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Active interventions to rehabilitate Caribbean coral reefs : The use of artificial substrates to increase the abundance of herbivorous fish and sea urchins

Hylkema, Alwin

Summary

Coral reefs are one of the most productive and biodiverse ecosystems providing ecosystem services such as fisheries, coastal protection, tourism and recreation. However, these services are threatened, as coral reefs worldwide are degrading because of multiple factors including overfishing, pollution, disease and climate change. The Caribbean region has been particularly affected and average cover of reef-building corals has been reduced by more than 80% since the 1970s.

Turf and macroalgae are the main competitors of corals for space, light and nutrients. Coral larvae need bare substrate or crustose coralline algae allow successful settlement. Turf and macroalgae can hamper coral recruitment by reducing available space and by outcompeting settlers. Herbivorous fish and invertebrates, in particular sea urchins, play a key role on coral reefs and contribute to coral settlement, survival and growth by grazing away encroaching algae and creating bare substrate. In the 1980s, an unknown disease decimated up to 98% of the Diadema antillarum sea urchins that up to then had been a prominent grazer on the reefs. As herbivorous fish were already low in abundance due to centuries of overfishing, Caribbean coral reefs rapidly became covered with algae, reducing coral cover and recruitment.

To reverse the trajectory of degradation in coral cover, active intervention methods should be considered to supplement passive conservation. This dissertation focusses on optimizing two active intervention methods: the deployment of artificial reefs to increase three-dimensional structure at eroded reef sites and the restoration of the sea urchin D. antillarum to increase the grazing pressure on taxa that compete with corals.

Artificial reefs are structures deployed on the sea bottom to mimic one or more ecosystem functions of a natural reef. To provide conservationists, marine park managers and researchers the opportunity to substantiate their choices of artificial reefs interventions, the available knowledge and experiences should be widely shared. The three most common purposes to deploy artificial reefs were 1) to create new dive sites, 2) to perform research and 3) to restore ecosystems. Ship wrecks, reef balls© and piles of concrete construction blocks were the most-often deployed artificial reef structures. As artificial reefs attract part of their marine organisms from surrounding habitats, their exploitation by fishers, without clear management, can adversely affect the fish stocks in the surrounding area and thus counteract any potential ecosystem benefits. To ensure artificial reefs and their fisheries do not negatively affect the surrounding ecosystem, we recommend to include artificial reefs, their fisheries and the surrounding ecosystem into future monitoring studies and to create no-take zones around artificial reefs that are not monitored.

To test the effect of artificial reef design on its habitat function for fish and coral, we compared the early fish colonization, one year after deployment, of three types of artificial reefs: reef balls®, layered cakes and piles of locally obtained basaltic rock. All artificial reef plots showed higher values for fish abundance, biomass and species richness than control plots covered by bare sand, which shows that artificial reefs can locally enhance the fish abundance and diversity. However, the effect differed among artificial reef plots. Fish abundance was 3.8 times higher on the layered cakes compared to the reef balls, while fish biomass was 4.6 times higher. Rock piles had intermediate values. Layered cakes harbored over five times more herbivorous fish compared to reef balls. We hypothesized that higher herbivorous fish abundance would result in a higher grazing intensity, and favorably impact the benthic community including coral recruitment, survival and growth. To test this, we compared the fish assemblage, territorial behavior and grazing intensity at reef balls and layered cakes 2.5 years after deployment. The higher fish biomass on layered cakes during early colonization disappeared during consecutive monitoring events. This might have been due to a higher abundance of territorial fish around the layered cakes: almost four times more chasing behavior was recorded compared to the reef balls. The difference in territorial behavior correlated with a more than five times lower fish grazing intensity on layered cakes. Coral recruitment, survival and growth was low at both reef types. Apparently, even the higher grazing intensity at the reef balls was not sufficient to prevent the development of a high cover of turf algae, which probably reduced coral development. Invertebrate grazing, for example by D. antillarum, should be facilitated to further enhance grazing pressure on turf algae.

Studying settlement rates of D. antillarum larvae can provide insight into the mechanisms constraining the natural recovery of this urchin species. Before the die-off, settlement recorded for Curaçao was high throughout the year and was characterized by multiple settlement peaks. Even though peak settlement rates for St. Eustatius were in the same order of magnitude as in Curaçao before the die-off, overall yearly settlement rates around St. Eustatius were still lower. Of all five collector types tested, bio ball collectors, consisting of plastic balls normally used in aquaculture filters, were the most effective and reproducible method to monitor D. antillarum settlement. High peak settlement rates indicated that there should be sufficient settlement potential for natural recovery of D. antillarum. The fact that very few juvenile and adult D. antillarum were observed on Saba and St. Eustatius, suggests that the larvae do not settle on the natural reef and that recovery of D. antillarum might be limited by the availability of settlement substrate. We proposed a new approach to restore D. antillarum populations which we term assisted natural recovery (ANR). ANR can accelerate succession by providing bio ball streamers that were attached to patch reefs shortly before the settlement season. After half a year, reefs with streamers had significantly higher D. antillarum recruit densities than control reefs. However, the recruit density remained low, possibly due to low post-settlement survival of settlers and high predation pressure on recruits.

Active intervention measures such as outlined above are expected to, next to reducing local and global threats, contribute substantially to the active rehabilitation of Caribbean coral reefs.