.qld.gov.au/business-priorities/fisheries/traditional-fishing

New South Wales – Recreational (Catch) Murphy et al. [2020].

New South Wales – Indigenous https://www.dpi.nsw.gov.au/fishing/aboriginal-fishing

Victoria – Commercial (catch) Silver trevally (Pseudocaranx georgianus) is not differentiated from other trevallies caught in Victorian commercial fisheries.

Victoria – Indigenous A person who identifies as Aboriginal or Torres Strait Islander is exempt from the need to obtain a Victorian recreational fishing licence, provided they comply with all other rules that apply to recreational fishers, including rules on equipment, catch limits, size limits and restricted areas. Traditional (non-commercial) fishing activities that are carried out by members of a traditional owner group entity under an agreement pursuant to Victoria’s Traditional Owner Settlement Act 2010 are also exempt from the need to hold a recreational fishing licence, subject to any conditions outlined in the agreement. Native title holders are also exempt from the need to obtain a recreational fishing licence under the provisions of the Commonwealth’s Native Title Act 1993.

Tasmania – Indigenous (management methods) In Tasmania, Indigenous persons engaged in traditional fishing activities in marine waters are exempt from holding recreational fishing licences, but must comply with all other fisheries rules as if they were licensed. If using pots, rings, set lines or gillnets, Indigenous fishers must obtain a unique identifying code (UIC). The policy document "Recognition of Aboriginal Fishing Activities” details application procedures for issuing a UIC.

Commercial catch of Silver Trevallies - note confidential catch not shown

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