Coral-algal phase shifts alter fish communities and reduce fisheries production

Abstract

Anthropogenic stress has been shown to reduce coral coverage in ecosystems all over the world. A phase shift towards an algae-dominated system may accompany coral loss. In this case, the composition of the reef-associated fish assemblage will change and human communities relying on reef fisheries for income and food security may be negatively impacted. We present a case study based on the Raja Ampat Archipelago in Eastern Indonesia. Using a dynamic food web model, we simulate the loss of coral reefs with accompanied transition towards an algae-dominated state and quantify the likely change in fish populations and fisheries productivity. One set of simulations represents extreme scenarios, including 100% loss of coral. In this experiment, ecosystem changes are driven by coral loss itself and a degree of habitat dependency by reef fish is assumed. An alternative simulation is presented without assumed habitat dependency, where changes to the ecosystem are driven by historical observations of reef fish communities when coral is lost. The coral-algal phase shift results in reduced biodiversity and ecosystem maturity. Relative increases in the biomass of small-bodied fish species mean higher productivity on reefs overall, but much reduced landings of traditionally targeted species.

Keywords: Ecopath with Ecosim; Raja Ampat; acidification; bleaching; coral disease; coral reefs; ecosystem-based management; reef fish.

Fig 1

Ecosim mediation functions. Vulnerability of prey vs. mediating group biomass: (A) tuna facilitating small pelagic predation by birds; (B) reef-building coral protection of reef fish and invertebrates; (C) cleaner wrasse symbiosis with large reef-associated fish; (D) sea grass and mangrove protection of juvenile reef fish. x- and y-axes are relative to model baseline values. Reproduced from Ainsworth et al., .

Fig 2

Mediation function boosts algal productivity when coral biomass is low.

Fig 3

Simulation results. (A) Transition from coral-dominated ecosystem (left) to algae-dominated ecosystem (right). Data represent end-points of 20-year simulations. Results are from the conjectural simulations (0%, 10% … 100% coral loss scenarios). Grey lines: biomass changes relative to initialization (2012) biomass; black hatches: reef fisheries productivity in kg C km⁻² yr⁻¹. (B) Biomass changes per trophic level under the 100% coral loss scenario (all species groups). Error bars show the range for species groups within these trophic levels. (C) Fisheries catch by fleet under the 100% coral loss scenario relative to 0% loss scenario. Catch for the year 2032 is compared.

Fig 4

Ecosystem biomass changes in Raja Ampat from 2012 to 2032 using Wilson et al., biomass forcing.