Fishing is a leading cause of disturbances in the marine realm with consequences such as trophic cascade (Mumby et al., 2006; Shears et al., 2008) and loss of habitats (Kaiser et al., 2002; Lundquist et al., 2018). Moreover, overfishing in targeted and by-catch fisheries causes population decline in several species of the megafauna, including sharks, seabirds, and marine mammals (Dulvy et al., 2014; White et al., 2017). The global expansion of fisheries to meet the demand for fisheries resource extraction and the continued development of gear technology both have expanded and intensified the activity, with fishing occurring even in the remotest parts of the ocean (Sala et al., 2018).

Globally, fishing activity requires robust management measures to mitigate its impacts on marine biodiversity. Although there are several frameworks concerned with improving fisheries management (e.g., Booth et al., 2020), the designation of marine protected areas (MPAs) is the most applied tool to maintain biodiversity and fisheries at a sustainable level (Lubchenco and Grorud-Colvert, 2015). Several countries, including Brazil, are making compelling cases for historical progress toward achieving the international targets for marine protection under the Convention on Biological Diversity and the Sustainable Development Goals (Friedlander et al., 2016; Magris and Pressey, 2018; Claudet et al., 2021). As a consequence, recent years have also seen an increase in the development of large-scale MPAs (i.e., larger than 100,000 km²) over offshore and deep areas (Boonzaier and Pauly, 2016), following an ad hoc, opportunistic process (O’Leary et al., 2018). However, for these conservation efforts to drive outcomes for biodiversity, they must translate into significant mitigation of human impacts, particularly derived from fishing activity. This requires overcoming the monitoring and enforcement challenges associated with the large-scale governance of dynamic and remote seascapes (De Santo, 2013; Brooks et al., 2019).

Overall, there are several ways of measuring MPA effectiveness, and several frameworks have been proposed (Pomeroy et al., 2005; Pajaro et al., 2010; Zupan et al., 2018a). Percentages of an area under protection (i.e., MPA coverage), although commonly used, are misleading indicators of conservation success (Roberts et al., 2018). Indicators of MPA management effectiveness are intended to show how well MPAs are working towards their objectives (Pajaro et al., 2010), but they are usually evaluated using only managers’ perceptions of good governance and MPA impacts (de Oliveira Júnior et al., 2021). Improvements of ecological conditions, such as the increase in species abundance, are seen as more accurate determinants of MPA effectiveness, but evaluations of offshore MPAs in remote areas are difficult due to data paucity and budgetary constraints related to the development of monitoring programs in such areas (Ban et al., 2017). Finally, a quantitative assessment of how well MPAs can abate the threatening processes provides an alternative, practical assessment of effectiveness (Zupan et al., 2018a) until detailed post-implementation monitoring data have been collected.

Vessel Monitoring System (VMS) data can help fill the gap in effectiveness assessments of large-scale, offshore MPAs and are widely used to evaluate fishing activity (Chang and Yuan, 2014; Delfour-Samama and Leboeuf, 2014; Rowlands et al., 2019). VMS can track vessel movements in near real-time using satellite transponders. Although the system is not tamper-proof (Appleby et al., 2018), it might be the only tool available to assess patterns of fishing activity and provides historical valuable information such as the vessel’s identity, position, and associated fishing gear. These data can thus provide a unique baseline for determining whether MPAs are effective at reducing threats in the absence of other monitoring tools. Here, I used a long-term, large dataset tracking the movements of commercial fishing vessels before and after the two of the largest MPAs in the southern Atlantic Ocean (within Brazil’s exclusive economic zone) were established – i.e., between the years 2015 and 2019 – to provide evidence of their effectiveness at reducing fishing pressure.

Considering the whole period from 2015 to 2019, I identified 28,226 total days of fishing activity in the SPSP region (93% of them within the area of the multiple-use MPA and about 7% within the no-take one) and 54,164 in the TMV region (82% of them within the area of the multiple-use MPA and about 18% within the no-take one).

For the SPSP region (Figure 1), I recorded a hotspot of likely fishing days on the northwestern portion of the multiple-use MPA, between the years 2015 and 2016, and on the southern portion of this MPA for the year 2017. Hotspots of fishing within the no-take MPA followed the same spatial pattern of the multiple-use one for the years prior to MPA establishment. After MPA creation, hotspots of fishing activity were well distributed in 2018, and more spatially concentrated on the western portions of both MPAs in 2019.

For the TMV region (Figure 2), I identified that hotspots of fishing activity clustered on the central parts of the region in 2017 and were well dispersed in the previous years. After MPA establishment, large areas of the multiple-use MPA could be identified as hotspots of fishing activity, mainly in its left half, closer to the mainland. While hotspots of fishing activity were identified within no-take MPAs in 2018, fishing activity was substantially reduced in 2019 for this MPA.

When I performed the correlation matrix analysis, three results emerged (Figure 3): (i) there was no agreement between the fishing activities occurring at each year within the multiple-use MPA in the SPSP region (Kendall coefficient: −0.09–0.14; Figure 3A); (ii) there was a strong agreement between the fishing activities occurring at several years before and after MPA establishment within the no-take MPA in the TMV region (Kendall coefficient: 0.94–0.96; Figure 3D); and (iii) there was only a substantial agreement between the fishing activities occurring in the years 2018–2019 within the no-take MPA in the SPSP region (Kendall coefficient = 1; Figure 3B), and within the multiple-use MPA in the TMV region (Kendall coefficient = 0.67; Figure 3C). While the first two cases imply that these specific MPAs might have little influence on the spatial patterns of fishing activity, the second situation indicates that the creation of those MPAs might have affected spatial patterns of fishing activity.

Analysis of fishing pressure within each MPA (Figure 4; top panels) revealed that the amount of fished area was reduced after MPA establishment for the SPSP region, which was not the case for the TMV region. The observed reduction in the SPSP region was followed by a decrease in the fishing intensity within the multiple-use MPA (middle panel) and an increase in the fishing density within the no-take MPA after their establishment (bottom panel). This was because fishing activity became more concentrated over smaller areas of this MPA. Overall, I also found a large increase in the fishing intensity within the multiple-use MPA for TMV (middle panel), which was also reported for the no-take MPA at least for the first year after MPA establishment (2018). When assessing the measure of fishing density for this region, I found that there was a small reduction of this measure in the no-take MPA. It was noticeable that fishing density seems not to change with MPA establishment for the multiple-use MPAs in both regions. I identified comparable and high levels of fishing pressure on the random areas that did not restrict commercial fisheries throughout the time assessed, regardless of the measure of fishing pressure used (Supplementary Figures 2–4).

The results presented here provide the first assessment of the conservation effectiveness of large-scale MPAs in the Southwestern Atlantic Ocean and contribute to the ongoing discussion about the benefits of this conservation strategy to mitigate threats from fishing (O’Leary et al., 2018). Results indicated that the effectiveness of the large-scale MPAs was variable and depended on the measure of fishing pressure used. Overall, there was a reduction in the fishing area and intensity in the SPSP region, but fishing became particularly intense over smaller areas (“squeeze factor”), particularly within the no-take MPA. On the other hand, while fishing area and intensity increased for the TMV region, a reduction in the fishing density was observed because the activity became spread over larger areas. More positively, a reduction in all measures of fishing pressure became apparent in 2019 for the no-take MPA in TMV. Fishing pressure is thus significant within these large-scale MPAs and monitoring and enforcement efforts to effectively promote their reduction over time needs to be encouraged.

I estimated that between 3 and 10% of the no-take MPA in the SPSP region, and between 12 and 38% of the no-take MPA in the TMV region remained potentially fished. This supports the existence of illegal fishing even in the remote places of the ocean as previously identified (Arias et al., 2016). Although there is some uncertainty in the VMS data to provide evidence of the magnitude of fishing activity as a result of the need to combine this technology with other forms of evidence gathering (Appleby et al., 2018), this is unlikely to change this result significantly. With the challenges associated with patrolling offshore and remote areas in the ocean, a more realistic approach to build evidence of illegal fishing would be to combine different data sources that are sufficient to lead to a prosecution, making enforcement effective.

As previously suggested (Magris and Pressey, 2018), the effects of multiple-use, large-scale MPAs have been marginal, at least in the short term. Indeed, multiple-use MPAs have been claimed to have a limited impact on biodiversity conservation (Giakoumi et al., 2017; Zupan et al., 2018b) when assessed in terms of improving biodiversity conditions in situ. On the other hand, some have argued that their contribution to ocean conservation would be to prevent mining expansion in the future (Giglio et al., 2018; Miller et al., 2018). While these conservation outcomes are not realized, several management recommendations could be derived for this category of MPAs to improve its effectiveness in the present. Areas identified as more important for biodiversity within their boundaries (Magris et al., 2020; Vilar et al., 2020) could be targeted for more strict fishing regulations through a zoning process. Moreover, fishing activity could be particularly required to adopt practices that reduce the risk of fishing mortality (Booth et al., 2019). Strengthening regulations and establishing adequate governance are key ingredients for increasing conservation benefits expected from effective MPAs.

Recent evidence has suggested that large-scale MPAs maintain fishing levels at a low level (White et al., 2020). The results presented here do not support this pattern at least for those MPAs affording partial protection. Multiple-use MPAs did not interfere in the spatial patterns of fishing activity over the time assessed and, in some instances, fishing intensity within no-take MPAs had even increased shortly after the creation of MPAs. These contrasting findings can be explained by the intrinsic difference between the sources of fishing detection systems (VMS versus Automatic Information Systems – AIS). At least in Brazil, the AIS system misses a considerable fraction of fishing vessels, rendering assessments based on that system misleading. For example, tracking the global footprint of fisheries using AIS across the national waters off Brazil, as well as other exclusive economic zones, has shown minimal fishing effort within this area (Kroodsma et al., 2018), which is a misreport of the activity.

A major challenge to quantifying the conservation effectiveness of large-scale MPAs is the dynamic context in which threats operate over vast areas and data availability. The assessment of the threat reduction capacity of these MPAs might be influenced by other environmental conditions such as ocean currents, temperature, and distance from the mainland. Ongoing efforts to gather and analyze data for their influence on the occurrence of fishing activity will possibly result in the revised estimates of the conservation effectiveness of large-scale MPAs, allowing more comprehensive assessments of their role in reducing fishing pressure.

Though there are venues for further development and refinement, this study constitutes an important first step in quantifying the effects of large-scale MPAs off Brazil. The case study highlights that, unlike other regions, fishing activity remains operating within multiple-use MPAs, and that avoiding illegal fishing within no-take MPAs is an urgent need. While remote sensing technologies provide spatially and temporally continuous assessment of fishing activities, it would need to be combined with other evidence-based tools on fishing effort for increasing existing levels of compliance and enforcement. The variations in fishing pressure among MPA types over time affirm the dynamic nature of managing offshore marine systems.

The processed data supporting the conclusions of this article will be made available by the authors, without undue reservation.

RAM was the sole author of this manuscript, developed the research question and protocol, conducted data collection and analysis, and wrote the content of the manuscript.

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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