Are MPAs effective in removing fishing pressure from benthic species and habitats?

Abstract

MPAs are expected to improve the conservation status of rare and important habitats and species. However, the reduction in anthropogenic pressure due to such management measures is rarely estimated. Although MPA networks may cover a large proportion of the seabed, designated areas that prohibit damaging fishing activity are often much smaller. This case study compares fishing pressure inside and outside areas covered by Scottish MPAs and MPA management measures, further relating its distribution to bottom ruggedness, which influences benthic communities and their exposure to mobile bottom fishing. While 7% of the study region was found to be within MPA boundaries, only 2.5% of the region was subject to management measures that restrict mobile bottom fishing. Taking historical levels of fishing as a benchmark, management measures have been applied to <0.6% of the swept area of existing mobile bottom fishing activity. This may be explained by the higher average seabed ruggedness within MPA management measures, which was also where some key species of conservation interest were found. These findings suggest that protection has been focussed in areas that already act as natural refugia for sensitive benthic species and lie away from the majority of fishing activity. While the measures do not reduce fishing pressure markedly, they do protect relatively pristine habitats from future fishing impacts. MPA management measures had higher average seabed ruggedness, suggesting a bias in protection for species and habitats within such areas.

Introduction

For decades, Marine Protected Areas (MPAs) have been designated for the conservation and recovery of marine biodiversity (Roberts et al., 2017; Wells et al., 2016). Although, by definition, the primary aim of an MPA must be nature conservation (Day et al., 2012), they can have other benefits such as improving commercial fish stocks (Kerwath et al., 2013) and promoting tourism (McCook et al., 2010). Various international policies drive MPA designation, most notably the Convention on Biological Diversity (CBD) Aichi Target 11 that calls for 10% of the global coastal and marine area to be effectively conserved by 2010 (CBD, 2010) and, in Europe, the EU Marine Strategy Framework Directive (MSFD) that requires member states to contribute to a coherent and representative network of MPAs. As a result, there has been a rapid increase worldwide in MPA coverage (Lubchenco and Grorud-Colvert, 2015), which at the time of publication stands at 4.8% of the world's oceans (http://www.mpatlas.org/). Both the IUCN (Day et al., 2012) and Aichi Biodiversity Target 11 (CBD, 2010) include qualifiers to emphasise that MPAs should improve conservation status. For example, Aichi target 11 includes elements such as MPAs being ecologically representative and effectively managed (Rees et al., 2018). The effectiveness of MPAs positively correlates with size, age and how much human activity is restricted (Edgar et al., 2014; Giakoumi et al., 2017; Hastings and Botsford, 2003). MPAs fall into various management categories ranging from marine reserves, where all human use and access is strictly limited, to areas where sustainable use of resources is permitted (Day et al., 2012). Approximately 2.2% of the global sea area is included in strongly protected marine reserves (www.mpatlas.org) and, in general, the placement of associated restrictions is biased towards areas with lower levels of extractive activities (Devillers et al., 2015). Hence, it is important to evaluate how MPAs have reduced the pressure on the biodiversity features that they are intended to conserve, rather than area covered.

Fishing, particularly from mobile bottom-contacting gear (hereafter mobile bottom gear), is thought to be the most wide spread anthropogenic pressure impacting benthic environments (Foden et al., 2011; Halpern et al., 2008; Kroodsma et al., 2018). Interactions between mobile bottom fishing gears and the seabed causes the removal of or damage to in-situ biota (Cook et al., 2013; Hiddink et al., 2017; Kaiser et al., 2006). There may also be changes to the structure and function of the benthic community, with fragile epi-faunal species being replaced by mobile scavengers or predators (Kenchington et al., 2007; Tillin et al., 2006) and long-lived species being replaced by short-lived species (Rijnsdorp et al., 2018). The exact physical effects of mobile bottom fishing depend on the types of gear used and the habitats fished. In sedimentary and biogenic habitats the first pass of a trawl can have the greatest impact (Cook et al., 2013; Hiddink et al., 2006), while in rockier habitats the effect can be incremental (Boulcott and Howell, 2011; Hinz et al., 2011). However, removing fishing usually leads to positive biological effects, although the speed and magnitude of a response can vary (Kaiser et al., 2006; Lester et al., 2009). While no-take marine reserves have not always led to increases in density or species richness (Lester et al., 2009), they are particularly relevant to fragile benthic fauna species that are often the focus of conservation efforts (Ardron et al., 2014).

Mobile bottom fishing is not uniformly distributed across the marine environment (Eigaard et al., 2017; Gerritsen et al., 2013; Hintzen et al., 2018) with sand and mud habitats tending to be preferred (Eigaard et al., 2017). Fishing gears are rarely towed across areas of high ruggedness due to potential damage to gear. Such rugged areas may provide a refuge for some contemporarily rare benthic species that historically had a wider distribution (Shepherd et al., 2012; Stirling et al., 2016), as well as species inhabiting reefs and harder ground. Locating MPAs in topographically complex areas is, therefore, unlikely to reduce exposure to fishing pressure. If such areas are predominant in MPA networks, statistics relating to total coverage are liable to overstate the effectiveness of spatial measures at reducing disturbance from towed, bottom-contacting gear. Moreover, such an approach may shift focus away from habitats and species associated with lower ruggedness and limit the ecological representativeness of the network.

As the Aichi target deadline approaches there have been several attempts to evaluate MPA networks. While area targets on the whole are likely to be met, many networks are generally not considered to be representative, adequate, well managed or implemented (Amengual and Alvarez-Beramengastegui, 2018; Jessen et al., 2017; Solandt, 2018). The provision of clear metrics relating to the placement of management measures and potential fishing displacement is paramount to the process of effective marine management, especially where there are strong stakeholder opinions. In Scottish Waters, an extensive network of 217 MPAs has been established to conserve or recover habitats and species that may benefit from area based management (The Scottish Government, 2018). Certain key benthic species and associated habitat were important in the designation of MPA sites. These included a number of species typical of harder grounds such as Northern sea fan (Swiftia pallida) as well as those found in sedimentary habitats such as fan mussel (Atrina fragilis) and ocean quahog (Arctica islandica). Burrowed mud was included because the associated biotopes are vulnerable to damage by trawling for Norwegian lobster (Nephrops norvegicus) and the habitat supports important populations of tall sea-pen (Funiculina quadrangularis) (OSPAR, 2010). While the process of designating Scottish MPAs up to implementation has followed a structured path involving input from stakeholders (Hopkins et al., 2016), there has been no evaluation of the likely effectiveness of measures. In most cases, the conservation action was to monitor the feature (a species or habitat) and implement measures where there was evidence of its deterioration. Therefore, the Scottish network provides a test case in designation and management practises serving the policy drivers behind MPAs.

In the present study we examine the extent to which current MPA designation and management could reduce fishing pressure on benthic species within MPAs in Scottish territorial waters. We compare fishing pressure inside and outside areas covered by MPAs and MPA management measures. As the preferred habitats of benthic species may vary in topographic complexity, we also relate the placement of management measures to seabed topography to identify any potential biases in the network. Specifically, we evaluate whether: (i) MPAs and MPA management measures are likely to reduce the level of fishing activity over benthic habitats, (ii) whether MPAs and MPA management measures are located in topographically complex areas and (iii) if the known locations of benthic species of conservation interest differ in their topographic complexity.