Into new depths: climate-driven habitat expansion of the endangered skate Dipturus chilensis (Chondrichthyes, Rajiformes)

Using species distribution models under various climate scenarios, this study projects that the endangered yellownose skate (*Dipturus chilensis*) will experience a net expansion of suitable habitat by 2100 due to a poleward shift toward Southern Chile, driven primarily by temperature and oxygen levels, which underscores the urgent need for climate-adaptive management strategies to protect emerging southern refugia.

The Gist

The Big Picture: A Skate Fish on the Move

Imagine the Yellownose Skate (Dipturus chilensis) as a slow-moving, ancient underwater turtle that lives on the sandy ocean floor off the coast of Chile. It's a bit like a "living fossil" that has been around for a long time, but it's currently in trouble. It's classified as Endangered, meaning it's on the brink of disappearing, mostly because humans have been catching too many of them for food.

Now, add a new problem: Climate Change. The ocean is getting warmer, and the water is losing some of its oxygen. This paper asks a simple but critical question: As the ocean changes, where will this skate fish go to survive?

The Detective Work: Using a "Digital Crystal Ball"

The researchers didn't just guess; they used a Species Distribution Model (SDM). Think of this model as a high-tech crystal ball or a sophisticated weather forecast, but instead of predicting rain, it predicts where fish can live.

They fed the computer two types of information:

1. The "Where": A massive map of 2,800+ places where the skate has been spotted in the past.
2. The "Conditions": Data on what the water feels like there (temperature, oxygen levels, salinity).

The computer learned the skate's "comfort zone." It figured out that this fish is a bit of a picky eater and a temperature-sensitive sleeper. It loves cool water and needs plenty of oxygen.

The Forecast: A Journey South

The researchers ran this crystal ball into the future, looking at three different scenarios for the years 2050 and 2100:

• The Green Scenario (SSP1-2.6): The world tries hard to stop climate change.
• The Yellow Scenario (SSP2-4.5): Things get a bit warmer, but not catastrophic.
• The Red Scenario (SSP5-8.5): The world keeps burning fossil fuels, and things get very hot.

Here is the surprising result:
Instead of the skate disappearing, the model predicts it will move south.

• The "Hot Zone" (Central Chile): The water up north is getting too warm and too low in oxygen. It's like a house where the air conditioning broke and the windows are stuck open. The skate will have to leave this neighborhood.
• The "Cool Zone" (Southern Chile): The water down south (in the Channels and Fjords) is cooler and fresher. It's like a natural air-conditioned sanctuary. The skate will migrate there to find a comfortable spot.

The Twist: More Space, But New Dangers

You might think, "Great! They are moving to a new home, so they are safe!" Not so fast.

The model shows that by the year 2100, the total amount of suitable habitat will actually increase. It's like the skate is moving from a small, crowded apartment into a massive, empty mansion in the south.

However, there are two big catches:

1. The Fishing Net Follows: Just because the fish moves to a new "mansion" doesn't mean the fishermen will stop. If the fish moves to a new area, the fishing boats might follow them there. It's like moving to a quiet suburb, only to find out the neighbors are starting a loud construction project right next door.
2. The "Slow Mover" Problem: Skates are not like tuna or sharks that can swim fast across the ocean. They are slow, bottom-dwelling creatures. They can't just zip away from danger. If their old home becomes too hot, they might not be able to reach the new cool home in time.

The Main Takeaway: We Need a Dynamic Plan

The paper concludes that we can't just draw a static line on a map and say, "This is the protected area." That line will become useless as the fish moves.

The Solution? We need Adaptive Management.
Imagine a dynamic fishing rulebook that changes every year based on where the fish actually are, rather than where they used to be. We need to:

• Protect the new southern "refuge" areas before fishermen arrive.
• Update fishing quotas and bans as the fish migrate.
• Accept that the ocean is changing, and our conservation plans must change with it.

In a Nutshell

The Yellownose Skate is packing its bags and moving south to escape the heat. While it might find a bigger, cooler home in the future, its survival depends entirely on whether we, the humans, can update our fishing rules fast enough to protect it in its new neighborhood. If we don't adapt, the skate could still vanish, even if the water looks perfect for it.

Technical Summary

1. Problem Statement

The yellownose skate (Dipturus chilensis) is an endangered, demersal elasmobranch endemic to the southeastern Pacific Ocean. The species faces a "double jeopardy" of intense historical fishing pressure (leading to a population collapse from ~5,000 tons in 2003 to ~537 tons in 2024) and increasing vulnerability to climate change. While overfishing is the primary historical driver of decline, the specific impacts of ocean warming, deoxygenation, and acidification on the distribution and habitat suitability of this temperate skate remain poorly understood. There is a critical need to forecast how climate change will alter the species' geographic range to inform adaptive fisheries management and conservation strategies in Chile.

2. Methodology

The study employed a robust Species Distribution Modeling (SDM) approach using the Maxent (Maximum Entropy) algorithm to project current and future habitat suitability.

• Occurrence Data:
  • Compiled a dataset of 2,826 records from GBIF, institutional repositories, scientific literature, and exploratory fisheries.
  • Data Cleaning: Restricted records to the Pacific Ocean (excluding Atlantic records misidentified as D. chilensis).
  • Bias Correction: Applied spatial thinning (11 km threshold) to reduce sampling bias in central Chile and generated a sampling bias layer using Kernel Density Estimation (KDE) to down-weight oversampled regions in the Maxent model.
• Environmental Predictors:
  • Utilized Bio-ORACLE v3.0 benthic layers (0.05° resolution) derived from CMIP6 Earth System Models.
  • Variable Selection: Initial screening of 12 variables (temperature, oxygen, salinity, pH, velocity, phytoplankton) was followed by collinearity analysis (VIF < 5, |rₛ| < 0.75).
  • Final Predictors: Maximum temperature, minimum dissolved oxygen, minimum temperature, and maximum salinity.
• Modeling Framework:
  • Scenarios: Projections were made for mid-century (2040–2050) and end-of-century (2090–2100) under three Shared Socioeconomic Pathways (SSPs):
    • SSP1-2.6 (Sustainable development)
    • SSP2-4.5 (Intermediate pathway)
    • SSP5-8.5 (Fossil-fueled development)
  • Validation: Models were run with 10 replicates using 70/30 training/testing splits. Performance was assessed via Area Under the Curve (AUC) and compared against null models (random occurrence sets) using Wilcoxon rank-sum tests.
  • Thresholding: Binary habitat maps were generated using the "Maximum training sensitivity plus specificity" (MTSS) threshold (0.3655).
• Study Area: Defined by four marine ecoregions: Central Chile, Araucanian, Chiloense, and Channels and Fjords of Southern Chile.

3. Key Results

• Model Performance: The empirical models demonstrated high predictive accuracy (Mean AUC = 0.94), significantly outperforming random expectations (Null AUC = 0.51; p < 0.0001).
• Environmental Drivers:
  • Maximum Temperature was the dominant predictor, contributing 57.4% to the model.
  • Minimum Dissolved Oxygen was the second most important variable (31.3%).
  • Minimum temperature (9.0%) and maximum salinity (0.8%) had minor contributions.
• Spatial Redistribution:
  • Poleward Shift: All scenarios predicted a consistent southward (poleward) shift of suitable habitat toward the Channels and Fjords of Southern Chile ecoregion.
  • Habitat Loss: Localized habitat loss was projected for the Central Chile and Araucanian ecoregions, particularly under the high-emission SSP5-8.5 scenario.
  • Habitat Gain: Significant expansion into the sub-Antarctic southern ecoregions was observed.
• Net Habitat Change:
  • Despite localized losses in the north, the total suitable habitat area is projected to increase by 2100 across all scenarios.
  • Current Area: ~132,000 km².
  • 2100 Projections:
    • SSP1-2.6: ~148,400 km²
    • SSP2-4.5: ~155,600 km²
    • SSP5-8.5: ~173,400 km² (largest expansion).

4. Key Contributions

1. First Climate Modeling for Regional Skate: This is the first study to model climate-driven habitat shifts for an endangered skate in the southeastern Pacific.
2. Quantification of Net Expansion: Contrary to the common narrative of climate-induced habitat loss for marine species, this study identifies a net gain in suitable habitat for D. chilensis due to the availability of cooler, oxygen-rich waters in southern Chile.
3. Identification of Critical Drivers: Confirms that thermal limits and oxygen availability are the primary constraints on the species' distribution, highlighting the physiological sensitivity of oviparous elasmobranchs to warming.
4. Methodological Rigor: Demonstrates the importance of correcting for sampling bias and using null models to validate SDM performance in data-limited marine environments.

5. Significance and Implications

• Conservation Paradox: While the projected expansion of habitat suggests a potential "climate refuge" in southern Chile, the authors caution that this does not equate to reduced extinction risk. The species remains Endangered due to its life-history traits (late maturity, low fecundity) and continued fishing pressure.
• Management Recommendations:
  • Dynamic Fisheries Management: Static quotas and protected areas may become ineffective as the species shifts poleward. Management must adapt to track the moving distribution.
  • Southern Refugia Protection: The emerging habitats in the Channels and Fjords of Southern Chile require immediate protection (e.g., Marine Protected Areas) to prevent "fishing the range shift."
  • Synergistic Threats: The study underscores that climate change acts synergistically with overfishing; without addressing fishing mortality, the species may not be able to colonize new suitable habitats effectively.
• Policy Impact: The findings provide a scientific basis for updating Chilean fisheries regulations to be climate-resilient, ensuring the long-term persistence of this emblematic species.