Lag Effects in the Impacts of Mass Coral Bleaching on Coral Reef Fish, Fisheries, and Ecosystems

Graham, N. A. J., et al., 2007. Conservation Biology

Original research (primary data)
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Recent episodes of coral bleaching have led to wide-scale loss of reef corals and raised concerns over the effectiveness of existing conservation and management efforts. The 1998 bleaching event was most severe in the western Indian Ocean, where coral declined by up to 90% in some locations. Using fisheries-independent data, we assessed the long-term impacts of this event on fishery target species in the Seychelles, the overall size structure of the fish assemblage, and the effectiveness of two marine protected areas (MPAs) in protecting fish communities. The biomass of fished species above the size retained in fish traps changed little between 1994 and 2005, indicating no current effect on fishery yields. Biomass remained higher in MPAs, indicating they were effective in protecting fish stocks. Nevertheless, the size structure of the fish communities, as described with size-spectra analysis, changed in both fished areas and MPAs, with a decline in smaller fish (<30 cm) and an increase in larger fish (>45 cm). We believe this represents a time-lag response to a reduction in reef structural complexity brought about because fishes are being lost through natural mortality and fishing, and are not being replaced by juveniles. This effect is expected to be greater in terms of fisheries productivity and, because congruent patterns are observed for herbivores, suggests that MPAs do not offer coral reefs long-term resilience to bleaching events. Corallivores and planktivores declined strikingly in abundance, particularly in MPAs, and this decline was associated with a similar pattern of decline in their preferred corals. We suggest that climate-mediated disturbances, such as coral bleaching, be at the fore of conservation planning for coral reefs.

Case studies

Basic information

  • Case ID: INT-128-1
  • Intervention type: Protection
  • Intervention description:

    Marine protected areas

  • Landscape/sea scape ecosystem management: Yes
  • Climate change impacts Effect of Nbs on CCI Effect measures
    Loss of food production  No effect Change in log abundance of individual size classes of fishery target species, biomass of target species
  • Approach implemented in the field: Yes
  • Specific location:

    The inner granitic islands of the Seychelles lie on the Mahe ́ Plateau --> Sainte Anne Marine National Park and Cousin Island Special Reserve

  • Country: Seychelles
  • Habitat/Biome type: Coral reefs |
  • Issue specific term: Not applicable


  • Notes on intervention effectivness: Effectiveness determined by comparing to fished/unprotected [control] sites While MPAs have higher biomass than non-fished areas (because of effective fishing restrictions), MPAs are not effective in sustaining the resilience of fish communities (including fishery targeted species) in areas that are affected by bleaching. So MPAs are effective in reducing fishing pressure, but they are not able to sustain the size structure of fish communities over time which is driven by a loss in coral reef structural diversity due to bleaching. When looking at the size structure of fish communities shows that there is an increase in large fish size classes brought about by loss of fish through natural mortality and fishing and the loss of reef structural complexity Mixed results for ecosystem because MPAs appear to benefit in the short term but they have similar declines in smaller size classes (including juveniles) and even greater declines in some functional groups
  • Is the assessment original?: Yes
  • Broadtype of intervention considered: Not applicable
  • Compare effectivness?: No
  • Compared to the non-NBS approach: Not applicable
  • Report greenhouse gas mitigation?: No
  • Impacts on GHG: Not applicable
  • Assess outcomes of the intervention on natural ecosystems: Yes
  • Impacts for the ecosystem: Mixed
  • Ecosystem measures: Structural complexity of the benthos, change in size spectra of fish communities, abundance of different functional feeding groups
  • Assess outcomes of the intervention on people: No
  • Impacts for people: Not reported
  • People measures:
  • Considers economic costs: No
  • Economic appraisal conducted: No
  • Economic appraisal described:
  • Economic costs of alternative considered: No
  • Compared to an alternative: Not reported

Evaluation methodology

  • Type of data: Quantitative
  • Is it experimental: Yes
  • Experimental evalution done: In-situ/field
  • Non-experimental evalution done: Not applicable
  • Study is systematic: