Coral reefs are declining worldwide due to the growing impact of stressors (such as overfishing, habitat destruction, pollution, and climate change), which threaten their biodiversity, ecological functions, and ecosystem services (e.g., Carpenter et al., 2008; Burke et al., 2011). The main management approach to counteracting coral reef decline is designation of marine protected areas (MPAs; e.g., Lester et al., 2009; Edgar et al., 2014) to reduce human endogenous (within-site) impacts, most notably over-fishing and habitat destruction. Well-managed and enforced coral-reef MPAs can enhance recovery following wide-scale natural disturbances, such as coral bleaching (Mumby and Harborne, 2010; De'ath et al., 2012; Gilmour et al., 2013). Moreover, MPAs may also benefit adjacent non-protected areas through the “spillover” of adult fish, or the export of larvae (e.g., Gaines et al., 2010; Russ and Alcala, 2011).

There is a growing body of literature showing that MPAs do not achieve all the expected conservation goals, such as reef community resistance to disturbances (Côté and Darling, 2010), or protection against exogenous stressors (Hilborn, 2015). Furthermore, in those cases in which MPAs are likely to fulfill the task of maintaining ecosystem health, or the recovery of degraded reefs (Abelson et al., 2016a), there are concerns regarding the socio-economic needs of local users, which in turn raise questions about the adequacy of MPAs as an ultimate solution (e.g., Christie, 2004; McClanahan et al., 2006). In this sense, many coral-reef MPAs are planned and evaluated by their ecological goals, while not meeting key social goals (e.g., Christie, 2004). Consequently, implementation of highly restricted MPAs and marine reserves may be challenging and prone to failure in areas where local communities greatly depend on reef ecosystem services (e.g., MacNeil et al., 2015). Furthermore, even in cases where MPAs can potentially contribute to remediating the reef, the time required for recovery may take many years or even decades (Carpenter et al., 2008; Blackwood et al., 2012; MacNeil et al., 2015). During this time, local communities are likely to suffer from an impaired livelihood under strict restrictions of “no-take” MPAs (i.e., marine reserves). Strict protection by MPAs may, therefore, be a problematic remedial solution if the basic requirements of the local communities that rely on the reefs' ecosystem services are not taken into account.

On the other hand, allowing unrestricted fishing activities is not a viable alternative as it may lead to a positive feedback, in which destructive fishing practices (e.g., cyanide and dynamite fishing) become more common as fish populations are depleted (e.g., McManus, 1997; White et al., 2000; Burke et al., 2011; Figure 1). A plausible outcome of this positive feedback is an accelerated decline of reefs to a point where basic ecosystem services are at risk (e.g., White et al., 2000), which may lead to “poverty traps” (Cinner et al., 2009, 2013). Such degradation cascades can already be witnessed in wide reef areas in South-East Asia (e.g., Burke et al., 2011; Hughes et al., 2013). Alternatively, in areas where fishing yields are still reasonable, fishing may remove herbivorous fish (e.g., Bejarano et al., 2013; MacNeil et al., 2015), potentially reducing ecological functions, altering trophic structures, and thereby threatening the reef's ecosystem integrity and resilience (Folke et al., 2004; Rogers et al., 2015), which may lead to “ecological vulnerability” and socio-economic traps (Cinner, 2011; Cinner et al., 2013).

To address the problem of fishing “business as usual” in areas where MPAs may be socially challenging or infeasible, a moderate fishery management approach has been proposed that can allow a certain exploitation while still maintaining essential ecosystem services (e.g., Siar et al., 1992; Bartlett et al., 2009; MacNeil et al., 2015). It is claimed that some forms of fishery management can enhance essential reef ecosystem functions and resilience, while also enabling the continuous consumption of reef fish necessary for the livelihood of local fishing communities (e.g., MacNeil et al., 2015). In reef areas where MPAs cannot be implemented, such fishery management tools may encounter less resistance and enhance collaboration among the different stakeholders and, therefore, can better achieve the goals of reef recovery without impairing the local community's livelihood.

Fishery management, however, may have some limitations as a countermeasure of reef degradation. These are mainly related to the relatively long time it takes to return the reef and the fish community to a state in which they enable essential ecosystem functions (e.g., Blackwood et al., 2012). Moreover, many reef sites are characterized by highly depleted fish communities (e.g., Burke et al., 2011; MacNeil et al., 2015), with fish biomass below 0.25 B₀ (where B₀ is reef fish biomass in the absence of fishing; see McClanahan et al., 2011; MacNeil et al., 2015), as a threshold below which multiple negative ecosystem effects of overfishing have been shown (MacNeil et al., 2015).

The recovery of such heavily depleted reefs is expected to take decades (Blackwood et al., 2012; MacNeil et al., 2015), and may impose some challenges and risks. First, the basic ecosystem functions and the structural complexity (estimated by measuring “rugosity” as a proxy for complexity; Alvarez-Filip et al., 2009) of the reef may continue to deteriorate due to the “detrimental cascading effects related to the removal of predators and herbivores” (Newton et al., 2007)—cascades which can lead to a further decline in the reef's fish community and fishery yields (Graham et al., 2006). Second, low resilience (as may be indicated by a low functional diversity of fish; Cheal et al., 2010), combined with a long recovery time of the reef fish community, exposes the reef to an increased risk of phase shift into a macroalgae-dominated reef, following mass coral mortality events. Disturbed reefs are prone to experience positive feedbacks, which can lead to stable states of low coral coverage that exhibit hysteresis (e.g., Mumby et al., 2007): A situation in which much higher values of the critical parameters (e.g., grazing rates) are needed for the degraded ecosystem to shift back to its original state, than the threshold values that caused the phase shift in the first place (Suding et al., 2004). Therefore, removing the stress (e.g., fishing) from a system does not necessarily ensure a return to a coral-dominated reef. Third, slow reef recovery implies reduced fishing yields for a long period, the outcome of which is a poor livelihood for the local communities. The long recovery time and the resulting poor livelihood are expected to pose socio-economic challenges, which may prevent compliance by the local stakeholders and consequently impede the establishment of suitable conditions for reef recovery.

The projected long recovery time, the reduced trophic level, and the socio-economic challenges related to the existing management approaches highlight the need for additional tools that can help to address, within a relatively short time, both the ecological and the socio-economic challenges; that is, to promote recovery of the reef ecosystem while sustaining the livelihood of its reliant coastal populations (e.g., Abelson et al., 2016b).

Fish population restocking is not suggested as an ultimate solution for all degraded reefs. It could, however, serve as an additional restoration tool in areas in which MPAs, or existing fishery management, are not likely to be effective as sole measures, due to the social–political circumstances or the progressively-depleted reef state. Moreover, fish restocking is not expected to fulfill its goal in conditions of low structural complexity, low coral recruitment, and/or high macroalgal cover reefs without artificial complexity enhancement, coral transplantation, and algal eradication, respectively (see Table 1; Abelson, 2006; Rogers et al., 2015; Abelson et al., 2016b).

One category of high-priority target reefs for restocking-based restoration is that of the degraded reefs that have undergone a phase shift from a coral-dominated state to domination by benthic algae (e.g., Bellwood et al., 2006; Bahartan et al., 2010), either macroalgae (Type I; Table 1; Figure 2A), or algal turfs (Type II; Table 1; Figure 2B), mainly due to over-fishing. Reefs dominated by macroalgae or algal turfs may suffer from limited recruitment, due to either adverse effects on corals and fish recruits, or a negative preference of potential recruits (Kuffner et al., 2006; Birrell et al., 2008; Dixson et al., 2014; Kelly et al., 2014). In such cases, the common reef restoration tool of coral transplantation is not likely to be effective in counteracting the potential damaging effects of algae on adult corals (Birrell et al., 2008; Dixson et al., 2014). On the other hand, intensive grazing and browsing activities following augmented populations of herbivorous fish may help in re-shifting the reef back to a coral dominated state through the removal of algae. An example of such rapid removal of algae was demonstrated in exclosures (exclusion experiments) in which fish herbivory reduced about half of a 3-year old macroalgal cover within 5 days (Bellwood et al., 2006).

Another category of target reefs for fish restocking implementation comprises degraded reefs of Type III (Table 1; Figure 2C), where the coral live-cover is high, but the reefs suffer from severely depleted fish communities, notably of herbivorous fish. Type III reefs, despite their high coral cover, may be of low resilience and under high risk of deterioration if exposed to mass coral mortality following extensive disturbances, such as crown-of-thorn starfish outbreaks and mass bleaching (e.g., Folke et al., 2004). In such reefs, fish restocking can be applied as a “proactive measure” to augment reef resilience and the livelihood of local fishing communities.

A third category of candidate reefs for fish restocking includes degraded reefs of Type IV (Table 1; Figures 2D,E), which are reefs of low coral cover and a high fraction of exposed rock, or crustose coralline algae (CCA). As in Type III, fish restocking can be applied as a “proactive measure” to augment reef resilience and the livelihood of local fishing communities. However, prior to implementing the restocking, artificial complexity enhancement and coral transplantation interventions should be considered (Table 1) to enable adequate shelter for the restocked fish.

Degraded reefs of Type V, which are structurally destroyed (Table 1; Figure 2F), require substrate stabilization interventions, in addition to the other available restoration methods. At present, substrate stabilization interventions are not considered to be effective on large spatial scales (Fox and Caldwell, 2006). Moreover, the combined restoration projects of such destroyed reefs are expected to entail a high cost.

There are diverse technical, ecological, and socio-economic aspects that may impede the potential success of restocking interventions in coral reefs, the majority of which are also relevant to other restoration interventions. These include, among others: High-cost production expenses, insufficient funding, lack of compliance of local stakeholders, below-threshold survival rates of the restocked fish, fish movement out of the target site, non-restricted illegal fishing, inadequate aquaculture technologies of relevant species, and adverse effects of the restocked fish on the target reef community. Additionally, there are potential adverse effects of aquaculture practices on coastal ecosystems, related to food sources (mainly of carnivorous fish), eutrophication (by mass culture to adult size) and the culturing of non-indigenous species, or genotypes. The “fish restocking” concept refers to indigenous genotypes (i.e., local broodstocks) of herbivorous fish cultured to juvenile stages, thus minimizing the abovementioned adverse effects. Moreover, if “Integrated Multi-Trophic Aquaculture” (IMTA) is applied (e.g., Soto, 2009), it is expected to further reduce the environmental impact and enhance the income of local stakeholders via aquaculture as an alternative source of livelihood.

These potential impediments and drawbacks can be assessed in the early stages by means of pre-launch experiments and analyses, to evaluate the site-specific success chances and cost-benefit of the proposed fish-restocking projects. These pre-launch steps (see Section Proposed implementation steps of restoration by restocking) can help in determining whether the proposed approach of restocking is applicable, and where, which reef types, and under what socio-economic circumstances.

In contrast, there are several arguments that may support the feasibility of fish restocking when appropriately planned and applied. These include:

Economic feasibility. The results of a recent model-based study suggest that restocking may be a financially beneficial restoration tool (Obolski et al., 2016), due to the high economic value of coral-reef services (Costanza et al., 2014) and the potentially low cost of restocking (Lorenzen et al., 2013; Obolski et al., 2016). When compared with coral transplantation (which is currently the major restoration tool; Edwards, 2010; Rinkevich, 2014), fish restocking has significantly lower costs and carries potential socio-economic benefits to coastal communities. These advantages should incentivize its implementation, alongside other tools, or as a major intervention, if applicable (Obolski et al., 2016).

The fish restocking management tool should be viewed as part of a comprehensive coral-reef management approach, which includes protection and fishing regulations, and requires the compliance and support of the local communities for its success (Ferse et al., 2010). To maximize the chances of success and minimize risks, it is proposed to consider a series of steps that would allow assessing and implementing critical parameters:

The reef degradation state and the appropriateness of the restocking tool. The reef state should fit one of the relevant degradation types (Table 1; Figure 2). In degraded reefs of type III, the restocking tool can be considered as a sole restoration intervention. However, in most degradation types further interventions, such as algal eradication, artificial complexity enhancement, coral transplantation, and substrate stabilization, may be required (Table 1; e.g., Rogers et al., 2015; Abelson et al., 2016a). The costs of these interventions may raise, however, questions regarding the scalability and implementation circumstances of combined restoration interventions.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer JD and handling Editor declared their shared affiliation, and the handling Editor states that the process nevertheless met the standards of a fair and objective review.