Vulnerability of Coral Reef Fisheries to a Loss of Structural Complexity

Abstract

Coral reefs face a diverse array of threats, from eutrophication and overfishing to climate change. As live corals are lost and their skeletons eroded, the structural complexity of reefs declines. This may have important consequences for the survival and growth of reef fish because complex habitats mediate predator-prey interactions [1, 2] and influence competition [3-5] through the provision of prey refugia. A positive correlation exists between structural complexity and reef fish abundance and diversity in both temperate and tropical ecosystems [6-10]. However, it is not clear how the diversity of available refugia interacts with individual predator-prey relationships to explain emergent properties at the community scale. Furthermore, we do not yet have the ability to predict how habitat loss might affect the productivity of whole reef communities and the fisheries they support. Using data from an unfished reserve in The Bahamas, we find that structural complexity is associated not only with increased fish biomass and abundance, but also with nonlinearities in the size spectra of fish, implying disproportionately high abundances of certain size classes. By developing a size spectrum food web model that links the vulnerability of prey to predation with the structural complexity of a reef, we show that these nonlinearities can be explained by size-structured prey refugia that reduce mortality rates and alter growth rates in different parts of the size spectrum. Fitting the model with data from a structurally complex habitat, we predict that a loss of complexity could cause more than a 3-fold reduction in fishery productivity.

Quantifying Multiscale Habitat Structural Complexity: A Cost-Effective Framework for Underwater 3D Modelling

“…In the absence of having access to an alternative and method proved highly accurate to recreate the three-dimensional structure of the reef, here we decided to use traditional metrics as proxy values for reef complexity (e.g., rugosity, substrate height). Although highly useful to understanding ecological processes and patterns [11,70,71], these metrics also introduce human error and noise [38] and, therefore, this variability will be reflected on the accuracy metrics. Therefore, the accuracy values here reported (79%-82%) for 3D reconstructions at reefscape scales reflects high fidelity of model estimates to traditional and ecologically relevant metrics of reef complexity, while using less detailed imaging techniques than the colony-scale exercise (e.g., downward facing video sequences from scooters or diver collected data from lawn-mowing patterns).…”

“…The relationship between coral reef structural complexity on key ecological processes, such as herbivory and predation, as well as fisheries productivity could be quantified, monitored, and predicted if this framework was adopted by a wide range of scientists [71,79]. Coral reefs face multiple threats, resulting in a decrease of live coral and, consequently, a decline in habitat structural complexity [7,77].…”

Tradeoffs between fisheries harvest and the resilience of coral reefs

“…Examples include kelp habitats in New England (42) and Tasmania (43) where overfishing drives algal deforestation by depleting predators of herbivorous urchins. Failure to prevent ecosystem overfishing will lead to a long-term reduction in habitat quality, which in turn will reduce biodiversity and the productivity of the fishery for future generations (44).…”

Accuracy and Precision of Habitat Structural Complexity Metrics Derived from Underwater Photogrammetry

“…Biogenic habitats provided by organisms such as oysters, mangroves, seagrass, macroalgae, and corals are critical to the integrity of their resident assemblages ( [1] and references therein). This is well studied in coral reef habitats where complexity is strongly related to important aspects of ecosystem function and service including coral cover, grazing pressure, fish density and biomass, and even tourism and shoreline protection [5][6][7].…”