Thresholds in Caribbean coral reefs: implications for ecosystem‐based fishery management

Abstract

Summary1. Ecosystem-based management of coral reef fisheries aims to sustainably deliver a diverse portfolio of ecosystem services. This goal can be undermined if the ecosystem shifts into a different state, with altered ecosystem functions and benefits to people. If levels of drivers that cause transitions between states are identified, management measures could be aimed at maintaining drivers below these levels to avoid ecosystem shifts. 2. Analysing data from a large number of Caribbean coral reefs (N = 2001), suites of nonlinear thresholds were identified between metrics of coral reef processes and structure along a gradient of total fish biomass (a proxy for fishing pressure). Several metrics (macroalgal cover, invertivorous fishes and fish species richness) associated with coral-dominated reefs exhibited thresholds at relatively high fish biomass levels (50-88% of unfished biomass). Other metrics (urchin biomass, ratio of macroalgal to coral cover, herbivorous fishes and coral cover) showed thresholds at lower fish biomass levels (28-37% of unfished biomass). 3. Ratios of total fish biomass in fishing areas to closed areas (unfished biomass) in the Caribbean indicate that reefs may generally be at risk for change at ratios between 0Á5 (coral dominated) and 0Á3 (macroalgal dominated). Similar relationships were found for coral reefs in the Indian Ocean. While these results illustrate thresholds at the scale of the entire Caribbean, assessing local reefs is advisable because biomass levels vary within the region, and reef trajectories depend on past, present and future local conditions. 4. Synthesis and applications. If the thresholds in this study are generalizable to scales relevant to management, it may be possible to produce sustainable yield while simultaneously maintaining coral-dominated reefs by restricting fishing mortality to levels that result in biomass ratios near 0Á5. Fishing down to biomass ratios near 0Á3 may increase the risk of overfishing (resulting in lower long-term yields) and transition to macroalgal-dominated reefs. Thresholds offer a simple and powerful way for managers to operationalize precautionary ecosystem-based fishery management by adaptively limiting fishing pressure in order to (i) maintain desirable coral reef conditions, (ii) establish a system-specific target for generating pretty good yield and (iii) maintain sustainable multi-species fishery yields.

Using Ecological Thresholds to Inform Resource Management: Current Options and Future Possibilities

“…Using these ecosystem features as a guide, practitioners can develop an integrated understanding of how drivers are likely to affect key features of the ecosystem and determine whether increasing or maintaining the type and intensity of pressures is likely to result in threshold behavior, and evaluate the relative impact of alternative management actions by being cognizant of which pressures drive ecosystem shifts, which are controllable, and how quickly a system may react to a change in drivers. For example, scientists and managers are currently developing ecosystem indicators for Caribbean coral reefs based on threshold responses of the ecosystem to overfishing (Karr et al, 2015). By analyzing monitoring data from a large number of sites that span a gradient of fishing intensity and reef condition, Karr et al found that lower fish biomass was correlated with several other ecosystem condition variables, including decreased fish diversity and coral cover, and increased macroalgal cover.…”

“…When fish biomass fell below the first threshold (50% of mean unfished biomass) they observed decreased fish species richness and increased macroalgal cover. When fish biomass fell below the second threshold (30% of mean unfished biomass) additional changes were detected, including reduced coral cover and urchin and herbivorous fish abundance and higher ratios of macroalgae to coral (see Figure 3 in Karr et al, 2015), indicative of a phase shift to algal dominance. Importantly, fish biomass thresholds and indicators were robust across scales (i.e., similar patterns are seen at local and regional scales) and can be used in data-limited systems.…”

“…Yet ignoring them is risky. Recent syntheses show that non-linear responses are common (Burkett et al, 2005;Karr et al, 2015;Hunsicker et al, in press;Kappel et al, in review), and examples from around the world show that incorporating information about threshold behavior into resource management can facilitate improved management outcomes (Kelly et al, 2014). Major advances in threshold science provide new opportunities for practitioners to employ a wide range of methods to examine the dynamics of ecosystems before and after they cross thresholds (Wang et al, 2012;Kéfi et al, 2014), monitor ecosystem indicators that provide early warning of impending change (Samhouri et al, 2009;Tomczak et al, 2013), and uncover the mechanisms and drivers responsible for ecosystem shifts (Scheffer et al, 2009).…”

The Dynamics and Resilience of Coral Reefs

“…Although precariousness has been suggested in the wider ecological literature Hodgson, McDonald & Hosken 2015), it has not been specifically measured for coral reefs. While the locations of unstable equilibria and thresholds have only been estimated for a very limited range of reefs, an interim approach might be to use the distance between a reef's current state and the 'tipping points' identified for some Indian Ocean and Caribbean reefs for average system state against fish biomass (McClanahan et al 2011;Karr et al 2015). One advantage is that precariousness does not require the user to undertake modelling themselves.…”

“…Therefore, there remains a need for studies to identify the proximate drivers of reef dynamics across a diversity of environments (Chollett et al 2012). Some of this can be achieved through globally-replicated experiments (Steneck, Arnold & Mumby 2014; and regional analyses of bivariate relationships (McClanahan et al 2011;Karr et al 2015), but it is critical that key processes are disaggregated and not lost as confounding effects in broad-scale studies ).…”