For Australia’s southern rock lobster fishery, a harvest strategy response to adapt to climate change

New strategy produced a notable increase in biomass and aligned with projection modeling

Commercially exploited lobster stocks are some of the highest unit value fisheries globally. The impacts of climate change on lobster species are widespread, affecting all aspects of the lifecycle including behavioral traits. The complex lifecycle of the southern rock lobster (Jasus edwardsii) – which supports important commercial and recreational fisheries across southeastern Australia – makes it particularly vulnerable to climate change. This includes an extended larval period lasting from 12 to 24 months through 11 stages of development as well as ontogenetic shifts in sociality and spatial distribution during juvenile maturation and development.

Given their economic importance, most lobster fisheries have extensive resource management measures. In some jurisdictions these include dedicated fishery management plans that detail specific harvest strategies with explicit decision rules around annual catch limits. In the face of environmental change, harvest strategies are now accounting for climate change impacts, particularly during the analytical model testing of proposed strategy options. Specifically, harvest strategies are increasingly aiming to target higher levels of fishery egg production to buffer against variability in recruitment expected under climate change scenarios.

There is now growing evidence to suggest that climate change has impacted on the productivity of J. edwardsii across its geographical range within southeastern Australia. These decreases occurred despite management dictating relatively stable levels of catch prior to the downturn. The synchronous and large-scale nature of the decline indicates an environment-driven ecosystem regime shift. In South Australia, this appears to reflect the effect of increasing ocean temperatures impacting on female lobster reproductive capacity.

This article – summarized from the original publication (Linnane, A. et al. 2024. Adapting to climate change in a spiny lobster (Jasus edwardsii) fishery: A harvest strategy response. Fisheries Research Volume 276, August 2024, 107054) – reports on a study reviewing how the adopted harvest strategy and updated shift recruitment indices were incorporated into the assessment process using trends in catch, catch per unit of effort (CPUE) and unexploited egg production (%UEP) as indicators of stock sustainability.

Study setup

In 2018, the harvest strategy for the southern rock lobster fishery was reviewed with the primary focus of achieving a target percentage of unexploited egg production (%UEP) within an agreed timeframe. The proposed harvest strategy was based on a tabular harvest control rule, where CPUE from the preceding season, as an indicator of lobster abundance, specified the following year’s total allowable commercial catch (TACC). To account for climate change impacts, the harvest strategy was assessed using a projection model where recruitment was sampled from post-regime shift years, thereby adopting a conservative approach to stock rebuilding.

For detailed information on the study design; models used for harvest strategy and recruitment evaluation; indicators and sensitivity testing, refer to the original article.

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Results and discussion

The effects of climate change on commercially exploited lobster species from a biological perspective have been extensively reviewed. Impacts include changes to the timing, level and duration of larval settlement, growth rates and corresponding size at maturity, juvenile abundances, distribution, and disease resistance. While largely perceived as having negative outcomes, some climate change impacts can be considered positive from a fishery perspective.

Researchers have suggested that increases in lifetime egg production can be associated with observed decreases in size at maturity within the Canadian lobster fishery. In addition, oceanographic modelling indicates that while climate change impacts are negative in some areas, they may have positive consequences in others, thereby suggesting a resilience in at least some fisheries to climate-induced changes.

While impacting on many stages of the reproductive cycle, it is the complex lifecycle of spiny lobsters that make the species particularly vulnerable to changes in recruitment. The larval phase J. edwardsii ranges from 1 to 2 years through eleven stages of deep-water oceanic development, thereby exposing the species to a range of environmental conditions for extended periods of time. While the exact causes remain poorly understood, there is growing evidence to suggest that increasing water temperature, acting on these early developmental stages, at least partly explain observed recruitment impacts. In South Australia, declines in fishery recruitment for this species coincide with decreases in levels of egg production which are likely to reflect temperature induced changes to size of maturity.

Management adaptations to climate change in lobster fisheries are varied but commonly involve incorporating changes in lobster recruitment into catch modelling. This process is particularly important during harvest strategy evaluation. Almost all of Australia’s spiny lobster fisheries are now managed under total allowable commercial catches (TACCs) with associated harvest control rules. These involve formal decision rules, based on quantitative performance indicators such as catch rate or model-estimated biomass, to set an annual catch limit. Within South Australia, the decision rule takes the form of discrete tables that use the relationship between catch rate and biomass to recommend a TACC. This approach assists in management communication in that the catch rate is easily understood and widely accepted by stakeholders as an indicator of abundance. In addition, it does not require running a population model to set an annual catch limit.

Projection modelling is the accepted approach to harvest strategy testing, of which, the assumptions around recruitment are known to be critical. To account for climate change impacts, this study tested harvest control rules (HCR) based on a low recruitment period (2000–2015) which had translated to poor fishery performance. By using this reference period, a conservative approach to TACC setting was taken with the aim of achieving the management objective of increasing fishery egg production. Sensitivity testing was included by further reducing the assumed level of recruitment in the projection runs.

The results from the testing showed positive outcomes in terms of catch rate and unexploited egg production (UEP). This largely reflected the impact of the HCR on catch and corresponding exploitation rates. Specifically, the harvest strategy allowed quota to fluctuate with abundance, but increases in catch were constrained as biomass increased with corresponding reductions in exploitation rates from 47 to 29 percent over the last six seasons, thereby allowing rebuilding to occur, despite the low levels of presumed recruitment. This conservative approach was deemed acceptable from a resource management perspective, and achieved the target of 20 percent UEP by 2035, thereby meeting the primary objective of the harvest strategy working group.

While projections were generally positive based on the 2000–2015 recruitment time series, sensitivity analyses indicated that if recruitment were to reduce further, the risks to sustainability are considerable. Specifically, under a 21 percent lower recruitment scenario, this strategy failed to meet the UEP target of 20 percent by 2035. Despite the absence of a stock-recruitment relation in many species, appropriate levels of egg production are deemed critical to the long-term viability of lobster resources.

In particular, there is strong evidence that fisheries with egg production levels above threshold levels are more resilient when faced with environmental perturbations resulting from climate change. For example, the Western Australia spiny lobster (Panulirus cygnus) recruitment failure from 2006 to 2012 was mediated by pre-emptive management that focused on protecting the breeding stock and maintaining high levels of egg production which ultimately ensured that resource sustainability was not compromised. Similarly, fisheries for the packhorse rock lobster (Sagmariasus verreauxi) in New South Wales and J. edwardsii in New Zealand have shown significant recovery when spawning stocks were protected with the aim of increasing overall egg production.

Overall, given the negative significant reduction in fishery performance under a reduced recruitment scenario, future research should attempt to quantify the impact of increased ocean warming on southern rock lobster recruitment levels. This could involve linking projections to future climate change models or developing a full management strategy evaluation which may allow for more thorough testing of some of the underlying hypotheses, such as the relationship between ocean temperature and lobster sexual maturity.

Perspectives

In 2019, the harvest strategy working group recommended the harvest strategy discussed above to the South Australian rock lobster Management Advisory Committee as the agreed harvest control rules (HCRs) for the Southern Zone fishery. Subsequently, the HCR was adopted and used to set the fishery total allowable commercial catches (TACCs) in the four subsequent fishing seasons (2019–2022). Over this period there has been a notable increase in biomass as observed by a 21 percent increase in legal-size catch rate in line with projection modelling. These results provide continued support for setting annual TACCs in proportion to the previous year’s CPUE based on an approximately constant exploitation rate as the basis for HCRs within South Australian rock lobster fisheries.

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