Aquaculture Structures Research Articles

Multiscale modelling of cage effects on the transport of effluents from open aquaculture systems.

Most Atlantic salmon mariculture operations use open sea cages for the grow out phase. The ultimate fate and effects of the effluents and the possibilities of disease transfer between fish farms are major concerns for farmers, governance and the general public alike. Numerical model systems applied to studying and managing effluents and disease transfer in mariculture must realistically resolve the hydrodynamics in the vicinity of the fish farms. In the present study, the effects of the aquaculture structures on the current patterns were introduced in the ocean model system SINMOD. The drag parameters for the ocean model were determined by comparing the simulation results from the ANSYS Fluent ® software suite and SINMOD in an idealized channel setting with uniform currents. The model was run for a number of realistic scenarios in high horizontal resolution (∼30 m) with sea cages influencing the flow field. Comparisons between extensive current measurements and the simulation results showed that the model system reproduced the current local current field well. By running simulation scenarios with and without the effects of the sea cages on the flow field, it was possible to assess the importance of such effects for numerical dispersal models and aquaculture environment interactions simulations and hence for assessment of environmental impacts.

A physical model approach to nonlinear vertical accelerations and mooring loads of an offshore aquaculture cage induced by wave-structure interactions

A grand aim of aquaculturists is to investigate the interactions between the oceanic environment and aquaculture structures, in order to make the structures more durable to loads caused by waves and current. To that end, we investigate the effects of extreme wave conditions by leveraging a physical model approach. We assume that the floating collar of the cage moves with the wave in heave, leading to extreme conditions. Thus, the wavelength is much larger than the size of the floating collar model and the wave frequency is much lower than the natural period of heave. For the floating collar model without netting, under wave-only conditions, the first- and second-harmonic components of the vertical acceleration are proportional to the wave amplitude ζa, and square of the wave amplitude ζa2, respectively. The third- and fourth-harmonic components exhibit oscillations at non-dimensional wave number νa≈0.055 due to the change of free surface. For the floating cage model under combined waves and current conditions, the first-harmonic component is more likely proportional to ζa, while the second-harmonic component shows erratic behavior. The effects of mooring loads in front and aft mooring lines are also assessed.

Optimising environmental conditions for nursery culture of juvenile Greenshell™ mussels (Perna canaliculus)

The nursery culture of early juvenile bivalves is costly, requiring the provision of optimal environmental conditions for efficient production. Nursery production of mussels is particularly challenging because of their innate secondary settlement behaviour which can be triggered by unfavourable environmental conditions, resulting in mass migration of juveniles off aquaculture structures. This is a pronounced problem for the aquaculture of the Greeenshell™ mussel (Perna canaliculus) where losses of over 80% of juveniles are typical. Consequently, we examined the growth, survival and retention of juvenile mussels of this species in relation to four key environmental conditions (i.e., photoperiod, aeration, water flow, and oxygen levels) over 25 days in nursery holding conditions. Maintaining vigorous aeration and high dissolved oxygen (100%) both resulted in higher growth (126.1 and 89% respectively), survival (92.5 and 89% respectively) and retention (97.7 and 83.8% respectively). In contrast, photoperiod had minimal effect on juvenile mussel performance. This study suggests that providing suitable key environmental conditions has the potential to greatly improve the efficiency of production of P. canaliculus during commercial nursery aquaculture production.

Drag coefficient parameter estimation for aquaculture systems

The flow conditions in and around a suspended canopy, resembling those formed by aquaculture structures such as rafts cages and longlines, were modeled using an augmented version of the hydrodynamic model Environmental Fluid Dynamics Code. The model was calibrated using vertical profiles of horizontal velocities, Reynolds stresses, and turbulent kinetic energies obtained from prior laboratory flume experiments. The parameter estimation code, PEST, was used to optimize various model parameters including horizontal momentum diffusivity, vertical eddy viscosity, turbulence closure constants, and, most importantly, depth-dependent drag coefficients. An increasing average drag coefficient was observed with decreasing canopy blockage ratio, and an empirical relationship for the vertical variation of drag coefficient was developed that may be appropriate for use in full-scale models of aquaculture systems. Overall, the calibrated canopy-turbulence parameters and drag-coefficient empiricisms may yield improved predictions of alterations to hydrodynamic and nutrient-transport conditions due to various aquaculture structures. Such predictions will help develop methods to minimize environmental impacts and to increase production from aquaculture farms.

Numerical simulation of flow through suspended and submerged canopy

Aquatic vegetation and marine aquaculture structures construct different forms of canopy. The existence of canopies produces drag resistance to water flow, alters ambient hydrodynamic and ecological environment. This work presents a numerical model to simulate flow through suspended and submerged canopies incorporating the interaction between the fluid field and the vegetation or structures. The numerical model is established based on Reynolds Average Navier–Stokes equations with additional canopy drag terms. Turbulence is modeled using two-equation k − ε model which takes into account the effect of canopies by an approximation of dispersive fluxes using the drag force produced by the canopy. The plant stem deformation and drag resistance are simulated accounting for both internal and external forces. An empirical formula for the bulk drag coefficient of the canopy region involving the shelter effect is proposed based on numerical simulation and laboratory experiment calibration. The numerical model is validated with existing data from field observation and laboratory experiments.

Ecological priming of artificial aquaculture structures: kelp farms as an example

AbstractThe continued development of the aquaculture industry is contributing to the proliferation of artificial structures in the marine environment. Observations of seaweed farms (infrastructure and biomass) suggest they act as a habitat for associated species. Seaweed farms differ from other forms of artificial infrastructure as the material deployed already has marine organisms (i.e. culture species) growing on it. This ‘priming’ of ropes with juvenile sporophytes may affect future development of communities by facilitating colonizing species or suppressing competitors. We call this process ‘ecological priming’: the provision of a biological platform that influences the successional development of specific communities. The communities that developed on ropes primed with Alaria esculenta individuals were compared with unprimed ropes to assess the ecological priming effect, at a commercial kelp farm in south-west Ireland. Species richness increased over two cultivation seasons and species composition was consistent between years, with distinct communities developing on primed and unprimed treatments. Timing of species occurrence on primed ropes was predictable with no predictable pattern occurring on unprimed ropes. Multivariate tests indicated distinct communities between treatments, with suppression of other algal species and potential facilitation of some species that have a particular association with A. esculenta on primed ropes. Communities from primed holdfasts contained a lower diversity of algal species compared with unprimed communities. Cultivated kelp holdfasts represent a habitat for distinct assemblages that reflect ecological priming of the substratum.

Filtration Rate of the Ascidian Ciona savignyi and Its Possible Impact

Ciona savignyi is an invertebrate filter feeder with an expanding invasive range in the world and a biofouling organism settling on artificial substrates such as fish net. In this study, we investigated the effect of body length and temperature on its filtration rate. As C. savignyi increased in size, filtration rate also increased. Optimal filtration occurred at a temperature of 24–25 °C. The effect of body length on filtration rate was expressed using an exponential function and the effect of temperature (15–27 °C) was calculated using a polynomial function. An equation to predict the filtration rate of C. savignyi was developed, by combining the terms for body length and temperature. The predicted filtration rates calculated using this power function showed good agreement with observed values. The filter-feeding ascidian, C. savignyi, is a biofouling organism that frequently grows on aquaculture structures. Using published data describing the numbers and weights of the cultured Japanese scallop (Mizuhopecten yessoensis) and of biofouling C. savignyi attached to pearl fishing nets in the Kugurizaka experimental fishing grounds, the filtration potential of both species was estimated as a case study. The results suggest that C. savignyi attached to the nets has the potential to negatively impact the growth of the Japanese scallop through competition for food.

First principle approach to the design of an open sea aquaculture system

ABSTRACTThis paper describes a procedure for designing flexible aquaculture structures at open sea. The procedure is demonstrated through the design of an innovative system for offshore aquaculture. The paper demonstrates how an understanding of the wave kinematics with respect to structural design yields a simple design method, which results with a product that meets the functional requirements and is capable of surviving violent sea conditions. Furthermore, the paper presents a novel and successful concept of an offshore aquaculture structure. The presented aquaculture system functions successfully since 2006, offshore the eastern coast of the Mediterranean Sea and survives extreme storms where several other concepts failed.

Impacts of aquaculture on fish biodiversity in the freshwater lake Laguna de Bay, Philippines

AbstractLaguna de Bay is the largest inland water body in the Philippines, being used predominantly for aquaculture and open water fisheries. Aquaculture in the lake began decades ago, with many changes in the lake ecosystem having occurred since that time. Most dominant species for fish culture are introduced species. Other invasive species were also introduced to the lake as escapees from land‐based aquaculture facilities. This study was conducted to monitor fish diversity in two adjacent, but distinctly different, sites in the lake, namely an open fishery area (OFS), with no adjacent aquaculture structures, and an aquaculture site (AQS), with cages for the culture of various commodities. Fish traps were installed at both sites, with the traps being sampled at least every 2 weeks from April 2013 to February 2015. The results of pairwise t‐tests indicated significantly higher Shannon–Wiener diversity index (H′), evenness (J′), Simpson's similarity index (D) and species richness (s) in OFS than in AQS. In terms of total catch per day, significantly greater fish biomass were obtained from AQS than from OFS. Introduced aquaculture species had a mean dominance of 83% and 47% in AQS and OFS, respectively. However, invasive species introduced from the ornamental fish trade exhibited a mean relative dominance of 10.3% in AQS and 13.5% in OFS. The relative dominance of native species was also significantly higher in OFS (41%) than in AQS (6.5%). The results of this study demonstrated the adverse impacts of aquaculture in regard to the species diversity of fish in localized areas in Laguna de Bay. The dependency of aquaculture on introduced fish species adversely impacted the natural fish population in the lake. Focusing on the culture of commercially important local species for aquaculture, rather than introduced species, will improve fish production of inland waters without accompanying adverse impacts on biodiversity.

Drag of Clean and Fouled Net Panels--Measurements and Parameterization of Fouling.

Biofouling is a serious problem in marine aquaculture and it has a number of negative impacts including increased forces on aquaculture structures and reduced water exchange across nets. This in turn affects the behavior of fish cages in waves and currents and has an impact on the water volume and quality inside net pens. Even though these negative effects are acknowledged by the research community and governmental institutions, there is limited knowledge about fouling related effects on the flow past nets, and more detailed investigations distinguishing between different fouling types have been called for. This study evaluates the effect of hydroids, an important fouling organism in Norwegian aquaculture, on the forces acting on net panels. Drag forces on clean and fouled nets were measured in a flume tank, and net solidity including effect of fouling were determined using image analysis. The relationship between net solidity and drag was assessed, and it was found that a solidity increase due to hydroids caused less additional drag than a similar increase caused by change in clean net parameters. For solidities tested in this study, the difference in drag force increase could be as high as 43% between fouled and clean nets with same solidity. The relationship between solidity and drag force is well described by exponential functions for clean as well as for fouled nets. A method is proposed to parameterize the effect of fouling in terms of an increase in net solidity. This allows existing numerical methods developed for clean nets to be used to model the effects of biofouling on nets. Measurements with other types of fouling can be added to build a database on effects of the accumulation of different fouling organisms on aquaculture nets.

Effects of Geoduck (Panopea generosaGould, 1850) Aquaculture Gear on Resident and Transient Macrofauna Communities of Puget Sound, Washington

In Washington state, commercial culture of geoducks (Panopea generosa) involves large-scale out-planting of juveniles to intertidal habitats, and installation of PVC tubes and netting to exclude predators and increase early survival. Structures associated with this nascent aquaculture method are examined to determine whether they affect patterns of use by resident and transient macrofauna. Results are summarized from regular surveys of aquaculture operations and reference beaches in 2009 to 2011 at three sites during three phases of culture: (1) pregear (-geoducks, -structure), (2) gear present ( geoducks, structures), and (3) postgear ( geoducks, -structures). Resident macroinvertebrates (infauna and epifauna) were sampled monthly (in most cases) using coring methods at low tide during all three phases. Differences in community composition between culture plots and reference areas were examined with permutational analysis of variance and homogeneity of multivariate dispersion tests. Scuba and shoreline transect surveys were used to examine habitat use by transient fish and macroinvertebrates. Analysis of similarity and complementary nonmetric multidimensional scaling were used to compare differences between species functional groups and habitat type during different aquaculture phases. Results suggest that resident and transient macrofauna respond differently to structures associated with geoduck aquaculture. No consistent differences in the community of resident macrofauna were observed at culture plots or reference areas at the three sites during any year. Conversely, total abundance of transient fish and macroinvertebrates were more than two times greater at culture plots than reference areas when aquaculture structures were in place. Community composition differed (analysis of similarity) between culture and reference plots during the gear-present phase, but did not persist to the next farming stage (postgear). Habitat complexity associated with shellfish aquaculture may attract some structure-associated transient species observed infrequently on reference beaches, and may displace other species that typically occur in areas lacking epibenthic structure. This study provides a first look at the effects of multiple phases of geoduck farming on macrofauna, and has important implications for the management of a rapidly expanding sector of the aquaculture industry.

The Effects of Ambient and Aquaculture Structure Hydrodynamics on the Food Supply and Demand of Mussel Rafts

ABSTRACT A field and modeling study of the food supply and demand of mussel (Mytilus edulis) rafts in Maine established the hydrodynamic and particle consumption characteristics of shellfish aquaculture structures. Mussels on rafts filtered about 8 × 106 L/h and consumed about 40 g chlorophyll a (chl a)/h under favorable conditions. Because of the drag of the culture ropes and predator nets, velocity inside the rafts was reduced by 75%–80% in relation to ambient conditions. Chlorophyll consumption by mussels increased with increased food (chl a) concentration and also with increased water velocity inside the rafts. Clearance rates per raft also increased with food concentration. Model results allow for an estimation of water flux and seston depletion within the rafts through the use of point measurements and correction factors. Water velocity measurements taken mid depth in the middle of the rafts underestimated the mean flow through the raft by 10%. Measurements of current velocity and chl a concentratio...

Environmental and biogeochemical changes following a decade's reclamation in the Dapeng (Tapong) Bay, southwestern Taiwan

This study examines the environmental and biogeochemical changes in Dapeng (formerly spelled Tapong) Bay, a semi-enclosed coastal lagoon in southwestern Taiwan, after two major reclamation works performed between 1999 and 2010. The lagoon was largely occupied by oyster culture racks and fish farming cages before December, 2002. Substantial external inputs of nutrients and organic carbon and the fairly long water exchange time (τ) (10 ± 2 days) caused the lagoon to enter a eutrophic state, particularly at the inner lagoon, which directly received nutrient inputs. However, the entire lagoon showed autotrophic, and the estimated net ecosystem production (NEP) during the first stage was 5.8 mol C m−2 yr−1. After January, 2003, the aquaculture structures were completely removed, and the τ decreased to 6 ± 2 days. The annual mean concentrations of dissolved oxygen increased, and nutrients decreased substantially, likely due to improved water exchange, absence of feeding and increased biological utilization. The NEP increased 37% to 7.7 mol C m−2 yr−1 after structure removal. The second reclamation work beginning from July, 2006, focused on establishing artificial wetlands for wastewater treatment and on dredging bottom sediment. Although the τ did not change significantly (8 ± 3 days), substantial decreases in nutrient concentrations and dissolved organic matter continued. The NEP (14.3 mol C m−2 yr−1) increased 85% compared to that in the second stage. The data suggest that the reclamations substantially improved water quality, carbon and nutrient biogeochemical processes and budgets in this semi-enclosed ecosystem.

Influence of ‘bouchot’ mussel culture on the benthic environment in a dynamic intertidal system

The present study examines the influence of mussel aquaculture structures (bou- chots) in the intertidal zone of the Chausey Archipelago (Normandy, France) where strong tidal currents are predicted to disperse biodeposits. Six sites were sampled to determine the spatial variability of organic matter content, sediment grain size, and benthic infaunal community char- acteristics. The influence of bouchots on the benthic environment was evident at both the meso- scale (i.e. km-scale, between farm and reference sites) and small-scale (i.e. m-scale, spatial struc- ture within farms but not reference sites). This influence was also apparent for sediment grain size distribution and most metrics of macrobenthic community structure, but not for sediment organic matter. Mussel culture structures likely modify natural flow patterns within farm sites, which increases shear stress around individual bouchots, thereby increasing the heterogeneity of ben- thic communities, altering the dominance of particular species within farms and differentiating them from reference sites.

The Engineering of Budj Bim and the Evolution of a Societal Structure in Aboriginal Australia

In the minds of most present day Australians, both indigenous and non-indigenous, “engineering” came to the continent with the first Europeans. However, if one looks at what “engineering” really is, then structures from history and pre-history, especially in the Middle East, which supplied water to growing settlements cannot be ignored; there were excellent examples present in Australia which may even pre-date those in many northern hemisphere civilisations. This paper looks particularly at the aquaculture structures built from the Budj Bim lava flows (Mt Eccles) in southwestern Victoria. The aquaculture industry produced a permanent food supply, a permanent settlement, food trading and a different form of governance than was generally found elsewhere in Australia. The works have now been recognised as an Engineering Heritage National Landmark by the Institution of Engineers Australia (commonly referred to as “Engineers Australia).

The engineering of Budj Bim and the evolution of a societal structure in Aboriginal Australia

AbstractIn the minds of most present day Australians, both indigenous and non-indigenous, “engineering” came to the continent with the first Europeans. However, if one looks at what “engineering” really is, then structures from history and pre-history, especially in the Middle East, which supplied water to growing settlements cannot be ignored; there were excellent examples present in Australia which may even pre-date those in many northern hemisphere civilisations. This paper looks particularly at the aquaculture structures built from the Budj Bim lava flows (Mt Eccles) in southwestern Victoria. The aquaculture industry produced a permanent food supply, a permanent settlement, food trading and a different form of governance than was generally found elsewhere in Australia. The works have now been recognised as an Engineering Heritage National Landmark by the Institution of Engineers Australia (commonly referred to as “Engineers Australia).

Factors affecting survivorship of defouled communities and the effect of fragmentation on establishment success

The environmental risks associated with the defouling of artificial structures (e.g., vessels, oil rigs, marina pontoons, aquaculture structures) in the marine environment are gaining international attention. This paper presents a series of laboratory- and field-based experiments that collectively aimed to elucidate biotic and abiotic factors that influence re-establishment success of biofouling organisms and fragmented colonial organisms defouled to the seabed. Reattachment success of colonial organisms experimentally fragmented was found to be species specific and dependent on fragment size. In both laboratory and field trials, some colonial ascidians had consistently greater reattachment success for larger size classes of fragments, while other encrusting and erect taxa showed poor reattachment capabilities. This study also revealed that sedimentation and turbidity are likely to have a strong influence on the survivorship of defouled material. Furthermore, for both high and low sedimentary environments, survivorship was found to be greater where predators were excluded. Despite risks posed by non-indigenous species (NIS), it is proposed that in-water defouling may be an appropriate management response in situations where a “do nothing” approach is potentially more detrimental. Moreover, results from this study suggest that environmental risks associated with defouling may be mitigated through appropriate defouling strategies (e.g., defouling location, frequency and method). In order to increase our predictive abilities for NIS establishment success resulting from in-water defouling, future studies should aim to further elucidate the relative importance of factors affecting survivorship of defouled material at locations where defouling is routinely undertaken.

Sea ducks and aquaculture: the cadmium connection

Elevated concentrations of cadmium have been reported in the kidneys of sea ducks that forage along the Pacific Northwest, and cadmium has been postulated as a possible cause of population declines. The blue mussel (Mytilus spp.) which occurs in dense numbers on aquaculture structures and are a primary prey item for sea ducks also contain elevated cadmium concentrations. To determine if foraging on mussels associated with aquaculture structures could pose a toxicological risk to sea ducks, amounts of cadmium ingested per body weight per day by a representative sea duck species, the surf scoter (Melanitta perspicillata), were estimated and compared to the reported avian cadmium NOAEL (no observable adverse effect level) and LOAEL (lowest observable adverse effect level). Results indicate that in some locations within the Pacific Northwest, sea ducks could be exposed to toxicologically significant levels of cadmium associated with mussels foraged from aquaculture structures. This raises the possibility that such exposure could be contributing to observed population declines in these species.

High-resolution computational fluid dynamics modelling of suspended shellfish structures

Aquaculture structures are responsible for flow disturbances that extend over a large range of scales. In the case of shellfish aquaculture, those scales extend from the size of an individual shellfish to the size of a whole farm with intermediate scales being the size of a shellfish structure or of a block of shellfish structures. The influence of block spacing and angle is investigated by the mean of a 2D high resolution computational fluid dynamics model. Block angle is found to be the main relevant parameter controlling to a large extent the mixing generated by the shellfish structures. Strong sensitivity is found for small angles. Nevertheless, it is shown that for a limited number of blocks, upstream turbulence still dominates the degree of the total mixing within the farm, and that total flow reduction is little affected by the orientation of the blocks. A simple analytical model is presented that predicts most of the numerical results.

Depth-Averaged Drag Coefficient for Modeling Flow through Suspended Canopies

Aquatic suspended canopies are porous obstacles that extend down from the free-surface but have a gap between the canopy and bed. Examples of suspended canopies include those formed by aquaculture structures or floating vegetation. The major difference between suspended canopies and the more common submerged canopies, which are located on the bottom boundary, is the influence of the bottom boundary layer beneath the suspended canopy. Data from laboratory experiments are presented which explore aspects of the flow through and beneath suspended canopies constructed from rigid cylinders. The experiments, using both acoustic Doppler and two-dimensional (2D) particle tracking velocimetry, give details of the flow structure that may be divided vertically into a bottom boundary layer, a canopy shear layer, and an internal canopy layer. The experimental data show that the penetration of the shear layer into the canopy is limited by the distance between the canopy and bottom boundary layer. Peaks in velocity spectra indicate an interaction between the bottom boundary and canopy shear layer. An analytical model is also developed that can be used to calculate a drag coefficient that includes the effect of both canopy drag and bed friction. This drag coefficient is suitable for use in 2D (depth-averaged) hydrodynamic modeling. The model also allows the average velocity within and beneath the canopy to be calculated, and is used to investigate the effect of canopy density and thickness on both total drag and bottom friction.