Silver Trevally NSW: Why Fishers Are Questioning The “Unsustainable” Label

For anyone reading the headlines, the story sounds simple.

Silver Trevally on the east coast are “depleted”, “overfished” and often described as “seriously overfished” in public guides that reference the trawl fishery off New South Wales and Victoria.

National stock reports and the latest NSW Stock Status Summary repeat that story. The joint NSW–Commonwealth assessment that feeds into these documents uses an integrated Stock Synthesis model and concludes that spawning biomass has been sitting near, or below, 20 per cent of the pre fishing level since the late 1990s, only starting to improve after 2019. On that basis, SAFS still lists the NSW component as Depleted and the Commonwealth as Overfished, while NSW DPI uses a more cautious label of Recovering.

That is the official picture.

On the water, many of us fishing Silver Trevally in NSW see something different.

This article is not a claim that the science is useless. It is a request for honesty about what the models can and cannot say, and for serious consideration of an alternative explanation for low landings that has never been properly tested.

1. What the science actually says about Silver Trevally in NSW

The modern story starts with Rowling and Raines in 2000. Their detailed study of Silver Trevally off NSW showed a long lived, slow growing fish that can reach 40 years of age, and by the late 1990s the age structure in commercial catches was clearly truncated, with a big shift to younger and smaller fish.

Using catch curves and yield per recruit models, they concluded the stock was growth overfished. In plain terms, the problem was that Trevally were being harvested too young and too small, not that recruitment had collapsed entirely. They recommended a higher size at first capture and changes to gear to let more fish reach larger sizes.

Fast forward nearly twenty years. Fowler and Chick’s 2019 NSW assessment relied mainly on:

• Historical trawl catches from 1955 onwards

• Standardised CPUE from trawl and trap fleets

• Market sampling of lengths in 12 years between 2004 and 2018

• Length based spawning potential ratio models to estimate how much spawning capacity remains

They used data limited methods, including an Optimised Catch Only Model and a length based SPR model, to argue that biomass had fallen to well below half of the level that would deliver maximum sustainable yield, and that spawning potential was seriously reduced.

The most recent step is the joint Commonwealth and NSW Stock Synthesis assessment completed in 2023. This is a full age and length structured model that pulls together:

• Catches from seven fleets across NSW, Victoria and Commonwealth zones off the east coast

• Three standardised CPUE series

• Length compositions from multiple fisheries and two years of fishery independent survey data

• One year of age at length data, plus growth and maturity parameters from earlier NSW work

The model estimates that spawning biomass fell below the 20 per cent limit point in the late 1990s or early 2000s and stayed there until at least 2021, although there has been a noticeable upward trend since 2019 as fishing mortality has dropped. Depending on the assumed natural mortality, the current biomass estimate sits either just above or just below the 20 per cent line.

That is why NSW DPI now calls the stock “recovering” rather than “sustainable”, while still reporting a SAFS status of “Depleted”.

So far, so serious. The question is whether these models tell the whole story for Silver Trevally in NSW.

2. The blind spot that matters: fewer boats, less water, more rules

If you only ever look at graphs of catch and modelled biomass, it is easy to forget how radically the fishery itself has changed since the 1980s.

The national Silver Trevallies reports in 2018 and 2020 admit that interpreting the collapse in NSW landings is “complicated” because of:

• Changes in how catches are reported between NSW and Commonwealth logbooks

• The creation of recreational fishing havens and marine parks, especially Batemans Marine Park, which reduced the area available to commercial Trevally fishing and likely influenced catch rates

• The introduction of a 300 millimetre minimum legal length in 2007, which immediately cut the quantity of legal catch that can be landed and altered the way fishers target the species

What the national and state assessments do not do is explicitly model the massive reduction in commercial businesses, vessel numbers and effective fishing effort in NSW. They measure effort in days and shots, not in the number of viable family businesses that still exist.

When you remove boats from the water through buybacks, close key grounds, shift effort into other species and increase the legal size of the target fish, you should expect recorded landings to fall. You also change the shape of CPUE time series in ways that are very difficult to untangle.

The Silver Trevallies reports acknowledge that closures and the higher size limit have “likely reduced catch and potentially influenced catch rates” and that this makes it hard to define reference points.

Yet in the stock assessment models, the dominant story is still that falling catches and low CPUE primarily reflect falling biomass.

3. On water evidence that does not look like a collapsed stock

Every fisher will have a different experience, but on the east coast many of us targeting Silver Trevally in NSW still see:

• Consistent shots on traditional grounds when we actually target the species

• A mix of sizes, including solid fish well above the legal minimum

• Regular presence of Trevally in areas where we have always expected to find them

Our own logbooks show that kilos of Trevally per hour of effective fishing time have not crashed in the way you would expect from a truly collapsed, recruitment impaired stock. What has changed much more is the number of days we can justify targeting Trevally, the number of grounds open to us, and the number of operators still in the game.

The national 2018 and 2020 reports concede that catch and CPUE are very hard to interpret because effort has declined, areas have been closed and the reference period used in older Commonwealth assessments may not have represented a true “target biomass” state in the first place.

If your model is tuned mainly on data from the years leading into overfishing, it will then treat any lower catch and effort later on as confirmation of depletion, even if the underlying stock has stabilised under tighter rules and fewer boats.

From the wheelhouse, that is exactly what many of us suspect is happening with Silver Trevally in NSW.

4. Why the current models for Silver Trevally in NSW are fragile

The joint Stock Synthesis assessment is a serious piece of work, but it still rests on a set of very strong assumptions. The NSW Stock Status Summary spells them out clearly:

• A single biological stock across the east coast

• Accurate commercial catch data by fleet

• CPUE indices that are linearly related to abundance

• Selectivity that is fixed through time for most fleets

• Natural mortality that does not change with time or age

The assessment team did test some alternative scenarios, such as removing the NSW trap CPUE, changing the steepness of the stock recruitment relationship and altering the weight placed on CPUE and length data. These sensitivity runs showed that the current biomass estimate moves by several percentage points, and that natural mortality assumptions are a major driver of whether the model sits just above or just below the 20 per cent limit.

What the assessment does not do is test a scenario where:

• Fleet size shrinks sharply

• Spatial closures and marine parks continue to remove grounds

• Catchability changes because remaining vessels fish differently and focus on different species

while the underlying abundance of Silver Trevally in NSW is more stable than the current model suggests.

The earlier NSW work that underpins many of the life history parameters for Trevally is also based on age samples taken in the late 1980s and 1990s. Since then, the main new information for NSW has been length data, not fresh age structure covering the last fifteen to twenty years.

In short, the current “depleted and recovering” label for Silver Trevally is based on models that assume catch and CPUE changes are mostly about biomass, even though the official reports themselves admit that management changes and area closures have made those indicators very difficult to interpret.

That does not mean the stock is certainly fine. It does mean the claim that Silver Trevally in NSW is unsustainable is not as watertight as some public messaging suggests.

5. A constructive way forward for Silver Trevally in NSW

If we want confidence in the status of Silver Trevally in NSW, we need to fix the blind spots, not just argue about labels. There are practical steps that could improve the situation for everyone.

1. Co designed monitoring using working boats NSW DPI could partner with willing operators on key grounds to collect high quality length and age data from current catches, not just rely on market sampling and old age structures. That would allow a proper check on whether age truncation is still as severe as it was in the 1990s.

2. Effort and catch per business, not just per day Rebuilding the history of the fishery in terms of active licences or active Trevally operators would help separate the effect of fleet reduction from changes in abundance. Published landings already show that historic catches over 1 000 tonnes in the 1980s have fallen to around 40 tonnes in 2019, and the national reports admit that a lot of the interpretation is confounded by reporting shifts and closures. Looking at tonnes per active business through time would be a simple and transparent way to test whether productivity per boat has truly collapsed.

3. Honest treatment of uncertainty in public guides Consumer guides that describe Silver Trevally as “seriously overfished” are basing that language on assessments that are themselves heavily qualified and now sit alongside a NSW label of “recovering”. At the very least, those guides should acknowledge that the science is uncertain, that management actions have reduced fishing mortality, and that the stock is projected to rebuild under current catch levels.

4. Industry at the table for future assessments Commercial fishers should be involved when the next round of Silver Trevally assessment is scoped. That includes defining the questions the model is meant to answer, agreeing on data priorities, and making sure that scenarios involving fleet reduction, effort redistribution and closures are tested alongside pure biomass speculation.

Closing thought

No one fishing for a living wants to go back to the days of growth overfishing on Silver Trevally in NSW. The minimum legal length, effort reductions and spatial protections are real changes, and they have cost our businesses dearly.

What we are asking for is simple.

If assessments and public guides are going to label a stock as depleted, overfished or unsustainable, they must properly account for the other forces that drive landings down. For Silver Trevally in NSW, that means taking fleet reduction, marine parks, recreational fishing havens and stricter rules as seriously as the models take catches and CPUE.

Until those alternative explanations are tested within the stock assessment itself, describing this fishery as unsustainable is at best premature, and at worst a misleading picture of what is actually happening on the water.