Climate analogues as a tool for marine aquaculture planning and adaptation

Technical Summary: Climate Analogues as a Tool for Marine Aquaculture Planning and Adaptation

Problem Statement
Global sea surface temperatures (SST) have risen by approximately 1°C since 1901 and are projected to continue increasing throughout the 21st century. While wild marine species can migrate to track suitable thermal conditions, farmed aquaculture species are confined to fixed locations, rendering them highly vulnerable to climate-induced thermal stress, disease, and mortality. Current aquaculture planning often lacks fine-scale, industry-grounded frameworks to identify future viable sites. Existing climate analogue studies have primarily focused on terrestrial conservation or broad agricultural crop suitability, leaving a gap in applying these metrics to marine food production systems, particularly for species operating outside their native ranges (e.g., Atlantic salmon in Tasmania).

Methodology
The authors developed a spatially explicit framework to identify "climate analogues"—future locations where SST conditions resemble those of current production sites. The study utilized Atlantic salmon (Salmo salar) aquaculture in Tasmania as a case study.

• Data Sources: The framework integrates daily SST projections (2015–2100) and historical runs (2003–2014) from nine Earth System Models (ESMs) within the CMIP6 ensemble. These were analyzed under five Shared Socioeconomic Pathways (SSPs), ranging from low emissions (SSP1-1.9) to high emissions (SSP5-8.5).
• Thermal Thresholds: Instead of using broad species thermal envelopes, the study defined a "realised operational envelope" based on empirical data from existing Tasmanian farms. Two critical thresholds were established:
  • 17°C: Onset of physiological stress.
  • 21.5°C: Sub-lethal limits and feeding cessation.
  • Classification: Sites were categorized as "Suitable" (thermal exceedances matching current operational sites, including those with reported mortality) or "Optimal" (matching sites with no reported mortality). Unsuitable sites exceeded these historical thresholds.
• Spatial Constraints: To ensure operational feasibility, the study applied exclusion layers for sensitive habitats (kelp forests, reefs), protected areas (IUCN categories Ia–IV), shallow depths (<10 m), and high-traffic shipping lanes.
• Confidence Metrics: Analogues were classified by confidence levels based on the agreement among ESMs (High: ≥75%, Medium: 50–75%, Low: 25–50%, No Analogue: <25%).
• Adaptation Triggers: A trigger point was defined as the moment the high-confidence analogue area in a jurisdiction falls below the current licensed finfish aquaculture footprint (25.88 km²), indicating the need for relocation or adaptation.

Key Results

• Decline in Suitability: Under high-emission scenarios (SSP5-8.5), suitable analogue areas are projected to decline by 25.5% by the 2050s and up to 77.1% by 2100. Under the most stringent mitigation pathway (SSP1-1.9), suitable analogues remain stable or expand slightly. Optimal analogues decline more sharply, with losses reaching 82% under SSP5-8.5.
• Jurisdictional Disparities: A distinct gradient exists across marine jurisdictions. State waters (0–3 nautical miles) are projected to lose all suitable and optimal analogues by mid-century under high-emission scenarios. In contrast, Commonwealth waters (3–200 nautical miles) retain significant viable areas (326,040–462,082 km²) by mid-century, even under severe warming.
• Impact of Spatial Constraints: Incorporating spatial constraints (e.g., shipping lanes, protected areas) significantly accelerates the timeline for adaptation. In State waters, the "constrained" adaptation trigger for optimal conditions is reached immediately (2021–2030), and for suitable conditions by the 2030s–2040s. This is approximately three decades earlier than "climate-only" projections, highlighting that competition with other ocean users reduces the window for adaptation.
• Geographic Shifts: Eastern and northern Tasmanian regions are projected to transition out of high-confidence analogue conditions earlier than southern and western regions.

Significance and Claims
The paper claims that this framework provides a robust, transferable tool for "climate-smart" aquaculture planning. Its primary contributions include:

1. Refining Relocation Planning: By linking specific physiological thresholds to current operational realities, the framework moves beyond theoretical suitability to identify practical relocation sites.
2. Quantifying Uncertainty: The inclusion of model agreement metrics allows stakeholders to distinguish between robust climate signals and uncertain outcomes, aiding in risk management.
3. Accelerating Adaptation Timelines: The study demonstrates that considering spatial constraints reveals that adaptation (specifically relocation to offshore Commonwealth waters) is required decades earlier than climate models alone suggest.
4. Policy Relevance: The framework offers industry and government stakeholders a lead time of several decades to implement targeted strategies, such as site relocation, offshore technology investment, or species diversification, thereby supporting food security and long-term economic planning in a warming ocean.

The authors emphasize that while the framework is demonstrated in Tasmania, it is designed for global application to other species and regions, particularly in rapidly growing production hubs. They note that the results are conditional futures dependent on global mitigation choices rather than fixed forecasts.