Management strategies need to incorporate more process-oriented measures when considering where to protect tropical reefs and how 24,42. The decoupling of productivity, a dynamic ecosystem rate, from standing biomass provides new opportunities to improve and adapt conservation strategies 35. We provide a new framework, accounting for both fish standing biomass and biomass productivity, to better guide management strategies on tropical reefs. We find that highly productive sites are usually located in areas of intermediate human pressures with high primary productivity. In contrast, degraded sites with both low biomass and productivity are primarily located in areas with high human pressure, as well as low primary productivity and lower temperature. However, almost every class was found in all sampled geographical areas (Fig. 3). No area was characterized by only “low biomass/low productivity” sites, or “high biomass” sites. Finally, sites exhibiting high fish biomass are mainly located in areas of low human pressure but high primary productivity and warmer waters. Yet, socio-economic context was highly variable through all management classes we explored.
The “high biomass” class is characterized by both high primary productivity and low gravity, confirming their status of intact or quasi-pristine reefs with few anthropogenic pressures. We urgently need to fully protect these sites as no-entry/no-take MPAs to allow the maintenance and reproduction of large individuals. In fact, high biomass areas are key in order to achieve long-term conservation of fish populations as they probably make important and irreplaceable sources of larvae and juveniles allowing the replenishment of depleted stocks on surrounding areas 43,44. Indeed, given the right conditions, accumulation of commercial species inside such well-protected MPAs can benefit fisheries through adult and larval-spillover 22,45−48.
In contrast, conservation efforts in “low biomass/productivity” sites, characterized by high human pressure and low primary productivity, may lead to a waste of time and money as these unproductive low-resilient sites may take long periods of time to eventually recover 18. Furthermore, local fisheries may not benefit from these reefs given poor standing biomass, productivity and reproduction potential. In a world where conservation operates on a tight budget 49, strategic placement of MPAs must avoid conservation failures which may lead to an erosion of trust, increasing risk of conflicts 23,50,51 and hindering future conservation endeavors 52–54, but see 55,56. As an alternative to fisheries, seaweed and bivalve aquaculture can achieve both socio-economic and ecological outcomes by reducing nutrient inputs and limiting ocean acidification but also provide food security for local communities 57,58.
The “high productivity” sites are usually located in areas with intermediate to high human pressure and high primary productivity (Fig. 2). Fishing pressure could thus remove large species contributing to high biomass and therefore increase biomass productivity boosted by small species. Furthermore, this concurs with studies showing that reef fish biomass is lowest at intermediate levels of economic development 27,59. These sites are also characterized by high primary productivity, which is consistent with previous studies demonstrating the importance of primary productivity on fish productivity on tropical reefs 34,60. The widespread degradation of tropical reefs is altering species composition and ecosystem functioning but giving birth to novel ecosystems 61–63. However, counterintuitively, several studies report that fisheries catches have maintained or even increased in these degraded sites 64–67. These artisanal fisheries are mostly sustained by an increase in herbivore fish biomass such as parrotfish (Scaridae) or rabbitfish (Siganidae). High productivity has been identified as a compensatory ecological mechanism, whereby fish populations characterized by high growth rate such as herbivores positively respond to size-selective fishing and biomass collapse through increased productivity 35. However, this compensatory mechanism may only be short term as intensive degradation may lead to a decrease in herbivore biomass 68. Therefore, in the face of future degradations, accounting for and monitoring reef fish biomass productivity is crucial to understand the response of tropical reefs to degradation and choose adapted management strategies.
Given the highly variable socio-economic context of these productive sites (Fig. 3), our framework suggests adaptive management, where protection intensity needs to be set according to local particularities. When gravity and dependence on marine ecosystems are high, the priority is to place local restrictions (industrial activities, destructive fishing practices, agricultural run-off...) on these sites through the establishment of partially protected MPAs or Other Effective Conservation Measures (OECMs) that can still sustain local fisheries. OECMs may be of particular relevance here given their importance for Indigenous and Community Conserved Areas (ICCAs) and ecosystem-based management 69–72.
In contrast, where human gravity and marine ecosystem dependence are low, placing no-take protected areas could be appropriate given their high recovery potential. Recent studies have warned about the “red herring” effect of certain partial MPAs that give the illusion of conservation without any real ecological or even social benefits 29. In certain settings, extractive activities can even be higher inside partially protected areas than outside 73. Furthermore, partial MPAs usually perform poorly in terms of ecological outcomes 74–76. However, their socio-economic outcomes, such as increased awareness or education, are mainly positive 29,77−79. This type of management may enable traditional fishing practices and improve food security of local communities 23,75,77,79,80. We argue that these protection measures should only be prioritized where dependency on reefs is high and when exclusion of all extractive activities is not feasible. Partial protection seems effective only when placed adjacent to fully protected areas and when social targets are clearly defined 76. Full protection should always be prioritized, when possible, given the benefits on biodiversity and fish biomass but also on local fisheries. Furthermore, a mix between full protection on undisturbed reefs and disturbed reefs may favor larval dispersal and fisheries yields 45,47. Access restriction and no-take areas are not mutually exclusive 81,82. Instead, they can be combined, whereby access restrictions can act as the incentive for establishing no-take areas when communities might not otherwise be willing to give-up areas for conservation 79,83.
According to our thresholds, “mid-range” sites do not fit in any of the management classes and neither exhibit high productivity nor high biomass. Here, management should focus on avoiding the shift toward the ecological trap of “low biomass/productivity”. For instance, expanding existing MPAs, in particular when targeting areas with high biomass, has been shown to exceed the cost of management 84. “Mid-range” sites where human gravity and dependence are low could therefore be included in existing sanctuaries through expansion. If well managed, these sites may still be able to partially deliver the goods and services needed during this new transitional period.
We acknowledge that our defined classes are based on thresholds that are, to some extent, arbitrarily chosen. Yet, these thresholds are necessary, in order to make a first step to incorporate process-oriented measures such as biomass productivity in the management of tropical reefs. However, this framework is not an end in itself, and should be tailored to each local context and its particularities.