Brandon J. Puckett

Oyster Demographics in a Network of No-Take Reserves: Recruitment, Growth, Survival, and Density Dependence

Abstract Central to ecology and resource management is knowledge of the spatiotemporal scales at which demographic rates vary and the ecological consequences of demographic variation, such as that due to density dependence. We quantified the spatiotemporal variation in eastern oyster Crassostrea virginica recruitment, density, growth, and survival and assessed density dependence within a network of no-take reserves in Pamlico Sound, North Carolina. From 2006 to 2008, average oyster recruitment and total density increased 15- and fivefold, respectively. The unprecedentedly high oyster densities in certain reserves (up to 6,500/m2 at the end of the study) modified demographic rates such that further density increases were regulated by density-dependent survival. Oyster demographic rates varied significantly among reserves at relatively small spatial scales (20 km). Certain reserves were the strong “recruiters,” others the fast “growers,” and yet others the high “survivors.” Cohort dynamics altered the demographic rank order such that the demographically “best” reserves varied intra- and interannually. From a management perspective, the prevalence of density-dependent survival suggests that the oysters in this system are habitat rather than recruitment limited, which may minimize the utility of stock enhancement programs. Addition of habitat (i.e., artificial reefs) should focus on reserves characterized by high recruitment but density-dependent growth and survival. This study (1) supports the efficacy of marine reserves in rapidly increasing the density and age—size structure of protected species, (2) highlights the need for spatially explicit demographic data to support multifaceted management objectives, and (3) when combined with evidence of reserve larval connectivity, provides support for applying metapopulation concepts to this reserve system.

SPATIOTEMPORAL VARIATION IN OYSTER FECUNDITY AND REPRODUCTIVE OUTPUT IN A NETWORK OF NO-TAKE RESERVES

ABSTRACT Adult fecundity and reproductive potential can be critical determinants of subsequent larval supply and juvenile recruitment, and important determinants of placed-based management, especially when selecting sites for marine reserves where larval export is an expected outcome. We quantified spatiotemporal variation in fecundity and reproductive potential of female oysters (Crassostrea virginica) within a network of no-take oyster broodstock reserves by sampling over 3 y at 3 spatial scales: (1) per capita, (2) per square meter, and (3) per reserve. A total of 2,596 oysters were collected using scuba from six reserves in Pamlico Sound, NC, during 2006 to 2008 and processed in the laboratory for fecundity. Per-capita fecundity ranged from 0–340,500 eggs, and increased exponentially with oyster size, peaking in May of all years. In general, per-capita fecundity was highest at more inland mesohaline reserves, whereas reproductive potential per square meter and reserve reproductive potential were highest at more seaward polyhaline reserves as a result of a combination of relatively high density, and large oyster size and reserve areas. All 3 reproductive metrics increased in general over time. These results suggest that inland broodstock reserves should be prioritized for stock enhancement/seeding—and more seaward reserves for reserve expansion—and highlight the need to consider spatiotemporal variation and the scale at which a key demographic rate (fecundity and reproductive potential) is expressed when assessing the efficacy and conservation/restoration targets of marine reserves.

Validation and Application of Lipofuscin-Based Age Determination for Chesapeake Bay Blue Crabs Callinectes sapidus

Abstract Quantifying lipofuscin (LF), a metabolic byproduct that accumulates in postmitotic cells, serves as one of the principal approaches for aging crustaceans, but the accuracy of this method remains an important issue. Here, we quantified LF accumulation as a function of chronological age and temperature (degree-days) in an economically important crustacean, the blue crab Callinectes sapidus, to test the accuracy of LF-based age estimates and determine the age-specific partial recruitment of juveniles to summer and fall commercial fisheries. Three known-age juvenile cohorts (63-83 d) were reared in ponds up to 1.8 years of age. Field collections were conducted in two subestuaries of Chesapeake Bay from June to October during 2003 and 2004. Lipofuscin accumulation oscillated seasonally in known-age cohorts. Significant (log e transformed) LF accumulation occurred at average intervals of 2.5 months, with the exception of winter months (mean temperature = 8°C). Seasonalized von Bertalanffy functions acc...

Oyster Density and Demographic Rates on Natural Intertidal Reefs and Hardened Shoreline Structures

ABSTRACT The ubiquitous loss of natural intertidal oyster reefs and associated ecosystem services has fueled restoration efforts throughout the world. Effective restoration requires an understanding of the distribution, density, and demographic rates (growth and survival) of oysters inhabiting existing natural reefs and how these may vary as a function of landscape-scale factors, such as tidal range and fetch distances. Furthermore, natural intertidal habitats are increasingly being replaced with hardened shoreline structures that may be colonized by oysters, yet little is known about habitat quality (as indexed by oyster density and demographic rates) of these hardened structures relative to natural habitats. The present study sought to compare oyster density, demographic rates, and population estimates (1) across estuarine landscape settings to inform natural intertidal oyster reef restoration (i.e., comparing natural intertidal reefs within adjacent water bodies that vary in tidal regimes and fetch distances) and (2) across natural habitats and human-made structures to assess variation in habitat quality between natural reefs and hardened shorelines. Oyster density, growth rates, and population estimates on natural intertidal reefs were greatest within the smaller, more tidally influenced Core Sound versus the larger, wind-driven Pamlico Sound, with no significant difference in survivorship identified between the two water bodies. Natural intertidal reefs and hardened shoreline structures were compared within Pamlico Sound only, with natural intertidal reefs hosting three to eight times higher oyster densities than hardened shoreline structures. When mean oyster density/m2 was multiplied by reef area to estimate population size, natural intertidal reefs within Pamlico Sound hosted considerably greater populations of oysters relative to hardened shorelines. The present study fills an existing need to understand oyster density and demographic rates on natural intertidal reefs and hardened shorelines to better inform future restoration and shoreline management scenarios.

Oyster Demographics in Harvested Reefs vs. No-Take Reserves: Implications for Larval Spillover and Restoration Success

Fishery species that reside in no-take, marine reserves often show striking increases in size and abundance relative to harvested areas, with the potential for larval spillover to harvested populations. The benefits of spillover, however, may not be realized if the populations or habitats outside of reserves are too degraded. We quantified oyster population density and demographics such as recruitment, growth, mortality, and potential larval output as a function of two types of oyster management strategies in Pamlico Sound, North Carolina, USA: (1) natural reefs + harvested and (2) restored reefs + harvested. We compared these data to demographic data collected as a function of a third type of management strategy, (3) restored reefs + protected from harvest. Mean oyster recruitment was ~12 times higher in restored + harvested reefs than in natural + harvested reefs. Mean total oyster density was ~8- to 72-times higher in restored + protected reefs than in restored + harvested or natural + harvested reefs, respectively. Moreover, harvested reefs exhibited truncated size structure, and few or no individuals greater than legal size (75 mm), whereas protected reefs typically had a polymodal size structure, including many large individuals. We estimate that restored + protected reefs have ~4 to 700 times greater potential larval output m-2 than restored + harvested or natural + harvested reefs, respectively. After accounting for total sound-wide areal coverage of each reef type, total potential larval output from restored + protected reefs was ~6 times greater than that from natural + harvested and restored + harvested reefs. Marine reserves can potentially subsidize harvested populations via larval spillover, however, in the case of oyster reefs in Pamlico Sound, the relatively degraded conditions of natural reefs (e.g., low vertical relief, low shell volume per square meter) may not provide much in the way of suitable settlement substrate to realize the benefits of larval spillover from reserves. Restoration of oyster reefs, even with a thin veneer of substrate, may improve settlement substrate to increase the benefits of larval spillover from reserves.

Density-dependent role of an invasive marsh grass, Phragmites australis, on ecosystem service provision

Invasive species can positively, neutrally, or negatively affect the provision of ecosystem services. The direction and magnitude of this effect can be a function of the invaders’ density and the service(s) of interest. We assessed the density-dependent effect of an invasive marsh grass, Phragmites australis, on three ecosystem services (plant diversity and community structure, shoreline stabilization, and carbon storage) in two oligohaline marshes within the North Carolina Coastal Reserve and National Estuarine Research Reserve System (NCNERR), USA. Plant species richness was equivalent among low, medium and high Phragmites density plots, and overall plant community composition did not vary significantly by Phragmites density. Shoreline change was most negative (landward retreat) where Phragmites density was highest (-0.40 ± 0.19 m yr-1 vs. -0.31 ± 0.10 for low density Phragmites) in the high energy marsh of Kitty Hawk Woods Reserve and most positive (soundward advance) where Phragmites density was highest (0.19 ± 0.05 m yr-1 vs. 0.12 ± 0.07 for low density Phragmites) in the lower energy marsh of Currituck Banks Reserve, although there was no significant effect of Phragmites density on shoreline change. In Currituck Banks, mean soil carbon content was approximately equivalent in cores extracted from low and high Phragmites density plots (23.23 ± 2.0 kg C m-3 vs. 22.81 ± 3.8). In Kitty Hawk Woods, mean soil carbon content was greater in low Phragmites density plots (36.63 ± 10.22 kg C m-3) than those with medium (13.99 ± 1.23 kg C m-3) or high density (21.61 ± 4.53 kg C m-3), but differences were not significant. These findings suggest an overall neutral density-dependent effect of Phragmites on three ecosystem services within two oligohaline marshes in different environmental settings within a protected reserve system. Moreover, the conceptual framework of this study can broadly inform an ecosystem services-based approach to invasive species management.

Integrating Larval Dispersal, Permitting, and Logistical Factors Within a Validated Habitat Suitability Index for Oyster Restoration

Habitat suitability index (HSI) models are increasingly used to guide ecological restoration. Successful restoration is a byproduct of several factors, including physical and biological processes, as well as permitting and logistical considerations. Rarely are factors from all of these categories included in HSI models, despite their combined relevance to common restoration goals such as population persistence. We developed a Geographic Information System (GIS)-based HSI for restoring persistent high-relief subtidal oyster (Crassostrea virginica) reefs protected from harvest (i.e., sanctuaries) in Pamlico Sound, North Carolina, USA. Expert stakeholder input identified 17 factors to include in the HSI. Factors primarily represented physical (e.g., salinity) and biological (e.g., larval dispersal) processes relevant to oyster restoration, but also included several relevant permitting (e.g., presence of seagrasses) and logistical (e.g., distance to restoration material stockpile sites) considerations. We validated the model with multiple years of oyster density data from existing sanctuaries, and compared HSI output with distributions of oyster reefs from the late 1800’s. Of the 17 factors included in the model, stakeholders identified four factors—salinity, larval export from existing oyster sanctuaries, larval import to existing sanctuaries, and dissolved oxygen—most critical to oyster sanctuary site selection. The HSI model provided a quantitative scale over which a vast water body (~6,000 km2) was narrowed down by 95% to a much smaller suite of optimal (top 1% HSI) and suitable (top 5% HSI) locations for oyster restoration. Optimal and suitable restoration locations were clustered in northeast and southwest Pamlico Sound. Oyster density in existing sanctuaries, normalized for time since reef restoration, was a positive exponential function of HSI, providing validation for the model. Only a small portion (10-20%) of historical reef locations overlapped with current, model-predicted optimal and suitable restoration habitat. We contend that stronger linkages between larval connectivity, landscape ecology, stakeholder engagement and spatial planning within HSI models can provide a more holistic, unified approach to restoration.