CORAL REEF HEALTH:

Fish as Indicators

Fish as Indicators of Coral Reef Ecosystem Status

We are pleased to share findings from our latest study, Fish as indicators of coral reef ecosystem status, published in Ecological Indicators in September 2025. Led by CORDIO Director Melita Samoilys and Bangor University Researcher Ronan Roche, the study investigated how reef fish can be used as indicators of coral reef ecosystem condition. The research compared fish metrics at both assemblage (whole community) and family levels, examining their relationships with reef benthic condition, structural complexity and human pressure. The findings provide an important contribution to global coral reef monitoring and conservation strategies.

The Study

Coral reefs are critical ecosystems that support biodiversity, food security and coastal protection, yet they are increasingly threatened by climate change and human pressures. Monitoring their health requires indicators that are both ecologically meaningful and feasible to measure. This study sought to test whether reef fish could serve that purpose at both assemblage and family levels, and how well their metrics related to key drivers of reef status.

Methods

The research drew on reef fish and benthic survey data from 77 sites in the western and central Indian Ocean, covering Kenya, Tanzania, Seychelles, Comoros, Madagascar, and Mauritius. Fish were surveyed using underwater visual census methods, recording biomass, abundance, and mean length across the whole assemblage as well as for four focal families: groupers (Epinephelidae), butterflyfish (Chaetodontidae), surgeonfish (Acanthuridae), and parrotfish (Scarinae). These metrics were then analysed against:

  • Benthic cover (coral vs. algal/rubble dominance)
  • Reef rugosity (structural complexity)
  • Human gravity (a measure of human population size and accessibility, used as a proxy for fishing pressure)

This design enabled the team to test how specific fish groups responded to habitat condition versus human pressure, and whether finer-scale family indicators provided more insight than broad assemblage measures.

Click the PDF above to read the full paper.

DOI: https://doi.org/10.1016/j.ecolind.2025.114137

Key Findings

The results showed that while reef fish are valuable indicators, their utility varies by family and metric. Human pressure emerged as the strongest and most consistent driver of fish indicators, while family-level responses provided clearer ecological signals than whole-community measures.

  • Groupers (Epinephelidae): Biomass and abundance reflected benthic condition, while mean length tracked reef structural complexity, making them reliable indicators of habitat quality.
  • Butterflyfish (Chaetodontidae): Biomass was closely tied to live coral cover, confirming their sensitivity as indicators of coral health.
  • Surgeonfish (Acanthuridae): Abundance responded to benthic condition, structural complexity, and human gravity, though without strong specificity. They remain useful for tracking turf-algae dynamics.
  • Parrotfish (Scarinae): Contrary to the widely accepted “parrotfish paradigm,” their indicators were shaped more by human pressure than benthic condition.
  • Assemblage-level indicators: Total biomass was a good indicator of human pressure but not benthic health, while mean trophic level showed mixed and difficult-to-interpret responses.

Recommendations

The study concludes that global monitoring frameworks should move beyond broad assemblage metrics and integrate family-level indicators to sharpen ecological interpretation. These metrics offer greater specificity and complement benthic cover measures, thereby strengthening global reef reporting.

  • Butterflyfish biomass, surgeonfish abundance, and grouper biomass → indicators of benthic condition
  • Grouper mean length → indicator of reef structural complexity
  • Parrotfish biomass or length → indicators of human pressure

Integrating these family-level fish indicators into frameworks such as the Global Coral Reef Monitoring Network (GCRMN) and the IUCN Red List of Ecosystems will provide a more accurate picture of coral reef ecosystem status and support effective conservation under the Kunming–Montreal Global Biodiversity Framework.