Marine protected areas are more effective but less reliable in protecting fish biomass than fish diversity

Highlights

• We develop the Conservation Status Index to test the efficiency of marine protection.

• The efficiency of marine protection depends on the ecoregion.

• The CSI_biomass shows the major efficiency in protected areas.

• The biomass of threatened species presents the highest contribution to the efficiency.

Abstract

Marine protected areas (MPAs) provide multiple conservation benefits, thus raising the question of how good and consistent they are at their roles. Here, we quantified three components, namely, diversity, biomass, and other relevant variables, in numerous protected and unprotected areas across four marine ecoregions in south-western Europe. We created a “global conservation status index” (CSI_global) as the sum of CSI_diversity, CSI_biomass, and CSI_relevant. We then tested whether CSI and its three components varied as a function of protection and marine ecoregion. MPA efficiency, defined as the effect size of protection on CSI_global, was unreliable and varied with geography. CSI_biomass and CSI_relevant contributed to the unreliability of MPA efficiency, while CSI_diversity was reliable. CSI_biomass showed the major efficiency in protected areas (60%). Biomass of threatened species was the single largest variable that contributed to MPA efficiency. Our easy-to-use approach can identify high- and low-efficient MPAs and help to clarify their actual roles.

Introduction

Marine protected areas (MPAs) are one of the main management tools for the current human-driven biodiversity crisis. With increasing anthropogenic pressures, MPAs are essential to preserve natural resources, biodiversity, and ecosystem properties (Micheli et al., 2012; Hilborn, 2016; Campbell et al., 2017). MPAs have steadily increased in the last decades to >5% of coastal areas under national jurisdictions and <1% of the high seas (Spalding et al., 2013) figures that keep on increasing with the establishment of some large MPAs, particularly in tropical waters (Devillers et al., 2015). MPAs are pivotal tools for coastal fisheries management and biodiversity conservation (Edgar et al., 2014). Yet, only 10% of the MPA surfaces are no-take zones, free of extraction, or habitat alteration activities, while 94% of MPAs allow fishing and other activities (Thomas et al., 2014; Costello and Ballantine, 2015; Campbell et al., 2017). Certainly, MPAs include a high range of areas, designs, uses, and management goals (Al-Abdulrazzak and Trombulak, 2012; Edgar et al., 2014; Pérez-Ruzafa et al., 2017), which could result in many benefits and varying degrees of protection efficiencies.

Besides MPAs, additional protection measures have been taken to contribute to biodiversity conservation, protection of threatened species, and restoration of fish stocks, including national parks, marine sanctuaries, natural parks, or natural monuments (Al-Abdulrazzak and Trombulak, 2012). Moreover, areas such as military zones can offer protection because of strong surveillance, highly restrictive access, and ban of extractive activities. All these protection measures can lead to the recovery of natural resources and other positive effects on natural communities (Russ et al., 2005; Weeks et al., 2010; Campbell et al., 2017).

The benefits of protection in marine communities are abundant, mostly focused on traits associated with diversity, biomass, or other relevant aspects related to protection. Protected areas are associated with larger species richness (Wantiez et al., 1997; Ciriaco et al., 1998; Edgar and Barrett, 1999; Barrett et al., 2007), larger trophic diversity (Shears and Babcock, 2003; Harmelin-Vivien et al., 2015), and larger functional diversity (Stelzenmüller et al., 2009; Villamor and Becerro, 2012; Guilhaumon et al., 2015) than unprotected areas. Protection also triggers an increase in fish biomass, particularly of commercial fish species (Barrett et al., 2007; Fenberg et al., 2012; Parravicini et al., 2014; Pérez-Ruzafa et al., 2017), likely as a result of decreased fishing pressures (e.g., larger biomass of fish over 20 cm; Edgar and Barrett, 1999; Barrett et al., 2007; Stuart-Smith et al., 2017). However, the biomass of other groups of species such as fish species in the IUCN Red List (Nieto et al., 2015) has received less attention (Willis et al., 2003; Afonso et al., 2011; Harmelin-Vivien et al., 2015). Other relevant variables unevenly used to assess the effects of marine protection include abundance of higher carnivores (Cole, 1994; Harmelin et al., 1995), vulnerability of fish community (Cheung et al., 2007; Stuart-Smith et al., 2017; Vasconcelos et al., 2017), and fish size (Shears and Babcock, 2003; Sciberras et al., 2013). Overall, most available evidence supports for a positive effect of protection on all these traits, providing ample MPA benefits in terms of fish diversity, fish biomass, and relevant traits of the fish community.

The variability in MPA traits and benefits also points toward the possibility that MPAs may be inconsistent in their multiple roles, leading to varying degrees and contrasting levels of efficiency (Dichmont et al., 2013; White et al., 2014). In fact, the positive effects of protection are unevenly spread across MPAs, and numerous studies fail to provide evidence for the expected beneficial effects of protection. Literature on the so-called paper parks provides ample evidence that MPAs can be inefficient (Bustamante et al., 2014; Gallacher et al., 2016) due to multiple factors (Rife et al., 2013; Edgar et al., 2014). Often, the protection effects of MPAs, e.g., increase in species richness or abundance, are noticeable after sufficiently long periods together with suitable control sites (Stobart et al., 2009; Chirico et al., 2017). Moreover, many studies that assessed MPA efficiency have focused on one rather than several benefits (McClanahan et al., 1999; Vanderklift et al., 2013), which could account for differences associated with the specifics of each benefit, MPA, or geographic region investigated (Caveen et al., 2015; Fletcher et al., 2015; Hughes et al., 2016).

Studies examining the effects of protection on multiple benefits over large geographic scales can provide opportunities to advance our understanding on how good MPAs are at achieving their multiple benefits and how reliable MPAs are at providing such benefits. In this study, we followed this approach to shed some light on the relationship between protection and their benefits. We used fish communities across south-western Europe to investigate how fish diversity, fish biomass, and other relevant protection-related benefits contributed to the overall differences between fish communities in protected and unprotected areas and tested whether these benefits were consistent or varied as a function of geography. We investigated >20 protected sites distributed in four marine ecoregions of the world (Spalding et al., 2007) in the Atlantic-Mediterranean confluence area. We used species richness, trophic diversity, and functional diversity to evaluate MPA benefits on fish diversity; biomass of commercial fish, biomass of large fish, and biomass of threatened fish species to evaluate MPA benefits on fish biomass; and fish vulnerability, fish size, and abundance of higher carnivores as other MPA benefits on fish communities. Our results showed evidence for a small but consistent protection effect on fish diversity as opposed to larger and geographically variable protection effects on biomass and other relevant variables that resulted in unreliable MPA efficiency in our study area.