Given the growing impacts of climate change on marine environments, leading fisheries scientist Andrew Penney advocates for greater recognition of changing population dynamics in fisheries modelling and management. 

By Catherine Norwood 

Climate change, warming oceans and associated ecosystem impacts are driving changes in fisheries faster and more dramatically than ever before. In 2023 and 2024, for example, rising ocean temperatures around Australia set consecutive new records. 

Leading fisheries scientist Andrew Penney says the degree of negative climate-related impacts on fisheries in Australia, and internationally, will be increasingly revealed as coming seasons unfold.  

“The most dramatic impact observed so far has been the total collapse of the Bering Sea snow crab population following a large and protracted heatwave in the area” says Andrew. “Since 2018 more than 10 billion snow crab have disappeared because of deep sea warming, starvation and predation.” 

Fishing effort is no longer the only influence on sustainability of fish stocks, and environmental changes are changing the actual productivity of fish stocks. 

One option for management to respond to this changing productivity, Andrew notes, is the use of dynamic management of fisheries to better reflect the changing dynamics of stocks due to environmental influences.  

This is among the key points he makes in a recent submission to the FRDC’s Board of Directors, reflecting on the challenges facing the future of Australia’s Commonwealth fisheries. 

Changing conditions  

Andrew says fisheries management has always had to deal with variability in stock dynamics. However, management processes, including stock assessments and harvest control rules, have tended to assume that fish stocks will rebuild to some historical average level if unfished. 

It has been assumed that recruitment, growth and natural mortality will all continue as they have in the past. Target reference levels for fish biomass, which provide the basis for harvest strategies and maximum sustainable yield targets, are based on this expectation. 

“But this has always been a mathematical convenience,” Andrew points out. “To deal with these environmentally driven changes, fisheries management needs to stop assuming that stocks are naturally ‘stable’ but have always been subject to change.” 

“This will require fisheries policy to acknowledge that changes to fishing mortality alone cannot mitigate environmentally driven declines,” he says. 

Dynamic reference points 

He says that, where the environment has caused a persistent decrease in stock productivity, the use of dynamic rather than static reference points for fish biomass targets in harvest control rules offers an alternative approach that could better respond to shifts in ecosystems and species productivity. It could also help to balance exploitation and conservation of stocks with reduced productivity.  

A number of species have populations that are failing to rebuild after years of decline, in spite of reduced fishing effort.

Using a dynamic reference point recognises that these stocks are unable to ‘recover’ to historical levels because of their reduced productivity, and allows them to be managed as smaller, less productive stocks, explains Andrew. 

Associated stocks 

Maintaining static reference points in the face of changing and potentially permanent environmental impacts “will likely result in repeated reductions in Total Allowable Catches (TACs) and, ultimately, closure of the fishery for those stocks, which often can’t be achieved without reducing catch of other stocks that are not necessarily at-risk” he adds. 

He points to the depleted Eastern Jackass Morwong (Nemadactylus macropterus) stock and the sustainably fished Tiger Flathead (Platycephalus richardsoni) as a case in point. These species cohabit the same locations and are caught together.  

Morwong stocks in the Southern and Eastern Scalefish and Shark Fishery (SESSF) have been declining since 1985 and have failed to recover despite constant reductions in fishing effort under harvest control rules and industry efforts to avoid the species. 

Stock assessments in 2014 showed a very little increase in morwong populations with the stock still considered to be depleted. This resulted in the closure of five areas that support Eastern Jackass Morwong populations. 

These closures have reduced harvests of Tiger Flathead, one of the key stocks in the SESSF, which has been sustainably harvested for 50 years. This has potentially affected the viability of fishers and reduced revenue collected from fishers for the management of the fishery. It has also reduced the catch data used in the assessment of stocks.  

Multi-species fisheries 

Andrew says in multi-species fisheries, such as the SESSF, harvesting must inevitably aim for a balance between over- and under-exploitation of different stocks caught together.  

“This could result in some stocks being fished at above target biomass levels, and others below target biomass levels, although still above limit levels.  

“Traditional management approaches applying separate single-species harvest strategies to each stock cannot simultaneously maintain all stocks in a multi-species fishery at target levels … when non-fishing effects are negatively affecting some stocks more than others.” 

Gathering evidence 

Andrew says key to the adoption of dynamic management will be evidence to separate fishing from non-fishing effects, to demonstrate which stocks have been negatively and persistently affected by environmental drivers. 

FRDC is already investing in a suite of research with this in mind, particularly in southeast Australia which is recognised as a global ocean warming hotspot. 

Projects underway include an update of the biological parameters of key commercial fish stocks which are used in stock assessments (2022-091). Some of these parameters date back more than 20 years and represent the static and historic data, which Andrew points out may no longer reflect the status of fish stocks, given changing environmental conditions. 

In northern Australia, FRDC is funding the use of CSIRO’s Models of Intermediate Complexity for Ecosystem assessments, or MICE (2023-063) in the Northern Prawn Fishery to account for climate impacts on Tiger Prawn populations. 

Efforts such as these to gather evidence of influences on fish stocks and the change taking place will be crucial in the development of more responsive management strategies that better reflect the dynamics of fisheries and environmental conditions, says Andrew.