High invasion risk of non-native fishes in the lower Tigris Basin (south-west Iran) with special reference to Shadegan International Wetland

An integrated risk assessment and species distribution modeling study reveals that 15 non-native fish species, particularly tilapias, pose a high invasion risk to the Shadegan International Wetland and lower Tigris Basin, with climate change potentially expanding their suitable habitats and underscoring the urgent need for coordinated transboundary management.

The Gist

Imagine the Shadegan International Wetland in southwest Iran as a bustling, ancient city. It's a vital hub for nature, home to unique local residents (native fish) and a critical source of food and water for the people living nearby. But recently, this city has been facing an influx of new, uninvited guests: non-native fish brought in by humans, either by accident or on purpose.

This paper is like a security audit conducted by a team of expert detectives to figure out which of these new guests are just harmless tourists and which ones are dangerous criminals planning to take over the city.

Here is the breakdown of their investigation, explained simply:

1. The Suspects (The 15 Fish)

The researchers looked at 15 different types of fish that have already entered the wetland. Some came in because people released them from pet tanks (like the Sailfin Molly), others were accidentally washed in from neighboring countries, and some were intentionally brought in to boost the local fish market (like the Common Carp).

2. The Security Check (The Risk Assessment)

To judge these fish, the team used a high-tech "invasion scanner" called AS-ISK. Think of this like a background check for a job applicant, but instead of asking about work experience, it asks questions like:

• "Can this fish survive in our hot, salty water?"
• "Does it eat the local food that native fish need?"
• "Does it have babies faster than the locals?"
• "Will it survive if the climate gets even hotter in the future?"

They gave each fish a risk score.

• Low Score: "Safe, just a tourist."
• Medium Score: "Keep an eye on them."
• High Score: "Danger! They might take over."

3. The Results: Who is the Biggest Threat?

The audit revealed that most of the new fish are dangerous.

• 13 out of 15 were flagged as High Risk.
• The top three "super-invaders" are all types of Tilapia (Redbelly, Blue, and Nile). These fish scored the highest, meaning they are tough, reproduce quickly, and can eat almost anything. They are like the "kingpins" of the invasion.
• Even the Goldfish and Prussian Carp (often seen as cute pets) were rated as high-risk because they can hybridize with native fish and ruin the local ecosystem.

4. The Climate Change Factor

The researchers also asked: "What happens if the weather gets hotter, as scientists predict?"

• For the Tilapias: The heat is like a superpower. Warmer water makes them even stronger and more likely to spread. Their risk score went up.
• For the Carp and other farmed fish: The heat is actually bad for them. If the water gets too hot or dries up, these fish might struggle to survive. Their risk score went down.

5. The Crystal Ball (Species Distribution Modeling)

The team used a "crystal ball" (computer modeling) to predict where these dangerous fish will go next.

• They found that the Tilapias aren't just sticking to one spot; they are likely to spread across the entire lower Tigris Basin.
• The maps showed that these fish are much more comfortable in the local environment than we thought, suggesting they will expand their territory rapidly if nothing stops them.

The Bottom Line: Why Should We Care?

The Shadegan Wetland is a Ramsar site, which is like a "World Heritage Site" for nature. It's home to rare, native fish that exist nowhere else on Earth. If the invasive Tilapias take over, they could push these unique local fish to extinction, just like a loud, aggressive crowd drowning out a quiet, precious conversation.

The Takeaway:
The paper sounds an alarm. We can't just wait and see. The authors are calling for a transboundary police force (cooperation between Iran, Iraq, and neighbors) to:

1. Watch the borders (stop new fish from entering).
2. Catch the criminals early (detect them before they spread).
3. Remove them (control the populations that are already there).

Without action, this beautiful wetland could lose its unique character and become a monoculture of invasive fish, hurting both nature and the local people who depend on it.

Technical Summary

1. Problem Statement

The study addresses the critical threat posed by biological invasions in freshwater ecosystems, specifically within the lower Tigris Basin in south-west Iran. This region, centered around the Shadegan International Wetland (SIW)—a Ramsar site and biodiversity hotspot—is under severe anthropogenic pressure from climate change, dam construction, and hydrological alteration.

While 15 non-native freshwater fish species have been documented in the area, there has been a lack of comprehensive, multi-species risk assessments. Previous management efforts have been reactive rather than proactive, and no systematic eradication or control measures have been implemented. The study aims to fill this knowledge gap by quantifying the invasiveness potential of these species under both current and projected future climate conditions to inform evidence-based biosecurity policies.

2. Methodology

The researchers employed a dual-methodological approach combining structured risk screening with predictive species distribution modeling.

A. Study Area

• Location: Shadegan International Wetland (SIW) and the catchments of the Jarrahi and Karun rivers (30°–32° N, 48°–50° E).
• Climate: Hot desert climate (BWh), with projections indicating significant warming trends.

B. Risk Screening (AS-ISK)

• Tool: The Aquatic Species Invasiveness Screening Kit (AS-ISK v2.4.1), a decision-support tool compliant with EU Regulation No. 1143/2014.
• Species Assessed: 15 extant non-native species, including Coptodon zillii, Oreochromis aureus, Oreochromis niloticus, Cyprinus carpio, Carassius auratus, and Gambusia holbrooki, among others.
• Components:
  1. Basic Risk Assessment (BRA): Evaluates the risk of introduction, establishment, dispersal, and impact under current conditions.
  2. Climate Change Assessment (CCA): Modifies the BRA outcome based on projected future climatic conditions (warming).
• Scoring: Scores were categorized using a generalized threshold of 14.7 (Low: <14.7; Medium: 14.7–72/82; High: >72/82). Confidence factors (CF) were calculated for all assessments.
• Statistical Analysis: Permutational ANOVA was used to test differences in confidence factors between BRA and BRA+CCA outcomes.

C. Species Distribution Modeling (SDM)

• Algorithm: MaxEnt (Maximum Entropy) was used to predict potential distributions.
• Data Inputs:
  • Occurrence records from both native and non-native ranges.
  • Environmental layers: Mean annual temperature, mean annual precipitation, Digital Elevation Model (DEM), slope, and flow accumulation.
• Validation: 75% of data used for training, 25% for testing (k-fold cross-validation). Model performance was evaluated using the Area Under the Curve (AUC).

3. Key Results

A. Risk Screening Outcomes

• BRA Results: 13 out of 15 species (86.7%) were ranked as High Risk, and 2 as Medium Risk.
• BRA+CCA Results (with Climate Change):
  • High Risk: 11 species (73.3%).
  • Medium Risk: 3 species (20.0%).
  • Low Risk: 1 species (6.7%).
• Top Scoring Species: The three highest-risk species were Redbelly Tilapia (Coptodon zillii), Blue Tilapia (Oreochromis aureus), and Nile Tilapia (Oreochromis niloticus). All three scored well above 40 in both BRA and BRA+CCA assessments.
• Climate Change Impact:
  • Increased Risk: For warm-tolerant species (e.g., Tilapias, Gambusia holbrooki, Heteropneustes fossilis), risk scores increased or remained high under future warming scenarios.
  • Decreased Risk: For cold-intolerant or farmed carp species (e.g., Ctenopharyngodon idella, Hypophthalmichthys spp.), risk scores decreased. The authors attribute this to projected water shortages and temperatures exceeding the thermal tolerance limits of these species, potentially reducing aquaculture propagule pressure.

B. Species Distribution Modeling

• Performance: The MaxEnt models for the top three species showed excellent predictive performance (Mean AUC = 0.82 ± 0.09).
• Key Predictors: Mean annual temperature and slope range were the primary drivers of distribution.
• Distribution Shifts: Models based on non-native occurrence data predicted a substantially wider potential distribution across the lower Tigris Basin compared to models based solely on native range data. This suggests these species have already adapted to broader environmental tolerances in their introduced ranges.

C. Confidence Factors

• The mean confidence factor for the screening was high (Total CF = 0.823).
• Confidence was significantly higher for the BRA (0.833) than for the CCA (0.742), indicating greater uncertainty in predicting specific climate-change-mediated impacts.

4. Key Contributions

1. First Multi-Species Appraisal: This is the first comprehensive, multi-species invasion risk assessment for the lower Tigris Basin.
2. Integration of Climate Projections: The study explicitly integrates future climate scenarios into risk screening, revealing that warming will likely exacerbate the threat of thermophilic invasive species (Tilapias) while potentially suppressing others.
3. Methodological Validation: The study demonstrates the utility of combining AS-ISK with MaxEnt modeling to provide a robust, evidence-based framework for prioritizing management actions in transboundary wetlands.
4. Identification of Priority Species: It definitively identifies the three Tilapia species as the highest priority for management, highlighting their capacity for rapid spread and ecological dominance.

5. Significance and Implications

• Ecological Threat: The findings indicate a high likelihood of continued spread and severe ecological impact on the SIW, threatening endemic species such as Arabibarbus grypus and Mesopotamichthys sharpeyi.
• Policy and Management: The results provide a scientific basis for:
  • Prioritization: Focusing monitoring and rapid response efforts on the three high-scoring Tilapia species.
  • Transboundary Governance: Emphasizing the need for coordinated management across Iran, Iraq, and other neighboring countries sharing the Tigris-Euphrates watershed, as hydrological connectivity facilitates the spread of invasive species.
  • Preventive Action: Strengthening regulations on aquaculture, the ornamental fish trade, and recreational fisheries to prevent further introductions.
• Conservation Urgency: The study underscores that without coordinated intervention, the degradation of this internationally important wetland system will accelerate, compromising its biodiversity and the socio-economic livelihoods dependent on it.