Sustainability of silver trevally

Sustainability of popular silver trevally stocks

MELBOURNE Ross Winstanley

During 2019, teams of fisheries scientists around the country conducted the fifth in a series of assessments of major Australian fish stocks. Among those covered in the Status of Australian Fish Stocks 2020 report were species taken by commercial and recreational fishers in two or more states. These included assessments of popular coastal and offshore trevally species: • Pseudocaranx georgianus, silver trevally • Pseudocaranx wrighti, skipjack trevally • Pseudocaranx dinjerra • Pseudocaranx dentex the main species taken commercially in most jurisdictions. Other species tend to be more common off certain states: P.wrighti occurs widely, from Victoria to WA, and P.dingerra occurs only off WA.

The SAFS report records P.dentex as “only present in Queensland”, however, other sources, such as Fishbase record this species as white trevally, occurring in subtropical and temperate waters in both hemispheres. In Australia, it occurs around to WA’s North West Cape.

Around Australia, these ‘silver trevallies’ are known as skipjack or skippies, silver bream, white or blue trevally, among other common names.

While one or two species predominate in catches,

Fig.1 illustrates the parallel record of catches from the two commercial fisheries over just part of their downward trajectories. Records show that combined catches rose from a low level in 1960, to average 1300 tonnes in the late 1980s. Note that, adding to fishing’s impact on the stocks, silver trevally are subject to high rates of post-capture discarding and release, owing to commercial marketing preferences and prescribed minimum size limits.

New South Wales

With evidence of a single P.georgianus species, the stock supporting NSW’s commercial fisheries once yielded annual catches of above 1000 tonnes. But since the 1980s, catches have of large fish in catches emphasise the impact of sustained fishing pressure – off NSW and elsewhere – in depleting the trevally stock.

Recreational catches of silver trevally have paralleled the declines measured in the commercial fisheries. Estimates have fallen from 87 tonnes in 2000/01, to 27 tonnes in 2013/14, and eight tonnes in 2017/18.

There is no good news in the SAFS report, which states that current levels of fishing pressure “are expected to prevent the stock recovering from a recruitment impaired state”. Worse still, in 2020 the NSW TAC Committee declared that the silver trevally stock was “Depleted below a biologically safe level”.

Based on substantial evidence and assessments, the silver trevally in NSW waters is classified as a “depleted stock”.

Commonwealth fishery

During the mid to late 1980s, the Commonwealth trawl and gill net fisheries annually landed 1200 to 1600 tonnes of ‘trevally’ annually, assumed to be silver trevally P.georgianus. In the decade to 2001, commercial catch rates suffered a “rapid decline”, approaching the level representing the stock at 20% of its original unfished level or biomass. Since then it has fluctuated close to that level.

Since 2013, catches have not exceeded 15% of the annual Total Allowable Catches set for the fishery. After recent TACs were gradually reduced, reaching 292 tonnes for 2019/20, the catch for that year was just 21 tonnes.

With all the above in mind, recreational fishers may be surprised that, under the applicable criteria, factors and terminology, the SAFS report found that “the stock is unlikely to be depleted”. It found that the available evidence points to classification of silver trevally in the Commonwealthmanaged fisheries as a “sustainable stock”. Queensland

Due largely to species identification issues, neither recreational nor commercial

There is a complex array of silver trevallies around Australia.

Photo courtesy of Gary Bell, OceanwideImages.

From southern Queensland to central Western Australia, silver trevallies fisheries are based on “a complex of species”, with the predominant or mix of species fished varying from state to state.

Trevallies are taken by angling in all states, spearfishing in NSW and Tasmania, and by recreational seine and gillnets in Tasmania. They are taken by most of the wide range of gear and methods used by commercial fishers: trawl, Danish seine, haul seine, hook, trap and gillnet.

To supplement the information on these species in the SAFS report, additional information on recreational catches has been taken from state surveys of recreational and charter fishing. THE ‘SILVER TREVALLIES’ COMPLEX

There is a complex array of ‘silver trevallies’ around Australia. The most commonly seen by anglers from NSW to WA is Pseudocaranx georgianus, which is assumed to be with so much potential for confused identification, the bottom line is that most recreational and commercial catch records list them as “silver trevally” or “trevally”. For example, the 2000-01 National Recreational Fishing Survey avoided the confusion around identifying and naming trevally species and combined silver trevallies with the various tropical trevallies, simply reporting them as the “trevally” species group.

The SAFS report takes a similar simplifying approach, presenting ‘silver trevally’ sustainability information at state and Commonwealth fisheries jurisdictions separately.

SILVER TREVALLY

Recreational fishers will find it puzzling to see the contrasting conclusions reached on the state of silver trevally as assessed under Australia’s two major trevally fisheries: the NSW combined commercial and recreational fisheries, and the Commonwealth-managed commercial trawl, hook and gillnet fisheries. declined steadily, to their lowest point in 2019 – 42 tonnes. The introduction of recreational fishing havens, marine parks and the 30cm legal size have played some part. However, the overall decline in catch rates and the diminishing proportion catches of silver trevallies are reported separately to the other trevally species in Queensland records. The SAFS report considers it “unlikely that the combined commercial and recreational catches exceeded 10t in 2019”.

With insufficient evidence to conclude otherwise, silver trevallies in Queensland waters are classified as an “undefined stock”. Victoria

After 1991, Victoria’s Bass Strait commercial gill net fishery moved from a state to a Commonwealthmanaged fishery. Until then, this fishery accounted for most of the state’s trevally landings, totalling more than 200 tonnes annually. Through the 1990s, state landings totalled less than 100 tonnes and declined further from 2001 as commercial licence numbers in the bays and inlets were reduced under a buy-out program, funded from Recreational Fishing Licences.

Over a period of 20 years,

there have been several commercial fishing licence buy-outs, culminating in the 2020 closure of net fishing in the Gippsland Lakes. These have had the effect of reallocating access to silver trevally (and other bay and inlet fish) to the recreational fishing sector.

With the closure of the Gippsland Lakes commercial fishery, Corner Inlet retains the last continuing seine net fishery, taking 70% of the state’s 44 tonne catch recorded for 2019/20. Unfortunately, the only estimate of Victorian recreational catch of ‘trevally’ was 37 tonnes, taken in 2000/01, before the buy-outs began.

Recent above average catch rates in the inlet fisheries indicate that “there are no local signs of depletion”, or that the stock’s spawning potential is impaired. A 2019 assessment observed that, while the stock “might be depleted” at a national level, fishing in Victorian waters is unlikely to be a major factor.

All the evidence leads to the classification of silver trevally in Victorian waters as a “sustainable stock”. Tasmania

Four estimates of annual recreational gillnet and line catches since 2000 have ranged between two tonnes in 2012/13 and nine tonnes in 2017/18.

Silver trevally are caught as a by-product of commercial gillnet and beach seine fishing. With the highest catch, eight tonnes, recorded in 1998, the

Sixty years of commercial fishing have had a significant impact on silver trevally

stocks. Photo courtesy of Kevin Rowling.

2018/19 catch of four tonnes was in line with the average over the previous 10 years.

Under this low level of catch and fishing effort, the stock is considered “unlikely to be depleted” and under no threat of spawning ‘impairment’.

On this basis, silver trevally in Tasmanian waters are classified as a “sustainable stock”. South Australia

The 2013/14 recreational fishing survey estimated the catch by anglers as 57,000 fish, with an estimated total weight of 15 tonnes, up from 12 tonnes in 2007/08. This ranks alongside the commercial catches by the multi gear/method/species Marine Scalefish Fishery in which trevallies are mainly by-catch species, ranging between four and 22 tonnes annually, during the past 20 years. The commercial catch in 2018/19 was five tonnes.

The SAFS assessment noted that the stock is “unlikely to be depleted”, and under no threat, from fishing, to its spawning capability.

On this basis, silver trevally in SA waters were classified as a “sustainable stock”.

Western Australia

Recreational private and charter catches of silver trevally have trended down, from 32 tonnes in 2011/12, to 15 tonnes in 2017/18, taken mainly from the West Coast Bioregion. The

Fig 1. Part of the decline in commercial catches of silver trevally from NSW and Commonwealth managed fisheries.

SPECIES LIMITS State Queensland

Family Carangidae

NSW

Silver trevally All other trevallies

Victoria

All trevallies

Tasmania All trevallies

SA

All trevallies

WA

All trevallies – 20 combined

30 cm 10 combined –

20 20 combined

20 10

24cm 20 combined

25cm 8

Current daily recreational catch limits around southern Australia. Note 1: additional fishing restrictions, apply in some states.

2017/18 survey of boat-based recreational fishing found that silver trevally were the sixth most commonly retained fish taken by WA anglers, as well as having a release rate of 40% or more.

Historically, since 1975, most annual commercial catches were below the estimated Maximum Sustainable Yield (66 tonnes). More recently, most silver trevally have been taken as a by-catch by commercial line fishing, with annual catches ranging from two to 10 tonnes since 2008.

As in other states, assessments indicate that current levels of fishing are unlikely to deplete the stock or to lessen its spawning capability. On the available evidence, silver trevally in WA waters is classified as a “sustainable stock”.

Note that the 2000-01 National Recreational Fishing Survey estimated the recreational catch of “trevally” to be 249 tonnes, reflecting the prominence of tropical trevallies in the state’s recreational fisheries. NATIONAL REPORTING

The Status of Australian Fish Stock Reports are a series of assessments of the biological sustainability of a broad range of wildcaught fish stocks against a nationally agreed reporting framework. The two-yearly reports examine whether the abundance of fish and the level of harvest from each stock is sustainable. Definitions of “sustainability” and quoted passages in this article are taken from the SAFS 2020 Reports.

More details about the status of Australian fish stocks are available at – www.fish.gov.au/reports

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