Oyster Density Research Articles

The hybrid approach to monitoring: a remote Rapid Assessment Protocol (RAP) complements existing monitoring tools for oyster restoration and management

Accurate and efficient monitoring of oyster reefs is critical for fisheries management and restoration. However, monitoring subtidal reefs requires labor‐intensive methods, such as sampling by SCUBA divers. A new Rapid Assessment Protocol (RAP) based on the collection of underwater imagery from random replicate points across a reef expands the toolkit for oyster reef monitoring. This study tests how the qualitative RAP compares to existing quantitative monitoring tools to inform approaches that combine methods and improve monitoring. We surveyed paired unrestored harvested reefs and restored sanctuary oyster reefs in the upper, middle, and lower Chesapeake Bay (United States) to compare the RAP to diver‐collected oyster metrics across management types and salinities. At each site, we compared the RAP's categorical habitat scores (based on percent cover and relative reef height in images) with physical metrics from synchronous diver collections, including live oyster density and biomass, on the same reef quadrats (n = 64). High RAP scores successfully captured high oyster density, biomass, reef height, rugosity, and multiple size classes. At lower RAP scores, the physical metrics often differed between RAP scores on unrestored harvested reefs, but not restored sanctuary reefs. This study highlights the value of underwater video in increasing monitoring efficiency with respect to time investment, while providing different habitat data than physical metrics. We describe how the RAP can be used in combination with more intensive, physical monitoring tools for restoration and fisheries management. The results of the RAP can inform oyster management in Chesapeake Bay and can be adapted for use worldwide.

Seven Years of Monitoring the Development of an Oyster Reef Living Shoreline

The implementation of nature-based solutions (NbS), including living shorelines, to mitigate estuarine habitat loss is increasing at a pace exceeding the evaluation of their long-term success. Constructed oyster reefs (CORs) made of shell, concrete, stone, and other materials are one living shoreline tactic that is widely utilized, yet few studies have been conducted to understand the development of CORs within the context of both physical and ecological parameters over longer time scales (4 + years). A COR-based living shoreline project at the Gandy’s Beach Preserve (GBP) in Delaware Bay, NJ, USA, had dual goals of coastal protection and habitat provisioning, which prompted the development of a goal-driven monitoring framework to track project objectives. Methods were developed to quantify the following multi-disciplinary metrics over 7 years: elevations of CORs, waves (height, period, and direction), shoreline elevations, change in extent of vegetation patches, oyster density and size, nekton richness and community composition, and horseshoe crab impingement. The CORs met most of their habitat provisioning objectives as they were colonized by a multi-generational population of shellfish and created habitat for nekton, while posing negligible hazards to horseshoe crabs. However, none of the coastal protection objectives was fully achieved including material stability, wave attenuation, and sediment elevation increase. Results highlight the value of longer-term monitoring to understand performance and the need to match the scale and type of NbS tactic(s) with both the scale of the landscape and the site-specific hydrodynamic conditions to meet project goals.

Combined Effects of Local and Regional Drivers on Oyster Spat Density and Growth in Eastern Australia

Up to 85% of shellfish reefs have been lost worldwide, resulting in declining ecosystem services, and increasing restoration demand. However, more information regarding the conditions which maximise oyster settlement and growth is required to optimise restoration. We deployed oyster settlement tiles at 21 intertidal sites throughout Moreton Bay, Australia; a region where > 96% of rock oyster reefs are lost and demand for restoration is high. We quantified effects of variables describing the spatial (from GIS), local habitat (using quadrats and water quality measures), and oyster predator (using underwater videography) characteristics of sites on oyster density and size on tiles. Oyster density was highest at sites with intermediate predator abundance and temperature, highest nearby invertebrate cover, and low and high values of turbidity and nearby rock and algae cover. Conversely, oyster size was highest at sites with intermediate predator density, higher fish species richness and turbidity, and lowest temperatures. Together, this showed that optimal restoration requires sites with 22 to 23 °C average water temperatures, between 10 and 15 oyster predators, and either low (< 2 NTU) or high (> 6 NTU) turbidity levels. Notably, we observed multiple peaks for several variables, suggesting the potential presence of multiple cryptic oyster species on settlement tiles. We found that oysters shared preferred environmental conditions with polychaetes, coralline algae, and tunicates, and were more prevalent and abundant at sites with lower turf algae, barnacle, and mussel cover. Identifying environmental variables influencing oyster population distribution, settlement, and growth can guide the selection and approach of oyster restoration sites.

The application of unoccupied aerial systems (UAS) for monitoring intertidal oyster density and abundance

The application of unoccupied aerial systems (UAS) for monitoring intertidal oyster density and abundance

Short-term effects of an unprecedented heatwave on intertidal bivalve populations: fisheries management surveys provide an incomplete picture

IntroductionCoastal marine ecosystems, are particularly susceptible to climate change. One such threat is atmospheric heatwaves, which are predicted to increase in frequency, duration, and intensity. Many intertidal organisms already live at the edge of their thermal tolerance limits and heatwaves can outstretch an organism’s ability to compensate in the short term. In June 2021 the Pacific Northwest region of North America, including the Salish Sea, experienced a significant atmospheric heatwave during some of the lowest tides of the year. This was followed by numerous reports of dead and dying intertidal marine organisms region-wide. A semi-quantitative rapid assessment found a range of both species- and location-specific effects but generally recorded widespread negative impacts to intertidal shellfish species across the Salish Sea. MethodsFollowing these results, we opportunistically analyzed data collected by intertidal bivalve resource managers from the region. These datasets allowed us to examine regional density and size data for clam and oyster populations before and after the heatwave to increase our quantitative understanding of heatwave effects. ResultsWe found a range of responses including positive and negative effects of the heatwave on clam and oyster density. While we generally found small changes in bivalve size, some site-species combinations displayed large shifts in size frequency. Many of our analyses did not indicate even moderate statistical support, even with large changes in the mean, driven in part by high variability in the data. Time intervals between surveys, ranging from 2 to over 25 months, had little effect on observed variability indicating that any heatwave-induced effects may be masked by variability inherent to the population ecology and/or survey methodology. DiscussionThis analysis has highlighted the need for intertidal resource managers, and the greater research community, to consider alternative survey approaches designed to constrain variability in order to detect the effects of acute or extreme events. With the effects of climate change predicted to become more intense, targeted survey approaches may be needed to detect the effects and implications of such events and to continue effective management of intertidal bivalves in the Salish Sea and worldwide.

Drone imagery and deep learning for mapping the density of wild Pacific oysters to manage their expansion into protected areas

The recent expansion of wild Pacific oysters already had negative repercussions on sites in Europe and has raised further concerns over their potential harmful impact on the balance of biomes within protected areas. Monitoring their colonisation, especially at early stages, has become an urgent ecological issue. Current efforts to monitor wild Pacific oysters rely on “walk-over” surveys that are highly laborious and often limited to specific areas of easy access. Remotely Piloted Aircraft Systems (RPAS), commonly known as drones, can provide an effective tool for surveying complex terrains and detect Pacific oysters. This study provides a novel workflow for automated detection, counting and mapping of individual Pacific oysters to estimate their density per square meter by using Convolutional Neural Networks (CNNs) applied to drone imagery. Drone photos were collected at low tides and altitudes of approximately 10 m across a variety of cases of rocky shore and mudflats scenarios. Using object detection, we compared how different Convolutional Neural Networks (CNNs) architectures including YOLOv5s, YOLOv5m, TPH-YOLOv5 and FR-CNN performed in the detection of Pacific oysters over the surveyed areas. We report the precision of our model at 88% with a difference in performance of 1% across the two sites. The workflow presented in this work proposes the use of grid maps to visualize the density of Pacific oysters per square meter towards ecological management and the creation of time series to identify trends.

Habitat Provision Differs Across Subtidal Reefs Varying in Location Within the Estuarine Landscape

Although position within the estuarine landscape is known to influence structural characteristics and ecological functioning of many habitats, the influence of position on subtidal oyster reef characteristics and functioning is relatively understudied. This study assessed habitat provision by oysters and benthic macrofauna on subtidal bar and patch oyster reefs within the northwestern Gulf of Mexico estuary. Bar reefs had greater and more variable oyster density, biomass, and volume, as well as dead shell material, compared to patch reefs. Benthic macrofaunal community composition differed between reef types, with bar reefs hosting 11× and 30× higher biomass of Porcellanidae and Ampithoidae crustaceans, and patch reefs supporting 34× and 47× higher biomass of Eunicidae annelids and Leptocheliidae crustaceans. The environmental variables most highly correlated with macrofaunal communities on both bar and patch reefs were sediment chlorophyll-a, volume of dead oyster shells, and depth. Despite differences in structural characteristics and position within the estuarine landscape, subtidal oyster reefs are often managed as a single habitat type. Our findings emphasize the importance of considering location in oyster reef conservation and restoration efforts to better predict and optimize faunal provision outcomes.

Location and reef size drive oyster reef restoration success

Optimizing habitat restoration success requires understanding how restoration location and design enhance the persistence and function of a restored habitat. Particular attention to the configuration of structure and its interaction with landscape‐scale processes is critical for enhancing the habitat value of restored areas. We monitored six subtidal restored oyster reefs in Pamlico Sound, North Carolina, United States, to identify how oyster demographics responded to initial habitat characteristics (e.g. reef area, volume, vertical relief, and perimeter‐to‐area ratio) and how changes in habitat characteristics over time altered suitability for oysters. Changes in reef habitat were measured by repeated mapping using bathymetric and side‐scanning sonar systems. A 2‐year time series of oyster demographic data quantified oyster response to habitat changes over time. All reefs provided habitat for the settlement and growth of oysters. Within 2 years of restoration, relative differences in oyster recruitment and survival emerged and were related to variations in reef location and two‐dimensional habitat characteristics among reefs, namely reef area and perimeter‐to‐area ratio. Larger reefs that were less fragmented resisted burial from sedimentation and enhanced oyster densities and biomass relative to smaller, more fragmented reefs that became heavily sedimented and failed to support oyster recruitment and survival. Positive feedback mechanisms between habitat characteristics and oyster recruitment success were established within 1 year of restoration and were likely driven by landscape‐scale processes such as sediment dynamics and larval supply. To improve restoration success, we recommend creating larger reef surfaces with low perimeter‐to‐area ratios in areas that promote habitat persistence.

Reversing functional extinction: successful restoration of eradicated oyster reefs

Functionally extinct ecosystems, those that have been locally eradicated save for remnant individuals, are unlikely to naturally recover over meaningful human time frames. However, ecosystem restoration provides opportunities to reverse functional extinction by rapidly addressing the physical and/or biological barriers that prevent natural recovery. Here, we assess the restoration progress of a native Flat oyster (Ostrea angasi) reef ecosystem in South Australia that was eradicated from the Australian mainland approximately 100 years ago. In the absence of any reference Flat oyster ecosystems in the region, restoration progress was assessed relative to ecological targets informed by a combination of local rocky reef ecosystems and an interim Flat oyster reference model informed by Australia's sole remaining O. angasi reef, in Tasmania. Two and half a years after the restoration was initiated via the construction of 14 boulder reefs, we observed densities of restored native adult O. angasi (192 ± 19 m−2; mean ± 1 SE) that exceeded oyster densities observed on the sole remaining natural reef. Communities of macroinvertebrates on the reef restoration represented approximately 60% of the biodiversity observed on healthy rocky reef reference systems, while ecological functions (e.g. filter feeding) are demonstrably increasing. The rate of recovery of this benthic ecosystem, from functionally extinct to a restored Flat oyster reef ecosystem within several years, demonstrates the latent resilience of degraded oyster communities and the capacity for effective marine restorations to achieve rapid ecological recoveries.

Increasing duration of heatwaves poses a threat to oyster sustainability in the Gulf of Mexico

The future of the wild oyster fishery in the northern Gulf of Mexico is largely uncertain due to changing environmental conditions and declining abundance of harvestable oysters. Specifically, rising temperatures can directly impact the physiological thresholds of the eastern oyster (Crassostrea virginica) at all life history stages and alter the narrow ecological niche this oyster occupies. The impact of rising temperatures is likely most pronounced during atmospheric heatwaves, defined as three or more days above the 90th percentile of daily maximum air temperatures, which have been shown to be increasing in frequency. Increasing exposure to high temperature extremes may contribute to and exacerbate an already declining oyster fishery. Critical to fishery health is recruitment i.e., the addition of new harvestable biomass, which is a dynamic process strongly driven by temperature. Here, we examine the relationship between heatwave characteristics and the prediction of poor oyster recruitment, measured as the abundance of post-larval oysters (e.g. spat) below the site-specific median density observed in historically productive oyster fisheries over 46-years (1976 – 2020) in Mobile Bay, Alabama and 21-years (1993 – 2014) in Apalachicola Bay, Florida. We acquired daily maximum air temperature measurements measured over 50 years (1970 – 2020) at weather monitoring stations adjacent to the bays to identify site specific annual heatwave events (maximum yearly air temperature, yearly and consecutive heatwave days, and number of annual heatwaves). Then, years with extreme heatwaves that exceeded the 75th percentile for the 50-year measurements were compared to years with non-extreme heatwave events. Years with extreme total heatwave days and extreme consecutive heatwave days were correlated with low post-larval oyster density. Across both bay systems, if consecutive heatwave days exceeded 11 days, then poor recruitment of oysters occurred 83 % of the time. Extreme heatwave duration as an indicator for poor recruitment has the potential to be a powerful tool for fishery managers to forecast recruitment and inform sustainable oyster harvest based on year-to-year variability in heatwave duration and long-term warming trends. Our findings illustrate how extreme temperatures can exacerbate multiple physiological and ecological stressors resulting in the loss of a keystone species for healthy and resilient coastal ecosystems.

Inorganic osmolytes and enzymatic biomarkers from the manabi oyster (Crassostrea cf. corteziensis) in response to saline stress

Abrupt drops in salinity that occur in tropical estuaries during the equatorial rainy season led to hyposaline conditions which may reduce the populational density of oysters. To assess the effect of saline stress on physiological and metabolic responses of the Manabi oyster (Crassostrea cf. corteziensis) was exposed to 35, 30, 20,10 and 5‰ concentrations during 96 h. Inorganic osmolytes, pH, salinity, haemocyanin and protein concentration in the plasma as well as the number of oysters with closed valves were recorded. Aspartate aminotransferase (AST), alanine aminotransferase (ALT), lactate dehydrogenase (LDH), alkaline phosphatase (ALP) and catalase (CAT) activity were analysed. Inorganic osmolytes and internal salinity were elevated in oysters exposed to 35, 10 and 5‰. A significant number of oysters with valve closure was observed in 10 and 5‰, which coincided with a decline in physiological pH and changes in haemocyanin concentrations. AST activity and AST/ALT ratio were reduced under 35, 10 and 5‰, and CAT increased in oysters exposed to 35‰; but protein concentration, LDH and ALP did not show significant variations. Metabolic adjustment and behavior of the Manabi oyster could explain tolerance and survival (at least for a short term) to hyposaline stress in tropical estuarine ecosystems.

Successful initial restoration of oyster habitat in the lower Hudson River Estuary, United States

The eastern oyster (Crassostrea virginica), has experienced dramatic declines throughout its range, and most U.S. states along the Atlantic and Gulf of Mexico coasts have restoration programs. Oyster restoration in the Hudson River Estuary has been a focus of management agencies for over two decades. The present project was initiated in 2013 after sampling for replacement of the Tappan Zee bridge determined live oysters would be lost during bridge construction, thus requiring mitigation measures. Preliminary studies identified three areas for pilot studies to inform the design of full‐scale efforts. A pilot study involving deployment of oyster shell‐filled gabions and two styles of Reef Balls, found all substrates supported oyster recruitment and growth but there were higher oyster densities on gabions. Some of the gabions, however, failed structurally necessitating a need for new design. For full‐scale restoration, a total of 422 modified gabions and 881 Mini‐Bay style Reef Balls were deployed across three sites which totaled approximately 2.4 ha (6 ac) in seafloor area. Both substrate types were heavily colonized by oysters and several other species at all three sites, essentially duplicating the findings of the pilot study. The final monitoring event in 2020 indicated a total of approximately 5.8 million live oysters were on the substrates deployed in 2018. This mitigation effort was the largest oyster habitat restoration project in, or north of, the New York Harbor region in recent decades based on restoration area and several oyster metrics of early success.

Seasonal variations in the diversity and benthic community structure of subtidal artificial oyster reefs adjacent to the Luanhe River Estuary, Bohai Sea

Artificial oyster reefs provide important spawning and nursery grounds for a variety of fishes and large mobile crustaceans. Between July 2016 and May 2017, seasonal surveys of species composition and community structure were performed in the artificial oyster reef area and control area adjacent to the Luanhe River Estuary in China. During the survey year, 56 species belonging to 50 genera, 45 families, and 19 orders were recorded. The dominant economically important fish and mobile crustaceans were Hexagrammos otakii, Pholis fangi, Sebastes schlegelii, Charybdis japonica, and Oratosquilla oratoria. Resident fishes belonged to the Cynoglossidae, Paralichthyidae, Pleuronectidae, and Gobiidae families. Seasonally important fish species included Lateolabrax japonicus, Konosirus punctatus, Thryssa kammalensis, Hexagrammos agrammus, and Acanthopagrus schlegelii. The ranges of H' values among stations were 1.18–2.16, 0.65–1.75, 1.18–2.06, and 0.62–1.92 in spring, summer, autumn, and winter, respectively. The benthic organisms present in the community of artificial oyster reef areas can be classified into groups according to month and season. The abundance biomass curves showed that the oyster reef area in spring, autumn, and winter experienced low disturbance, whereas the community structure in summer was subject to large variations from external disturbance. We also found that as the age of the oyster reefs increased, the percentage of oysters in the low shell height group (< 40 mm) decreased. The oyster density was 324 ind/m2 for the reef created in 2016, 724 ind/m2 for the reef created in 2015, and 364 ind/m2 for the reef created in 2013. These findings can be used to develop suitable management strategies for the sustainable maintenance of artificial oyster reef ecosystems.

Characterizing canopy complexity of natural and restored intertidal oyster reefs (Crassostrea virginica) with a novel laser‐scanning method

The structural complexity of oyster reef canopy plays a major role in promoting biodiversity, balancing the sediment budget, and modulating hydrodynamics in estuarine systems. Although oyster canopy structure is both spatially and temporally heterogeneous, oyster canopies are generally characterized using simple first‐order quantities, like oyster density, which may lack the ability to sufficiently parameterize reef roughness. In this study, a novel laser‐scan approach was used to map the surface of intact reference and restored reefs (restoration age: 1–4 years) during low tide, when the oyster canopy was fully exposed. Measurements were used to estimate hydrodynamically relevant roughness characteristics over the entire reef surface (&gt;140 m2; 0.50 m resolution), providing estimates of the canopy height (hc), standard deviation (), rugosity index (R), and fractal dimension (D). Average canopy heights ranged from 3.6 to 4.9 cm, with canopy height standard deviations between 1.4 and 2.0 cm. Mean rugosity indices and fractal dimensions were relatively low on the youngest (1 year) restored reef (R = 1.28;D = 2.67), with substantial increases observed for more mature reef canopies (4 years:R = 1.56;D = 2.71). Structural complexity was consistently greater on reef margins than in reef interiors. Increases in complexity were linked to restoration age, with older reefs exhibiting more complex oyster canopies. The highest fractal dimension was observed on the intact reference reef, highlighting the importance of sustained reef growth for maintaining higher‐order structural complexity. Results provide spatially explicit surface roughness characterizations for healthy, intertidal oyster reefs, with applications in both restoration science and natural and nature‐based feature design.

Assessment of Infection Prevalence and Intensity of Disease-Causing Parasitic Protozoans Perkinsus marinus and Haplosporidium nelsoni in Georgia Oysters.

Eastern oysters, Crassostrea virginica, are ecologically and economically important coastal species which provide a commercially valuable food product while also improving water quality through filtration, protecting shorelines, and providing habitat. The protozoan parasites Perkinsus marinus and Haplosporidium nesloni commonly infect oysters along the United States Atlantic and Gulf coasts and have been linked to poor oyster health and mass mortality events. In this study, wild oysters were collected from multiple reefs within four tidal creeks along the coast of Georgia to investigate P. marinus and H. nelsoni prevalence and intensity, their potential impact on oyster health, and identify possible drivers of the parasites. A second study occurred on four sites on Sapelo Island, Georgia, with continuous water quality monitoring data to further elucidate potential drivers. Oyster density and condition index, a proxy for health, were measured, and parasites were quantified using a TaqMan probe based quantitative real-time PCR within gill tissue. Real-time PCR showed that 86% of oysters tested were infected by one or both parasites in the coast-wide survey, and 93% of oysters from Sapelo Island were also infected by one or both parasites. Prevalence and infection intensity for both P. marinus and H. nelsoni varied across sites. Overall impacts on oysters were complex-intensity was not linked to oyster metrics in the coastwide study, but oyster condition was negatively correlated with P. marinus prevalence in the Sapelo Island study. Several relationships between both parasites and water quality parameters were identified, providing valuable information about potential drivers that should be investigated further.

Moderate recreational oyster harvest has variable impacts on biophysical properties of reefs and reef‐associated fauna

AbstractDespite the popularity of recreational fishing, our understanding of the impacts of this type of harvest within marine ecosystems generally lags behind the well‐documented effects of commercial fisheries. Intertidal oysters serve as ecosystem engineers within otherwise soft‐bottom estuaries but may be disproportionately susceptible to harvest because of their sessile nature and accessibility at low tide. We used a Before–After–Control–Impact design to investigate the effects of recreational oyster “picking” on the physical structure provided by reefs and the community of reef‐associated mobile fauna. Despite reductions in oyster density on harvested reefs following the fishing season, we did not observe changes in other biophysical properties of reefs. Similarly, we found variable impacts of oyster harvest on the community of reef‐associated fauna, with community diversity changing seasonally on closed reefs but remaining low throughout on harvested reefs. Direct relationships between change in oyster density through time and changes in reef rugosity, faunal diversity, and faunal density were also variable. While oyster harvest removes the physical structure provided by these foundation species, the level of recreational harvest currently occurring in our study system apparently does not exceed the threshold which leads to cascading changes in reef‐associated fauna.

Demography of Oysters Pre‐ and Postcollapse in Apalachicola Bay, Florida, Using Stage‐Based Counts

AbstractThe collapse of oyster populations and the fisheries they support has been a worldwide phenomenon, but studies of oyster demography in situ prior to and after the collapse have been rare. We used time series of stage‐based counts of eastern oysters Crassostrea virginica in Apalachicola Bay, Florida, to help understand how abundance and demographic rates may have changed in the decade after the 2012 collapse relative to the period before the collapse. We relied on a Bayesian hierarchical model in which the latent stage structure of the oyster population (i.e., densities of spat, sublegal oysters, and legal oysters) was governed by a system process and where the count data represented summaries of that latent structure. Count data were sufficient to conduct this on two large oyster bars that had some of the highest precollapse oyster densities. We also examined nine other bars with less data for any temporal trends in postcollapse abundance that might be associated with recent restoration efforts. Among the 11 bars examined, oyster densities were often increasing prior to the collapse and were very low, without detectable trends, afterward. Based on our demographic analyses, mortality rates of Apalachicola Bay oysters in the decade after the collapse generally exceeded (often greatly so) those during the precollapse period for all oyster stages. On the other hand, spat settlement rates apparently were increasing prior to the collapse and remained high during the postcollapse period. Simulations of postcollapse demography suggest that without improved survival rates, further declines of the oyster population can be expected. We discuss these findings in light of ongoing restoration and management efforts and suggest ways in which rapid transitions to undesirable socio‐ecological regimes might be avoided in the future.

First-Year Performance of the Pervious Oyster Shell Habitat (POSH) along Two Energetic Shorelines in Northeast Florida

Novel living shoreline methods are being developed to minimize negative environmental impact while maintaining strength and effectiveness in high-energy systems. The “Pervious Oyster Shell Habitat” (POSH) is a novel structure composed of oyster shells bound by a thin layer of Portland cement into the shape of a dome. The structure’s makeup greatly reduces its environmental impact while providing optimal substrate for the provision of oyster reef habitat. Previous laboratory testing has demonstrated that the structure is robust, and this follow-up study assesses the structure’s performance in the estuarine environment. Oyster and barnacle densities were compared between POSH modules and the industry standard “Oyster Ball” model Reef BallTM along two energetic shorelines in northeast Florida. Oyster densities on the POSH were high and significantly greater than on the Oyster Ball at both sites. Barnacle densities did not differ between structures and did not appear to affect oyster recruitment. The size distribution of oysters on POSH and Oyster Ball modules was measured to assess the demographics and growth of oysters over time. Overall, demographics were similar among the two structures. Differences in oyster densities and demographics were greater at our more energetic site. Results show that the POSH can be an optimal structure for early oyster recruitment and reef development in energetic systems and should be considered by restoration stakeholders.

Experimental estimation of ladder dredge efficiency for capture of European flat oysters over mixed sediment

Fishing gear-based landings or survey methods are often used to make assessments of species stock abundance. In order to convert catch into abundance values, estimates or assumptions are made on the catch efficiency of the gear-based method. This is the case in areas where flat oysters, Ostrea edulis, are surveyed for fisheries and conservation objectives in a range of projects across Europe. Flat oyster dredge efficiency assumptions vary widely from 5–30% in published studies and uncertainty in what is an appropriate efficiency estimate has led some survey teams to switch to Catch-per-unit-effort (CPUE), where CPUE is also of concern should catch efficiency change with shellfish density, ground type or some other unmeasured variable such as shellfish distribution. We undertook an experimental approach to estimate dredge efficiency in a standard ladder dredge used to harvest and survey adult flat oysters in the UK and Ireland. The dredge efficiency trials assessed how efficiency was influenced by oyster density (between 1 and 2.2 oysters m2), distribution (clumped vs uniform) and ground types across a gradient of more hard to more soft surface sediments. Dredge efficiency was significantly affected by oyster distribution, but also density and ground hardness as well as their interactions. While a median value between 7 and 10% seems an appropriate universal ladder dredge efficiency to adopt, ground type and distribution had such an effect that local conditions may effect this considerably. Catch efficiency was negatively density-dependent, this makes CPUE methods challenging where oyster densities are likely to vary. Practitioners, regulators and researchers conducting surveys can improve CPUE approaches through standard techniques and knowledge of how catch efficiency varies as we have presented here.

Catch per Unit Effort, Density and Size Distribution of the Oysters Pinctada capensis and Saccostrea cucullata (Class Bivalvea) on Inhaca Island, Southern Mozambique.

Oysters are important resources for the daily diet, a source of economic income for many coastal communities and a delicacy for the tourism industry. In this study, the oysters (Pinctada capensis and Saccostrea cucullata) were investigated with the aim to assess the catch per unit effort, density and size composition. The study was conducted over a three-year period on Inhaca Island, Southern Mozambique. For both species, perception of fishery trends from collectors was carried out through habitat censuses and interviews. Transects, quadrats and daily fisheries catches approaches were used. Results show that P. capensis is the most exploited on the island. A total of 72.1% of respondents pointed that the oyster P. capensis is decreasing, due to excessive catching (75.4%) followed by natural death (24.6%), while 20.9% affirmed that the resource is stable and 7.0% are unaware about the resource trend. Oyster densities, sizes and catches per unit effort were higher in less accessible areas only for P. capensis. The present study provides valuable baseline information to recommend best practices to improve the exploitation, and access the need for introduction of aquaculture, towards the sustainable management and conservation of oysters, and ultimately to ameliorate people's livelihoods.