Periodically harvested closures require full protection of vulnerable species and longer closure periods

Highlights

• Periodically harvested closures (PHCs) can increase targeted fish biomass.

• PHCs will need at least 3 years of closure to provide long-term benefits.

• Highly vulnerable species will not benefit from PHC management.

• PHCs should be combined with conventional fisheries management strategies.

• No-take marine reserves should be used for conservation rather than PHCs.

Abstract

Periodically harvested closures (PHCs) are small fisheries closures with objectives such as sustaining fisheries and conserving biodiversity and have become one of the most common forms of nearshore marine management in the Western Pacific. Although PHCs can provide both short-term conservation and fisheries benefits, their potential as a long-term management strategy remains unclear. Through empirical assessment of a single harvest event in each of five PHCs, we determined whether targeted fishes that differ in their vulnerability to fishing recovered to pre-harvest conditions (the state prior to last harvest) and demonstrated post-harvest recovery benefits after 1 year of re-closure. For low and moderately vulnerable species, two PHCs provided significant pre-harvest benefits and one provided significant post-harvest recovery benefits, suggesting a contribution to longer-term sustainability. PHCs with a combination of high compliance and longer closing times are more likely to provide fisheries benefits and recover from harvest events, however, no benefits were observed across any PHCs for highly vulnerable species. We recommend PHCs have longer closure periods before being harvested and species that are highly vulnerable to fishing (e.g. large species of; grouper, wrasse and parrotfish) are avoided during harvests to avoid overexploitation and increase the sustainability of small-scale fisheries.

Introduction

In an attempt to recover fisheries resources and provide food security to communities in the Western Pacific, locally-managed marine areas have been widely promoted (Govan, 2009, Jupiter et al., 2014). Periodically harvested closures (PHCs) have become one of the most common forms of fisheries management used in locally-managed marine areas, with over 1000 closures estimated across the Western Pacific (H. Govan, pers. comm.). PHCs are generally small fisheries closures (e.g., median area of 1 km² in Melanesia; Govan et al., 2009), with periodic harvest regimes that make them functionally similar to rotational closures (Cohen and Foale, 2013). Historically they have been applied in Pacific coastal communities to increase catch efficiency and provide for socioeconomic and cultural needs, while objectives such as sustaining small-scale fisheries and conservation of biodiversity have been proposed more recently (Cohen and Foale, 2013, Jupiter et al., 2014, Jupiter et al., 2012). The widespread use of PHCs in a region where small-scale fisheries are essential for food security (Bell et al., 2009), highlights the importance of understanding the best practice and trade-offs of PHCs for fisheries management and conservation strategies.

PHCs vary markedly in the way they are managed, in particular the time they are closed versus open to fishing, which has resulted in variation in their ability to increase the abundance, size or biomass of targeted species (Bartlett et al., 2009, Cinner et al., 2006, Goetze et al., 2015, Jupiter et al., 2012). However, a recent meta-analysis found that PHCs across Melanesia were capable of providing pre-harvest protection benefits through increased abundance and biomass of targeted species, which translated into harvest benefits when opened to fishing (Goetze, 2016). The meta-analysis found that these benefits are greater in PHCs that are large, have high compliance and are closed to fishing for long periods. However, variation in these factors within Fijian PHCs has resulted in inconsistent outcomes for the abundance, size and biomass of targeted species (Goetze et al., in review). While there is some evidence for well-managed and designed PHCs providing short-term fisheries benefits prior to harvesting, a large proportion of the biomass of targeted species is usually removed during harvest events (Goetze, 2016). The ability of PHCs to recover from high levels of harvesting and their role in sustaining fisheries has not been explored empirically.

Similar to no-take marine reserves (hereafter referred to as marine reserves), recovery of targeted biomass within a PHC is expected to occur through multiple mechanisms, the importance of which will vary depending on the length of time that the area is protected (Russ and Alcala, 2003). Recruitment, the addition of juveniles, growth of the existing population and migration/movement across PHC boundaries are some of these mechanisms. The rapid changes in fishing pressure associated with opening and closing PHCs makes it particularly important to account for migration/movement across PHC boundaries. For example, “spill-in” of targeted species into protected areas can occur when fishing pressure outside is high (Eggleston and Parsons, 2008) or a “bail-out” effect can occur when there is a sudden increase in fishing pressure within PHC boundaries (Jupiter et al., 2012). This highlights the importance of monitoring both PHCs and sites open to fishing across the entire harvesting regime when investigating recovery dynamics.

Assessing the implications of PHCs for long-term fisheries management and conservation requires understanding how species that vary in their vulnerability to fishing are affected by harvest regimes. Long term studies using marine reserves have been used to assess how coral reef fish recover from the effects of fishing and suggest that decadal time scales may be required for the full recovery of fish assemblages in heavily fished areas (McClanahan et al., 2007, McClanahan and Graham, 2015, Russ and Alcala, 2004). In addition, coral reef fishes have a broad range of life history traits that influence their vulnerability to overfishing including: maximum size; growth rate; maximum age; age of sexual maturity; and rates of mortality (Abesamis et al., 2014, Cheung et al., 2005, Jennings et al., 1999, Russ and Alcala, 1998). Recovery trajectories will thus not only depend on the local fishing intensity, but also on the life history traits and vulnerabilities of targeted fish species, with higher vulnerabilities generally resulting in slower recovery (Abesamis et al., 2014, Claudet et al., 2010, McClanahan and Humphries, 2012). For example, Abesamis et al. (2014) use marine reserves to show that the full recovery of large predators in overfished regions may take between 20 and 40 years, while smaller-bodied herbivores may recover within 10 years.

The recovery trajectories of coral reef fishes observed in marine reserves is applicable to PHCs during the no-take closure periods. Abesamis et al. (2014) related the recovery trajectories observed in marine reserves to the management of PHCs and estimated that a 10% removal of stock will require several years of recovery for less vulnerable species (e.g., small parrotfish), while moderately to highly vulnerable species (e.g., large groupers) may take more than a decade. This suggests that certain species will be better suited to the strategy of periodic harvesting and collecting data on the recovery trajectories of target species across different levels of vulnerability will be essential to ensure the long-term sustainability of the harvesting regime within PHCs. We estimated the biomass of targeted species immediately (1–2 days) before, after and 1 year after a harvest event, inside and outside of five PHCs across Fiji with varying management strategies. We aimed to determine if targeted fish biomass within PHCs would recover to pre-harvest conditions and provide post-harvest protection benefits after 1 year of re-closure, a common closure time across Melanesia (Goetze, 2016). Additionally, we assessed how targeted species with low, moderate and high vulnerabilities to fishing were impacted and whether they recovered from harvest events. We hypothesised that species with high vulnerability were likely to benefit least from PHCs, and that magnitude of recovery would decrease with increasing vulnerability.