The Effectiveness of Providing Shell Substrate for the Restoration of Adult Mussel Reefs

Abstract

Providing benthic substrate is the most common method used for oyster reef restoration. The physical relief from the seabed, increased habitat complexity, and attachment surfaces have been shown to improve oyster health, recruitment, and survival. While the addition of shell material is an effective substrate for oyster restoration, its usefulness in mussel restoration has been understudied. This study tested the effectiveness of adding shell substrate to two different types of soft sediment for the restoration of adult green-lipped mussels. Over 10 t of shell was used to create a 10 cm layer on the seabed in replicated experimental plots at the two locations. 10 t of live adult mussels were deployed onto the shell substrate and an additional 10 t of mussels onto adjacent soft sediment control plots. A year after deploying the live mussels, mussel survival across all plots was 80.6 ± 6.5%, with no differences between mussel plots with or without the added shell substrate for either of the two locations. This study emphasizes the importance of context-dependency, revealing promising avenues for future research, and indicates that for adult green-lipped mussels the addition of a shell substrate appears to provide little advantage for adult mussel restoration at high deployment densities.

1. Introduction

Reef-building shellfish, like oysters and mussels, create biogenic habitats that play an ecologically significant role in coastal environments globally. The structure provided by shellfish reefs is important for mitigating storm damage [1], stabilizing benthic sediments [2], and reducing water turbidity [3]. These structures also act as important juvenile fish nurseries [4,5] and provide habitat for many organisms, increasing biodiversity and secondary productivity [6,7,8]. Despite their importance, 85% of oyster reefs have been lost globally [9], while 53% of mussel reefs have been lost in Europe, North America, and Australia [10]. The global loss of shellfish populations is largely due to anthropogenic stressors like overharvesting, disease, and pollution [3,9,11,12]. Recognizing this global loss of shellfish reefs has led to the widespread emergence of restoration efforts [10,13,14]. However, shellfish restoration has been largely focused on oysters, particularly Crassostrea virginica (Gmelin 1791) in the USA, while the restoration of mussels has received less attention in terms of both research and practice [15].

Oyster restoration is frequently based upon providing additional hard substrate in the benthic environment to promote increased settlement and establishment of oysters [16,17,18]. The addition of substrate is necessary for oyster restoration as it provides hard substrate for oyster larvae to settle and reduces predation [14,18,19]. Materials used as hard substrate are most commonly limestone, concrete, shell materials as whole bivalve shells or shell hash [18], or a combination of these materials, such as mixing shells with cement [20]. However, oyster shells are used in the highest number of restoration projects [18] due to the shells being a natural product which retain chemical properties that may promote larval settlement [21,22,23]. Due to the high demand of shells for restoration, there are shell recycling programs working with aquaculture processors and restaurants to recover shell by-product for use in restoration instead of ending up in landfill (e.g., The Nature Conservancy’s ‘Shuck don’t Chuck’ for oyster shell in Australia; [24]). In areas with sufficient natural larval supply the shell is deployed to the seafloor and oyster larvae attach directly onto the shell [25], but in areas with limited larval supply, oysters are settled onto the shells in a hatchery and then translocated to the seabed for restoration [4,26]. This helps to significantly reduce restoration cost by using larvae on shell instead of raising and transporting adult oysters. This process has been proven in many locations to efficiently restore sessile oysters [4,18,25], where they attach permanently to the shell material at the end of the larval stage.

Unlike oysters, mussels do not permanently attach to substrate, but they can instead anchor themselves to substrate with byssal threads that can be detached to allow some movement, with new byssus attachments made once repositioning has been completed [27,28]. Mussels are commonly found attached to a wide variety of hard surfaces on the seafloor, such as the shells of dead or live shellfish, including conspecifics [29,30,31], rocks [32], and different types of suspended aquaculture ropes [33]. Although the benefits of providing hard substrate are clear for oyster restoration, the value of this technique is largely unknown for the restoration of non-sessile shellfish, like mussels. The ability of mussels to reattach byssus to conspecifics and new substrate is a behavior that is used in mussel restoration. For example, mussels can be detached from aquaculture infrastructure and then relayed onto seabed locations where they reattach to substrate or conspecifics to form a restored mussel reef [34,35,36].

In soft sediment environments the addition of substrate may benefit the establishment of mussels by providing anchorage material and thus helping to prevent dislodgement by currents and/or predators [35,37]. There are a small number of studies that have directly assessed the importance of providing hard substrate for mussel restoration which have produced conflicting results that may be species-specific. While some studies suggest the availability of hard substrate for the attachment of mussels is critical for their successful establishment (e.g., Mytilus edulis (Linnaeus, 1758); [37,38]), the results of other studies suggest hard substrate may not be necessary for establishing adult mussels (e.g., Modiolus modiolus (Linnaeus, 1758); [39,40]), but may have benefits for juvenile recruitment (e.g., Perna canaliculus (Gmelin 1971); [31]). Further research is necessary to better understand if the addition of a hard substrate to soft-sediment habitats will provide benefits for mussel restoration.

The resident population of the endemic, green-lipped mussel (Perna canaliculus) in Pelorus Sound/Te Hoiere, Aotearoa-New Zealand, were extensively overharvested last century, with no recovery of the population since dredge fishing and intertidal hand-gathering ceased more than 40 years ago [41]. The reasons for this lack of recovery are unclear but may relate to the loss of adult mussel beds and increased sediment accumulation rates from land run-off [41]. This location was chosen for this restoration study, with the aim to determine whether the addition of shell substrate at two locations alters the effectiveness of restoring adult mussels into these habitats.

2. Methods

2.1. Study Area

Pelorus Sound/Te Hoiere is a drowned river valley system that is located within the Marlborough Sounds at the top of the South Island of Aotearoa-New Zealand (Figure 1). Two locations within Pelorus Sound that differed in soft sediment composition were used for the experimental deployment of shell substrate: (1) Kenepuru Entrance, and (2) Fairy Bay (Figure 1). Locations were chosen based on historical data that indicated that mussels were historically present in both locations [42,43]. In November 2019, prior to the shell and mussel deployments, sediment grain size and organic content at each site was characterized from sediment samples taken by a Ponar grab (scoop area 0.05 m², maximum depth 15 cm) at three points within each of the two locations. Sediment samples were pooled for each location and stored at −20 °C until analysis where organic content was determined using from ash free dry weight [44] and grain size samples were run on a Malvern mastersizer 3000 and classified using the Wentworth grain size classification [45].

2.2. Shell Substrate and Mussel Deployment

At each location six plots on the seafloor were randomly assigned as either a shell plot (shell substrate and mussels added, n = 3, Figure 2c,d) or a soft sediment control plot (mussels added on soft sediment, n = 3, Figure 2e,f), with each plot ~4 m apart and at 10–12 m water depth. In October 2020, a total of 10 t of steam-cleaned, green-lipped mussel shells of 90–110 mm in length were collected as a by-product of processing cultured mussels. At each location, bags of shells were lowered on the marked shell plots by a crane from a barge to 5 m depth and emptied to form large rectangular plots (~4 × 8 m) on the seabed that each contained ~1.5 t of shell forming a layer on the seabed of approximately 10 cm. To allow time for the shell substrate to settle, three months later, in January 2021, a total of 20 t of adult, green-lipped mussels (mean shell length 106.0 ± 0.9 SE mm), grown on a mussel farm from wild spat collected in Pelorus Sound/Te Hoiere three years earlier, were harvested for translocation to the two study locations. During harvesting, all conspicuous fouling organisms were removed by hand, to reduce the risk of spreading non-indigenous pest species. The 20 t of mussels were divided into 24 individual bulk bags weighing approximately 800 kg each. Two bags of mussels (1.6 t in total) were released consecutively into the center of each of the three shell plots, and the three soft sediment control plots at each location. All mussels were installed in the plots within 24 h of harvest from the mussel farm.

2.3. Sampling Design

Sampling of the mussels was conducted at deployment (January 2021) and six and twelve months after deployment (July 2021 and January 2022, respectively). At each sampling event mussel survival and density were estimated using an adapted method from Benjamin et al. [8] and Wilcox [31] where divers placed a 0.25 × 0.25 m quadrat three times along a transect located through the center of each restored plot lengthwise, avoiding areas that were 0.5 m from the edge of the plot. To determine mussel density and survival the mussels inside each quadrat were counted by divers and assessed as dead or alive. The shell substrate was distinguishable from the dead mussels as the substrate consisted of weathered half shells and the newly dead mussels were clean with the hinges still intact. In order to assess recruitment in the mussel plots, divers paid close attention to look for any juvenile mussels by recording any mussels < 50 mm inside each quadrat. Mussel survival for each plot was calculated as the proportion of live mussels out of the total of both live and dead mussels recorded for each plot. The area of each plot of mussels was determined by measuring the length and width of the mussel plot at three positions along the rectangular plot to estimate the average area covered. At deployment, plot area was not measured for three plots at Fairy Bay (two shell and one soft sediment), and one mussel plot on soft sediment at Kenepuru Entrance was not sampled at all due to field logistic constraints. All plots were completely sampled at 6 and 12 months.

A sample of 15 haphazardly selected mussels from each restored plot were collected at each sampling event to determine condition (an indicator of mussel health status: [46]) and shell length. Mussels were not collected at one shell plot at Kenepuru Entrance at the 6-month sampling due to field logistical constraints. Mussels were stored at −20 °C until processing, where they were dried at 60 °C for 48 h and then weighed. The dry condition index was then determined using the following formula: $Dry Weight of Flesh\times 100/Dry Weight of Shell$.

At each of the three sampling occasions, the abundance of a common mussel predator in this region, the eleven-armed sea star, Coscinasterias muricata (Verrill, 1867), was estimated by divers counting the number of sea stars located 1 m either side of the central transect in each plot. Additionally, while swimming the central transect divers briefly recorded visual observations of macroalgal growth and other benthic invertebrates.

2.4. Statistical Analyses

To determine differences among shell plots and plots of soft sediment alone, firstly assumptions of normality and equivalence of variance in the data were assessed visually using a quartile-quartile plot and with a Shapiro–Wilks test. Three-way repeated measure ANOVAs, with location, sampling time, and treatment (i.e., with and without shell substrate) as factors were used to compare the response variables of mussel growth and plot area. The effect of the addition of shell substrate on mussel survival, density, condition, and sea star counts were analysed using linear mixed-effects models (LME) using the R package lme4 [47]. In these models, the fixed effects were sampling time, location and benthic type, and the random factor was plot number to account for the lack of independent replicates over the two sampling periods. To obtain the p-values of all the fixed effects in the models, the ANOVA function from the R package ‘car’ was used [48]. Significance was further examined for each variable using pairwise Wilcoxon tests. All tests were performed using R statistical software version 3.2.3 [49]. As this study did not test multiple locations with the same habitat type, the study was not undertaken to test the effect of habitat type on the mussel metrics.

3. Results

3.1. Sediment Composition

Seafloor sediments prior to the mussel deployment at Fairy Bay comprised 23.8% mud (particle size < 63 µM), 13.0% very fine sand (particle size 63–125 µM), 23.2% fine sand (particle size 125–250 µM), 20.8% medium sand (particle size 250–500 µM), and 18.7% coarse sand (particle size > 500 µM). Kenepuru Entrance comprised 89.5% mud (particle size < 63 µM), 5.5% very fine sand (particle size 63–125 µM), 1.5% fine sand (particle size 125–250 µM), 1.8% medium sand (particle size 250–500 µM), and 1.7% coarse sand (particle size > 500 µM). Sediment organic content at Fairy Bay was 3.1% and 7.1% at Kenepuru Entrance.

3.2. Mussel Survival

After a year on the seafloor, mussel survival was consistent (79.8–82.3%) at the two locations and between the two benthic types (i.e., with and without shell substrate; Figure 3). There were no detectable differences between locations (LME, F_(1,8) = 0.464, p > 0.05) and treatment (i.e., with and without shell substrate; LME, F_(1,8) = 0.006, p > 0.05), whereas survival differed significantly between 6- and 12-months (LME, F_(1,20) = 55.134; p < 0.001, 6 months 96.3 ± 2.1%, 12 months 80.6 ± 6.5%).

3.3. Mussel Density and Area

There were no detectable differences in mussel density or the area covered by the mussels recorded by location or treatment throughout the experiment (density: location, LME, F_(1,8) = 3.882, p > 0.05, treatment, F_(1,8) = 0.000, p > 0.05; area: location, 3-way ANOVA, F_(1,5) = 1.475, p > 0.05, treatment, F_(1,5) = 2.408, p > 0.05). However, sampling date influenced both mussel density and area (density LME, F_(1,20) = 81.813, p < 0.001; area 3-way ANOVA, F_(1,16) = 5.579, p = 0.03) where at deployment mussel density was high and area low, and throughout the experiment density decreased and the area occupied by the mussels increased at each subsequent sampling event (overall means: deployment 30.8 ± 3.3 m², 844 ± 91 mussels m⁻²; 6 months 32.3 ± 3.4 m², 562 ± 34 mussels m⁻²; 12 months 43.2 ± 4.9 m², 245 ± 18 mussels m⁻²; Figure 4) demonstrating the dynamic nature of these mobile mussels within the plots.

3.4. Mussel Growth and Condition

Sampling time significantly affected mussel condition and growth (condition, LME, F_(1,466) = 551.95, p < 0.001; length, 3-way ANOVA, F_(1,454)= 22.06, p < 0.001), but benthic type and location had no effect on mussel condition (benthic type, LME, F_(1,8) = 0.166, p > 0.05; location, LME, F_(1,8) = 4.664, p = 0.06) or growth (benthic type, 3-way ANOVA, F_(1,4) = 0.007, p > 0.05; location 3-way ANOVA, F_(1,4) = 0.015, p > 0.05). At deployment, the mussels had a mean condition index of 13.8 ± 0.3 and a mean length of 106.0 ± 0.9 mm with no differences between locations or benthic types (Pairwise Wilcoxon test; p > 0.05). At 6 months mussel condition decreased at both locations and benthic types as compared with deployment (Pairwise Wilcoxon test; p < 0.001, Figure 4) and failed to grow significantly post-deployment (Pairwise t-test; p > 0.05). At 12 months mussel condition remained unchanged from the 6-month assessment, however, the mussels had grown significantly, irrespective of benthic type (111.1 ± 0.7 mm; Pairwise t-test; both p < 0.001; Figure 4).

3.5. Sea Star Abundance

There were no detectable differences in sea star abundance between benthic types within each location (LME, F_(1,8) = 1.207, p > 0.05, Figure 4), although sea star abundance differed significantly between the two locations and over the sampling period (LME, interaction effect, F_(1,20) = 14.394; p = 0.001). At the deployment of the live mussels, only 2 sea stars were recorded overall at Fairy Bay (0.02 ± 0.2 sea stars m⁻²) and none at Kenepuru Entrance. Following deployment, at the 6-month sampling, sea star density increased but only at Fairy Bay (1.1 ± 0.3 sea stars m⁻², Pairwise Wilcoxon test; p = 0.01). However, at the 12-month sampling both locations had attracted greater densities of sea stars as compared with deployment (Fairy Bay 1.4 ± 0.1 sea stars m⁻², Kenepuru Entrance 0.3 ± 0.1 sea stars m⁻², Pairwise Wilcoxon test; p < 0.02 for both), despite non-detectable differences between the 6- and 12-month sampling at either location (Pairwise Wilcoxon test; p > 0.05, Figure 4). At both the 6- and 12-month sampling there was a higher density of sea stars recorded at the sandier Fairy Bay location than at Kenepuru Entrance dominated by mud (Pairwise Wilcoxon test; p ≤ 0.02 for both, Figure 4).

3.6. Visual Observations

At the mussel deployment, three months after the shell deployment, the shell material had settled and become biofouled, including patches of emergent macroalgae growing on it (Figure 2b). At the muddier location, Kenepuru Entrance, at the 12-month sampling (which was 15 months after the shell was deposited), the shell appeared to be completely sunk into the seabed and no longer protruded 10 cm above the sediment whereas at the sandier Fairy Bay location, the shell remained elevated throughout the study, providing the mussels relief from the seabed. Mobile epibenthic animals were commonly encountered by divers on the plots at both study locations, including crabs, seahorses, nudibranchs, sea cucumbers, sea stars and triplefin fish. At deployment, the mussels were spread unevenly throughout the plot area due to the deployment method of upending bags of mussels in mid-water, resulting in greater density in the center of the plots and lower density around the edges. However, the mussels self-organized over time, spreading out to form a uniform density within the plots (Figure 4a). This was evident in the reduced variation in mussel density estimates between 6 months and 12 months and the provision of shell substrate did not appear to significantly affect this behavior (Figure 4a). At Kenepuru Entrance the live mussels on soft sediment visually had higher amounts of sediment on the mussels than the mussels on the shell substrate. Divers did not detect any evidence of mussels < 50 mm (considered new mussel recruits) at any of the sampling events.

4. Discussion

This study tested the benefits of providing shell substrate for establishing mussels on soft sediment habitats as a potential rehabilitation or restoration tool. The results indicate that the addition of shell substrate to soft sediment made no detectable difference to the establishment of adult mussels in either of the two locations. There were no differences in mussel survival, growth, condition, or the abundance of sea stars between restored mussel plots with and without shell substrate. There were also no differences recorded in the density of mussels or areal extent of the beds, indicating that the provision of shell substrate did not appear to affect mussel aggregation behavior.

Identifying ways to increase survival of restored mussels is critical for ensuring the efficacy of rehabilitation or restoration efforts. In oyster restoration the addition of substrate has been shown to improve adult oyster survival by providing relief from the seabed and protection from dislodgement [25,50]. After a year, the overall mussel survival in this study at both locations in Pelorus Sound/Te Hoiere was high, with and without shell (80.6%) as compared with one green-lipped mussel restoration effort in the Hauraki Gulf, Aotearoa-New Zealand that saw 26.2% mussel survival after two years [34]. The lack of a detectable treatment effect indicates that the addition of shell substrate provided no negative or beneficial effects on mussel survival. These findings are similar to a smaller scale (0.25 m²) short term study on green-lipped mussels in the North Island of New Zealand [31] and a study on Modiolus modiolus in Ireland [39], both of which found the addition of substrate to have no impact on mussel survival. The locations in those studies, as well as this study, were subtidal and protected from heavy wave action. Mussels can use adjacent conspecifics for the attachment of byssal threads to provide anchorage in soft-sediment environments [51]. In low energy subtidal environments, such as in the current study, it appears that this anchorage is sufficient, but in higher energy environments, additional substrate may be important for preventing dislodgement [37,38]. For example, in high energy benthic environments, both in a flume and in situ intertidally, the addition of shell substrate was found to be critical for preventing dislodgement of blue mussels (Mytilus edulis; [37]). Additionally, when transplanting juvenile blue mussels, the addition of oyster shell substrate was shown to increase retention in a wave-exposed intertidal mudflat [38]. Thus, it is possible that the benefits of providing substrate to aid mussel restoration may be contingent on the hydrodynamics and depth of the environment to be restored.

Recruitment of juvenile mussels may benefit from the addition of substrate in mussel restoration, particularly on soft sediment. In this study mussel recruitment was not observed in any of the restored mussel plots in the year following deployment, but many studies have shown that juvenile mussels do not have the capacity to settle onto soft sediment alone and prefer to attach to adults or to shell materials (e.g., [30,31]). Additionally, recruitment onto and amongst adult mussels has been shown to provide protection from predation [52]. In oyster restoration, the addition of hard substrate increases larval recruitment even without the addition of live adult oysters [19,25], partly due to the chemical attraction of larvae to settle on conspecific shell [21,22,23]. Few studies have investigated whether mussels use conspecific chemical cues to promote recruitment to adult beds, although markedly increased settlement has been recorded in the vicinity of restored mussel beds versus adjacent soft sediment habitats without mussels [34] and chemical cues from algae have been shown to increase mussel larvae settlement [53]. Despite active larvae settlement on artificial collectors in the same study area [54,55], no mussel recruitment was observed in any of the restored mussel plots within the year of sampling. However, as our visual assessments may have been compromised due to low water visibility, further study is recommended to better assess the role that the addition of shell substrate may have on P. canaliculus recruitment.

The addition of substrate in oyster restoration has been shown to decrease predation pressure and increase survival, particularly due to the increase in habitat complexity [19]. However, in our study the addition of shell had no effect on the abundance of sea star predators associated with plots of mussels deployed on soft sediment. Sea star predation has been a significant cause of mortality of mussels restored onto some soft sediment locations in New Zealand [34,56]. A previous mussel restoration study in the same region examined five different locations with differing sediment characteristics and found that reduced sediment mud content can be an important predictor of sea star numbers, with the least muddy location having high abundances of sea stars (~19 m⁻²) after 5 months [36]. That finding appears to be corroborated in this study with more sea stars found at the sandier, Fairy Bay location. However, the low sea star numbers (0–2 per m²) at both locations indicates that the chosen locations were suitable for mussel restoration trials. There was no increase in sea stars between the 6- and 12-month sampling, indicating that more sea stars do not appear to be migrating into the mussel plots over time. Similarly, in a mussel restoration study in the North Island of New Zealand, most sea star predators arrived in the restored mussel beds in the first nine months after deployment [34].

High sediment accumulation has been shown to negatively affect mussel health [57]. Initially the shell substrate visually appeared to reduce sediment accumulation compared to adjacent soft sediment as it provided a 10 cm relief from the seabed. However, after 15 months the relief remained in the sandy location (Fairy Bay), while in the muddy location (Kenepuru Entrance) the shell appeared to have sunk into the soft seabed and no longer provided the mussels with relief. Mussels have the ability to move and re-aggregate to elevate themselves above the soft sediment by attaching to conspecifics to form complex mobile clumps [58,59,60,61]. However, the stress of having to filter large amounts of suspended sediment from the water may explain some of the mussel mortality at the Kenepuru Entrance location as predator abundances were lower there than at Fairy Bay.

In oyster restoration, providing substate with high relief from the seabed has been shown to optimize flow rates, creating healthier physiological conditions [25], preventing burial by sediment, and promoting larval recruitment [62]. A reef height of 0.3 m has been shown as a threshold for oysters, where lower reefs experienced heavy sediment deposition and reefs elevated ≥ 0.3 m showed increased oyster density and survival over 2 years [63]. Therefore, it may be possible that our 0.1 m elevation was insufficient to produce marked effects. Shellfish restoration efforts have had issues with burial by sediment or unhealthy environmental conditions [17,31,64]. While it is possible that providing substrate with higher relief may overcome these barriers and create healthier conditions to increase long term transplantation survival, additional research is needed in this area. This study does, however, indicate that to achieve the same elevation on different benthic environments more shell volume may be required in locations with higher mud content to prevent the shell from sinking into the soft sediment.

The ideal density for restoring shellfish is an ongoing question for restoration managers. For both oysters and mussels, deployment densities are highly variable across restoration studies [65]. In mussels this may be due to limited resources and lack of clarity (i.e., variability between species and the lack of remaining natural beds to study). It might also be that in high energy soft sediment environments, where the only attachment is conspecifics [30], the deployment density needs to be higher than at locations where attachment substrate is not readily available, such as in locations where seabed disturbance has removed or buried shell material. For example, a restoration study using M. edulis found mussel density was double on soft sediment after one month compared to on shell substrate, despite initially deploying the mussels at the same weight [37]. Additionally, a study on M. modiolus showed that the addition of shell substrate limited mussel aggregation [40]. However, in the current study, the mussels had the same mean density of 245 ± 18 mussels m⁻² on both shell and soft sediment after 12 months and the spatial extent of the restored mussels did not differ. The mean density recorded on all mussel plots in this study was similar to a restoration experiment performed in the same study area on P. canaliculus, where after two years mussels in four different locations had a mean density of 267 ± 15 mussels m⁻² [36]. It is possible that the results of this current study would have been different if the mussels were deployed in lower densities and utilized the shell substrate to form stabilizing clumps, but at the high densities used in this study it appears that the addition of shell substrate had no impact on mussel density or their areal extent.

The addition of substrate is widely used for efficient oyster restoration and has the potential to be used for other bivalves, like mussels, to overcome restoration barriers. However, the results of this study indicate that the addition of shell substrate did not influence the establishment of adult mussels deployed at high density in two low energy subtidal soft-sediment environments. Visual observations recorded a reduced amount of resuspended sediment and an increase in macroalgal growth on the shell substrate versus on adjacent soft sediment before the mussels were deployed, but these potential benefits require further quantification. As mussel restoration barriers can occur, including dislodgment from heavy wave action and burial by sediment, further study of the addition of substrate for mussel restoration should explore the benefits of substrate with different levels of elevation. The addition of substrate with higher elevation may create healthier conditions for restored mussels as has been shown for oysters, so locations with high mud content and sediment accumulation may benefit from the addition of higher shell substrate to help increase restoration efficiency. Additionally, the use of shell substrate may prove more beneficial when mussels are deployed at lower densities and utilize the shell for attachment and security. Overall, this study indicates that the efficacy of shell substrate for mussels may be context dependent, and that for adult P. canaliculus the addition of shell material, as a benthic substrate at low relief, appears to provide little advantage for adult mussel restoration at high deployment densities in low energy subtidal environments.