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Effects of size-dependent allocation of energy to maintenance, growth, and reproduction on rehabilitation success in overexploited intertidal mussels Perna perna (L.).

ABSTRACT Mussels play an important supplementary role in the diet of coastal communities in many parts of the world. Over the last three decades, exploitation pressure on the intertidal brown mussel Perna perna on the southeast coast of South Africa has become unsustainable, eliminating mussels from substantial areas of suitable habitat and creating the need to rehabilitate denuded shores. The effect of mussel size on the effectiveness of the rehabilitation technique was tested, hypothesizing that mussel size may influence the effectiveness of rehabilitation due to size-dependent energy allocation (e.g., growth versus reproduction). Immature, small (1-2 cm), and sexually mature larger mussels (3-4 cm) were deployed at four sites, separated by distances of about 100 m, and sampled after 2, 4, and 6 mo. It was found that there were no significant differences between the two size classes in survival rates, which were always high (-60-80%). There were frequent size effects on the other variables measured, but these tended to depend on the criteria used. Small mussels showed significantly greater growth in terms of shell length, increase in weight, and percentage increase in dry weight. Similarly, small mussels had weaker byssal attachment strength than large mussels, but higher tenacity when attachment strength was normalized to shell area. Small mussels also generally had higher condition indices than large mussels. This was probably related to spawning by adults and the difference was not always significant. For large mussels, maintenance, including byssal thread production to resist dislodgement, seemed to have higher priority than shell growth and condition. The absence of significant differences in tenacity between size classes would explain their similar survival rates. No difference was found between size classes in the numbers of new recruits appearing in experimental patches. Deciding which size class is better for rehabilitation depends on the criterion used. Overall, it was found that ontogenetic reallocation of energy reserves affected neither biomass increase nor survival, probably the most meaningful definitions of success. Given their greater availability and efficiency at increasing biomass, it is suggested that on balance small mussels are likely to be more effective in restoring mussel populations.

KEY WORDS: mussels, Perna perna, ecological rehabilitation, overexploitation, natural resources

INTRODUCTION

Along much of the coast of the southeast coast of South Africa, shellfish are harvested for subsistence purposes. The primary target species is the intertidal mussel Perna perna, which comprises up to 95% of harvested intertidal shellfish (Lasiak 1992), and is severely overexploited in many places (Lasiak & Barnard 1995, Dye et al. 1997; L. Macala, personal observation). Harvesters collect mussels as small as 30-40 mm (Lasiak & Dye 1989) when they are only just sexually mature. Overexploitation is so severe that individuals that are capable of reproduction are removed and the species is often virtually eliminated from suitable habitat, making natural recovery impossible (Underwood 1993). The problem is exacerbated by two factors. First, recruitment rates in this region are markedly lower than along the rest of the country's coastline (Harris et al. 1998) and even where they exist within marine-protected areas, mussel populations are recruitment limited (Reaugh-Flower et al. 2011). Second, following overexploitation, mussel beds are replaced by beds of coralline macroalgae (Dye 1992) that intercept settling mussel larvae, which are then incapable of transferring onto the primary substratum (Erlandsson et al. 2008). The replacement of P. perna by coralline macroalgae results in a radical rearrangement of the whole biota (Lasiak & Field 1995) and the new configuration has medium/ long-term (>20 y) stability because the interception of settling mussel larvae prevents or at least severely delays natural recovery (Erlandsson et al. 2011). The only short-term management alternative is artificial rehabilitation of mussel beds and an effective technique involving reseeding of mussels onto the shore has been developed by Dye and Dyantyi (2002). The Mussel Rehabilitation Project, sponsored by the South African Department of Agriculture, Forestry and Fisheries, has used this technique in collaboration with local communities and the results have been very site specific, with high growth rates and 80% cover within a year at some sites, but slow growth, high mortality, and only 5% cover after a year at others (personal observation; M. G. Calvo-Ugarteburu, unpublished data).

Several parameters, which are related to energy allocation, are likely to affect the success of rehabilitation because mature and immature mussels are known to locate energy differently (Ren & Ross 2005). Growth depends on the amount of energy remaining after metabolic maintenance (Ren & Ross 2005) and how that energy is apportioned internally. Young mussels grow very rapidly (Hamburger et al. 1983, McQuaid & Lindsay 2000), whereas adults channel energy not only into growth and maintenance, a high proportion also goes into reproduction (Bayne et al. 1983, Matzelle at al. 2014). Perna perna of less than 25 mm are sexually immature (Rius et al. 2006), and they allocate limited energy to the development of the reproductive organs rather than the production of gametes (Ren & Ross 2005). According to net production models, assimilated energy is immediately available for maintenance, then for growth and any remaining energy is reserved (Ren & Ross 2005). Maintenance competes with and is more important than growth, which is retarded if all available energy is expended on maintenance (Kooijman 2000). Energy allocated for reproduction is stored before it is converted into eggs or sperm and released during spawning. Shafee (1989) described gamete/gonad development as a stage when gonads start to develop and gametogenesis becomes apparent and small groups of germinal cells are scattered in the mantle. Energy spent on attaining maturity by juveniles is spent on reproduction by adults. This means that large mussels require some energy for maintenance (including the production of byssal threads for the stronger attachment needed due to an increased surface area and associated drag) and for reproduction, which reduces the proportion of energy available for allocation to growth (Kooijman 2000, Ren & Ross 2005). Besides the energy required for reproduction by large mussels, there can be a direct relationship between maintenance costs and structural volume, so that larger adult mussels are assumed to use proportionally more energy on maintenance than small mussels (Kooijman 2000). Consequently, reproduction competes with byssal thread development and attachment strength in terms of energy allocation (Carrington 2002, Zardi et al. 2007). Carrington (2002) found that byssal thread development halts and tenacity decreases after the onset of gamete development, recovering after the release of the gametes when energy is again available for attachment. Similarly, in a study of Perna perna and the invasive mussel Mytilus galloprovincialis, Zardi et al. (2007) found that, for both species, peaks in the gonad index were preceded by gamete production during periods of low hydrodynamic stress, whereas during high hydrodynamic stress, energy was redirected toward byssus production, indicating a trade-off between the two. Given the different energetic priorities of sexually mature and immature individuals, our study asked whether the size of mussels used for reseeding shores during the rehabilitation process would influence the success of rehabilitation. It was hypothesized that small, sexually immature mussels would be more effective for rehabilitation in terms of survival and percentage increase in dry mass than large, sexually mature mussels due to their different energetic priorities.

To test this, the rehabilitation success of different age classes of mussels was measured by comparing the performance of small, sexually immature (1-2 cm) and larger, sexually mature (3-4 cm) individuals. "Success" was estimated using several energetically interlinked parameters. These were survival, attachment strength, growth, and condition.

MATERIALS AND METHODS

Study Sites and Approach

The study was carried over austral spring/summer (September 2010-February 2011) on rocky intertidal platforms at four sites separated by 100-150 m near Three Sisters on the south coast of South Africa (33[degrees] 33' 39" S, 27[degrees] 01' 17" E). All sites were on very flat wave-cut platforms. Wave action at each site was measured using dynamometers and showed very similar levels among all sites (L. Macala, unpublished data). Two size classes of Perna perna (small and sexually immature, 1-2 cm and large, sexually mature, 3-4 cm) were used. Thirty-six patches of mussels were attached at each site in haphazard positions separated by 0.5-5 m over a total area of about 105 [m.sup.2] (15 X 7 m) at each site in September 2010, with 18 patches of small and 18 patches of large mussels per site. Each patch comprised 40 mussels kept in a loose mesh bag (1-cm mesh). The mussels were allowed to attach to the rocks by covering them for a month with a 35 cm length of PVC drainage pipe that was cut in half along its length and attached to the rock so as to cover the bag of mussels. This piping had an extremely coarse mesh of about 1 X 5 cm. Piping was fixed onto the shore using coach screws with nylon plugs. Care was taken to ensure that mussels were held firmly under the pipes but could still open their valves. Pipes were removed after a month, but mussels were still kept in the mesh bags, which were secured using eye bolts. The mesh protected mussels, allowing them to reattach if they were dislodged during pipe removal. The use of mesh bags was necessary as during preliminary experiments all mussels were washed away overnight immediately after removal of the pipes. Four patches of each size class of mussel were removed from each site on each of three occasions to measure attachment strength, growth, condition index (CI), and survival. The first patches were collected 1 mo after the removal of the PVC drainage pipe (occasion 1, October 2010), the second after 3 mo (occasion 2, December 2010), and the last after 6 mo (occasion 3, February 2011). As patches were sampled destructively and each was used only on a single occasion, the effect of occasion/time was included in the analysis. All analyses were conducted using GMAV 5 software (1997).

Survival

Each mussel patch contained 40 mussels at deployment. Survival was determined for each of the four patches removed on each occasion. Data were analyzed using three-way analysis of variance (ANOVA) to determine the effects of site (random, 4 levels), size (fixed, 2 levels), and time (fixed, 3 levels) on survival.

Attachment Strength/ Tenacity

Four patches of each size class of mussels were removed from each site on each of the three occasions. Attachment strength was measured for five mussels from each of the removed patches using a fish hook attached to a spring balance by 15 cm of 25-kg fishing line. A small hole (1.5-2 mm) was drilled through the posterior lip of each mussel shell. The fish hook was inserted through the hole and the mussel was removed from the rock by pulling normal to the substratum, allowing measurement of the force required to dislodge each individual (Caro et al. 2008). Readings (mass, in kilograms) were converted to force as F= m X a, where a (m/[s.sup.2]) is the acceleration due to gravity. Number of replicates was 5 mussels X 4 patches = 20 per size class per site on each occasion. Four-way ANOVA was used to determine the effects of site (random), size (fixed and orthogonal), time (fixed and orthogonal), and patch (nested in size, site, and time).

Because the force exerted on a mussel by wave action depends on size, tenacity was calculated for the same individuals by dividing attachment strength by shell planform area (Bell & Gosline 1997).

Growth

This was measured in two ways: as change in shell length and as percentage increase in dry mass. For shell length, on initial set up of the experiment, all mussels were dried with a cloth and marked at the growing edge using Tipex as a tag. When the tags were dry, they were coated with super glue. As some tags were lost, growth checks were used as secondary markers for shell length at the time of translocation to the experimental plots. The growth history of mussels is frequently recorded on the shell in the form of growth increments that are seasonal or caused by disturbance, such as translocation (Seed 1969, Seed & Richardson 1990). This was corroborated by comparing Tipex tags and growth checks for 120 individuals. Growth was measured as shell increments, from the growth check or Tippex tag to the posterior growing edge of the mussel. Growth after deployment can be detected from growth increments but growth increments do not show how long growth was delayed after disturbance.

Vernier calipers were used to determine shell growth from Tipex marks or growth checks, defined as the difference between length at collection and length at deployment (Millstein & O'Clair 2001) and analyzed as growth per day using four-way ANO VA as above. Number of replicates was 10 mussels per patch per size class per site for each occasion.

Percentage Change in Dry Mass/Biomass

For percentage increase in dry mass, the final mass of the same individuals was measured directly after drying to constant mass at 60[degrees]C. The initial dry mass was estimated by deriving regressions relating the final dry mass and shell length. Separate regressions were derived for each occasion.

These were y = [0.0054x.sup.2.5475], [R.sup.2] = 0.80; y = [0.0039x.sup.2.7644], [R.sup.2] = 0.82; y = [0.00061x.sup.2.3341], [R.sup.2] = 0.83, where y is the initial dry mass and x the initial shell length. Mass change was calculated by subtracting initial mass estimated from the regression from the measured final mass. Change in dry mass was divided by estimated initial dry mass, multiplied by 100 to calculate percentage change in dry mass.

Percentage increase in dry mass

= [change in dry mass(g)/estimated initial dry mass(g)] x 100

Condition Index

The method of Davenport and Chen (1987) was used to determine CI. Ten mussels from each removed clump were placed in boiling water to remove the flesh from the shell. The flesh was dried to constant mass in an oven at 60[degrees]C for 48 h, and then shell and flesh were weighed separately (Steffani & Branch 2003). Four-way ANOVA was used to determine the effect of site, size, time, and patch on CI.

CI was calculated as:

CI = [dry flesh mass(g)/shell mass(g)] x 100

Number of replicates was 10 mussels per patch per size class at each site.

Recruitment

New recruits (<5 mm) present in each cleared plot were counted, taking particular care to check the byssal threads.

RESULTS

There were significant spatial effects of site for all tested variables, except survival and tenacity, and of patch in all nested analyses, but there was no hypotheses concerning either site or patch.

Survival

Neither site nor size had a significant effect on survival of mussels (P = 0.076, 0.668, respectively). There was a difference in survival among occasions; survival decreased from occasion 1 to 3 (Fig. 1), reaching about 60% by the end of the experiment at 6 mo (significant time effect, [F.sub.2, 6] = 12.09, P = 0.008).

Attachment Strength

Attachment strength increased over time for both size classes, and on all three occasions large mussels attached significantly more strongly than small mussels (Fig. 2), giving a significant size effect ([F.sub.1, 3] = 83.36, P = 0.003). Mussels at site 1 generally had stronger attachment than those at other sites. Attachment strength differed significantly among patches ([F.sub.72, 384] = 1.55; P < 0.005).

Tenacity

There was a significant three-way site X size X time interaction ([F.sub.6, 456] = 4.07, P = 0.0006), but small mussels had higher tenacity than large mussels in all comparisons (mean values 9.06 and 6.80 N/[mm.sup.2], respectively). There was no significant effect of site or occasion or the size X time interaction (Fig. 3).

Growth

Site, size, time, and patch all affected growth in shell length significantly (P < 0.01 in all cases) with no significant interactions. Small mussels grew faster than large mussels (mean 0.037 and 0.011 mm per day, respectively; [F.sub.1, 3] = 115.74. P < 0.005). Growth was highest on occasion 2, followed by occasion 3, in contrast to other parameters, which showed a steady increase from occasion 1 to 3 (Fig. 4).

Percentage Change in Dry Mass/ Biomass

Absolute increase in biomass and percentage change in biomass showed exactly the same responses. Both were significantly affected by all main factors, including patch, as well as the size X time and site X time interactions. Absolute and percentage increase in dry mass were markedly higher for small mussels (mean for percentage increase 119.35% and 21.99% for small and large individuals, respectively, Figs. 5 and 6), though the magnitude of difference differed among sites and occasions (size X time, [F.sub.2, 6] = 9.98, P = 0.01; site X time, [F.sub.6, 6] = 3.88, P = 0.002). Presumably, biomass was accumulated by immature mussels, but was lost as gametes in the case of mature animals.

Condition Index

Again, all main factors were significant, as was the site X time interaction. Small mussels had significantly ([F.sub.1, 3] = 11.99; P < 0.05) better CI values than large mussels, but condition differed significantly between the first and last occasions for both size classes (Fig. 7). In the case of large mussels, this resulted from a steady decline (CI = 6.2-4.5), but for small mussels, there was a drop from occasion 1 to occasion 2, with no change thereafter (CI = 6.9-5.3).

Recruitment

As expected, numbers of new recruits present in patches were extremely low (maximum <10) when they were cleared and differed among sites ([F.sub.3, 6] = 6.06, P = 0.001), and with time ([F.sub.2, 6] = 7.68, P = 0.02), but size had no effect ([F.sub.1, 3] = 4.01, P = 0.34).

DISCUSSION

The root problem of overexploitation can be addressed through the creation of marine-protected areas or the rigorous imposition of size classes, bag limits, and so on, but in situations where overexploitation has already occurred, rehabilitation can be highly successful with up to 80% cover achieved within a year on shores where mussels had been virtually eliminated (Calvo Ugarteburu, unpublished data). This study asked whether ontogenetic changes in energy allocation between mature and immature mussels affect their usefulness in the rehabilitation of overexploited shores. It was found that even small differences in size between mature and immature mussels result in differences in their performance. Size did not influence survival or the number of new recruits in cleared patches (unpublished data), but smaller mussels showed better condition and tenacity, faster shell growth, and proportionally more rapid increases in biomass. Coupled with the fact that small individuals are much easier to find locally (personal observation), it is believed that this makes them better candidates for rehabilitation.

Survival

The first criterion for success is simple survival of transplanted individuals, and no difference was found between the two size classes in this study. Mussel larvae suffer intense mortality through cannibalism as they attempt to settle among the filter-feeding adults (Porri et al. 2008) and very small recruits (2-10 mm) in South Africa experience high rates of predation from both pelagic and benthic predators (Plass-Johnson et al. 2010). For mussels larger than such early recruits, the main predators are octopuses (Smale & Buchan 1981, McQuaid 1994) and birds (Griffiths & Hockey 1987), but overall rates of predation on larger mussels are very low in South Africa (Branch & Steffani 2004, Nicastro et al. 2007). The main cause of mortality is competition for space (Griffiths & Hockey 1987), which is not limiting on these denuded shores. Given similar levels of survival, several criteria were used to evaluate success and clear differences were found between size classes that presumably reflect ontogenetic shifts in energy allocation.

Dynamic energy budget (DEB) models assume that assimilated energy is allocated first for maintenance and then to other processes such as reproduction and growth (Kooijman 2000, Ren & Ross 2005). Adult mussels tend to emphasize reproduction at the expense of growth and large mussels can expend more than 90% of assimilated energy on reproduction (Seed & Suchanek 1992), though energy allocation may change in the face of environmental stress (Petes et al. 2007). Immature mussels channel energy into the development of the reproductive organs rather than gametes and the proportion used for this is minor (Ren & Ross 2005). In models for bivalves, reproduction and growth are assumed to compete for energy resources (e.g., Ren & Ross 2005 for Perna canaliculus and Rosland et al. 2009 for other bivalves) and DEB models based on these assumptions provide predictions of growth and reproduction for Perna perna that prove accurate when validated against field measurements (Tagliarolo et al. 2016). The effects of size that was observed were in agreement with the DEB models and suggest different energetic imperatives for adults and juveniles, a variation on the "principle of allocation" described by Steyermark (2002) that essentially reflects the balance among maintenance, growth, and reproduction (e.g., Hawkins et al. 1986). The results also aligned well with the predictions of these models in terms of condition and growth.

CI and Growth

Condition index relates the amount of shell to the quantity of living tissue (Davenport & Chen 1987) and is affected by spawning. This makes it a potentially useful measure for identifying the probability of rehabilitation success, assuming that the CI of reproductive adults would be higher at sites that lend themselves to successful rehabilitation. Reproduction is highly stressful for adults (Myrand et al. 2000) and spawning results in marked decreases in adult condition (Dix & Ferguson 1984). Small mussels showed better CI than adults, though both showed a gradual decline in condition across the duration of the experiment. For small mussels, this was a drop only between the first two sampling occasions after which condition stabilized. Large mussels showed a stronger and more consistent decline probably due to the effects of spawning. In this region, the study species shows trickle spawning with intermittent peaks. The timing can differ strongly among sites separated by tens of kilometers, but spawning is frequent during the spring-summer months of the study period (McQuaid & Phillips 2006, Zardi et al. 2007). The decline in the condition of large mussels was particularly marked shortly after translocation on occasion 1 and probably indicates stress-induced spawning.

Sexually immature individuals can theoretically make a higher proportion of energy available for growth and small mussels showed faster growth in shell length. Because they exhibited higher shell growth than large mussels, small mussels are expected to have similarly larger increases in dry mass. In fact, small individuals were more efficient at accumulating biomass, showing higher rates of increase when this was measured as a percentage change.

Attachment Strength

A second potential cause of mortality is removal through wave action. Attachment strength is related to the number and tensile strength of the byssal threads (including the plaque) and is a critical adaptive response for mussels, allowing them to resist dislodgment (Caro et al. 2008). Tenacity depends on both attachment strength and the surface area exposed to wave forces and is directly linked to the probability of dislodgement (Carrington et al. 2009). Working on Mytilus californianus, Denny (1987) found that tenacity increased slightly with size, in this study attachment strength was greater for large mussels and tenacity clearly greater for small individuals. Attachment strength is achieved by diverting energy into byssal thread production (Cheung et al. 2006) and increases rapidly in response to wave action (Carrington et al. 2008). In energy budget models, maintenance (including byssal thread production) is regarded as having a higher priority than growth (Kooijman 2000). Byssal production forms a substantial part of maintenance costs (Hawkins & Bayne 1985) and can compete for energy with reproduction so that channeling energy into attachment may limit reproduction in large mussels and rapid byssal production in response to seasonal increases in wave action coincides with low reproductive output (Zardi et al. 2007). Large individuals had stronger attachment strength, which reflects the production of more byssal threads. The likelihood of removal is, however, linked to tenacity, which is attachment strength normalized to the planar area of the shell exposed to lift or drag (Carrington 2002) and small mussels were more tenacious than large mussels.

Spatial Effects

There were no hypotheses concerning the effects of site or patch, but both had frequent effects. These could have been due to differences in wave exposure at the site scale (sites were separated by hundreds of meters) and/or local hydrodynamics at the patch scale (patches were separated by cm to m). In general, any natural landscape has features of spatial heterogeneity that influence ecological processes and patch dynamics (Levin 1992), the degree of influence depending on the nature and scale of heterogeneity (Miller & Etter 2008). The importance of site as a major determinant of mussel growth has long been recognized and demonstrated through transplant experiments (Dickie et al. 1984) and indeed it is already known that some sites can be rehabilitated much more effectively than others (unpublished data). Wave exposure has strong effects on the need to allocate energy to avoid dislodgement and is known to affect growth rates directly (McQuaid & Lindsay 2000; McQuaid et al. 2000) and differences among sites in water flux and wave action may explain why some shores can be rehabilitated easily and others not at all. In this case, however, the four sites showed no consistent differences (tested using ANOVA) in either maximum wave force, measured using dynamometers (n = 6 per site, measured monthly for 6 mo), or bulk water flux, estimated using cement balls (n = 6, placed on the shore for 24 h each month). Hydrodynamics were not measured at the patch scale, and, although patches were placed only 0.5-5 m apart at the same shore level on relatively flat shores, it is known that very small-scale changes in hydrodynamics can affect growth rates. Hydrodynamics have been experimentally manipulated at centimeter scales, with strong effects on Perna perna growth rates (McQuaid & Mostert 2010), presumably through effects on the delivery of food particles, and there were differences in growth among patches of the same-sized mussels at the same sites, presumably reflecting the effects of patch position on microhydrodynamics within sites.

Recruitment

Recruited larvae determine the distribution, density, and abundance of sedentary benthic populations (Underwood & Fairweather 1989, Underwood & Keough 2001) and in the case of mussels, larvae can settle among macroalgae or within existing adult beds (McQuaid & Lindsay 2005), particularly among the byssal threads (Lasiak & Barnard 1995). The marine mussel Mytilus edulis is believed to exhibit a primary/secondary settlement pattern that allows plantigrades to relocate from algae onto adult beds after a period of growth (Bayne 1964). This behavior is not shown by Perna perna, however, so that larvae settling onto macroalgae are believed to be lost when they grow too large to maintain themselves there (Erlandsson et al. 2008). This is unfortunate as overexploitation of mussels in this region results in their replacement with dense cover of coralline macroalgae (Lasiak & Field 1995), so that almost the only settlement site for larvae is algae and natural recovery is extremely delayed. Experimentally cleared plots in the region have remained covered by algae for c. 20 y (A. H. Dye, unpublished data).

Theoretically, at the metapopulation level large mussels could have the advantage of their reproductive output and the attraction to larvae of their more complex byssal structure, especially as recruitment rates are particularly low in the study region and these populations are recruitment-limited (Reaugh-Flower et al. 2011). Previous studies have indicated, however, that dense populations of large adults in nearby marine-protected areas export the larvae of the infauna associated with mussel beds, but that recruitment of mussels themselves was not enhanced within or close to MPA compared with more distant sites (Cole et al. 2011). Similarly, densities of recruits within our experimental patches showed differences among sites, but no significant effect of mussel size.

CONCLUSION

Overall, with quite small differences in size reflecting whether individuals were sexually mature, differences were found in the performance of mussels during experimental rehabilitation of shores. The differences between age classes presumably reflect an emphasis on growth for juvenile mussels and on reproduction coupled with the need for greater attachment strength in adults. In the case of growth, mussels compete aggressively with one another for primary space and small mussels showed much faster shell growth. This presumably reflects the need to dominate physical space. In the case of avoiding dislodgement by wave action, the larger surface area of adult mussels requires stronger attachment strength to compensate for the greater drag they experience.

In terms of management and mussel rehabilitation, there was no difference in survival or accumulation of new recruits between patches of large and small mussels. Large mussels will produce larvae, with the potential to seed existing beds, but the fact that protected beds in local MPA do not show self-recruitment or increase recruitment at nearby shores (Cole et al. 2011) suggests that this advantage is minimal. The object of rehabilitation in this situation is to provide a sustainable source of food for local fisher folk, rather than to conserve mussels. In this case, large mussels appear to channel energy into reproduction and into byssal production to achieve high tenacity. Neither of which has immediate benefits for local harvesters. In contrast, small mussels are more efficient at producing new biomass, suggesting that they are better able to fulfill the objectives of restoring overexploited mussel populations.

ACKNOWLEDGMENTS

This work is based on research supported by the South African Research Chairs Initiative of the Department of Science and Technology and the National Research Foundation, the Swedish Research Council and SIDA/Sarec, Rhodes University and the South African Department of Agriculture, Forest and Fisheries. We thank S. Baldanzi, F. Porri, V. Cole, C. Von Der Meden, A. Ludford, P. Cramb and L. Johnson for their help in the field. Thanks to the Almighty God, Jehovah for giving us an opportunity to study the complexity of his creation.

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L. MACALA * AND C. D. MCQUAID

Department of Zoology and Entomology, Life Science Building, Barratt Complex, African Street, Rhodes University, P.O. Box 94, Grahamstown 6140, South Africa

* Corresponding author. E-mail: lukholomacala@yahoo.com

DOI: 10.2983/035.036.0103

Caption: Figure 1. Mean (+SE) survival among occasions and between size classes. Letters indicate homogenous groups among occasions. There was a significant interaction between size and time. Large mussels survived better on occasion 2.

Caption: Figure 2. Mean (+SE) attachment strength increased over time, large mussels attached more strongly than small mussels. Letters indicate homogenous groups among occasions.

Caption: Figure 3. Mean (+SE) tenacity. Small mussels had greater tenacity than large mussels. Occasion had no significant effect.

Caption: Figure 4. Mean (+SE) growth per day showing higher growth by small mussels than large mussels. The effect of occasion was significant. Letters indicate homogenous groups among occasions.

Caption: Figure 5. Mean (+SE) increase in absolute biomass. Values increased overtime, small mussels had greater increase in biomass than large mussels. Letters indicate homogenous groups among occasions.

Caption: Figure 6. Mean (+SE) percentage gain in dry weight, small mussels had higher percentage change in dry weight than large mussels. Letters indicate homogenous groups among occasions.

Caption: Figure 7. Change in mean (+SE) CI over time. Letters indicate homogenous groups among occasions.
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Author:Macala, L.; McQuaid, C.D.
Publication:Journal of Shellfish Research
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Date:Apr 1, 2017
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