Cédric Bacher

Characterization of intertidal flat hydrodynamics

Abstract The paper reviews the different physical forcings that control tidal flat hydrodynamics. Tidal propagation and cross-shore or long-shore currents, tidal asymmetry, wind-induced circulation, wave propagation and drainage processes are successively considered. Some simple methods are described for estimating cross-shore currents and wave-induced bottom shear stresses, and the results obtained are compared to field measurements on three contrasted sites in Europe. In particular the cross-shore current is shown uniform in the lower part of the flat, and decreasing towards the shore. Bottom friction-induced wave attenuation is simply formulated on gently sloping beds, leading to a maximum wave height that a flat can experience; it is proportional to the water height according to the ratio between the slope and the wave friction factor. The maximum related shear stress occurs at high water and is also proportional to the water depth. Maximum tidal velocities are very similar in the three sites where bottom sediment is muddy, suggesting a relationship between physical stresses and sediment characteristics. The consequences of physical forcings on sediment transport are listed. The bottom shear stress is suggested as the relevant parameter for comparing tidal and wave effects. In general, tide induces onshore sediment transport, whereas waves and drainage favour offshore transport. The processes leading to a possible tidal equilibrium profile are analysed: they involve the intrinsic asymmetry that favours net deposition at high water, and an ebb dominance generated by the resulting bottom profile convexity. Eroding waves are likely to upset such a balance; this equilibrium then reduces to a trend for the system.

Allometric relationships and effects of temperature on clearance and oxygen consumption rates of Crassostrea gigas (Thunberg).

Abstract Clearance and oxygen consumption rates of Crassostrea gigas were investigated with animals of 5–200 g total wet weight (0.1–3 g dry tissue weight), and at different temperatures (5–32 °C) after 10 days acclimation. During this period significant mortalities were observed at 32 °C, which may be close to the upper thermal limit for this species. For each temperature, allometric relationships between physiological rates and the dry weight (DW, g) of the animal were estimated. Clearance rate (CR, 1·h−1) was maximal at 19 °C; oxygen consumption rate (VO2, mgO2·h−1) increased over the range of experimental temperatures (T, °C). Two statistical models are proposed: CR = [a − (b ∗ (T − c) 2 )] ∗ DW d and VO 2 = [a + (b ∗ c T )] ∗ DW d . However, neither model is appropriate during the reproductive period.

Modelling the effect of food depletion on scallop growth in Sungo Bay (China)

Sungo Bay (China) has a mean depth of 10 m, a total area of 140 km 2 and is occupied by several types of aquaculture, whilst opening to the ocean. The production of scallops (Chlamys farreri) cultured on long lines is estimated to exceed 50 000 tonnes (total weight) per year. Selection of sites for scallop growth and determination of suitable rearing densities have become important issues. In this study, we focused on the local scale (e.g. 1000 m) where rearing density, food concentration and hydrodynamics interact. We have developed a depletion model coupling a detailed model of C. farreri feeding and growth and a one-dimensional horizontal transport equation. The model was applied to assess the effect of some environmental parameters (e.g. food availability, temperature, hydrodynamism) and spatial variability on growth, and to assess the effect of density according to a wide range of hydrodynamical and environmental conditions. In the simulations, food concentrations always enabled a substantial weight increase with a final weight above 1.5 g dry weight. Compared to a reference situation without depletion, a density of 50 ind m –3 decreased growth between 0% and 100%, depending on current velocity when maximum current velocity was below 20 cm s –1 . The mean ratio between food available inside and outside the cultivated area (depletion factor) varied with the percentage of variation in scallop growth that was due to density. Our model suggests that scallop growth was correlated with maximum current velocity for a given density and current velocity below 20 cm s –1 . The model was integrated within a Geographical Information System (GIS) to assist in making decisions related to appropriate scallop densities suitable for aquaculture at different locations throughout the bay. Concepts (depletion), methods (coupling hydrodynamics and growth models), and the underlying framework (GIS) are all generic, and can be applied to different sites and ecosystems where local interactions must be taken into account.

Growth model of the Pacific oyster, Crassostrea gigas, cultured in Thau Lagoon (Méditerranée, France)

Abstract We developed a growth model for the oyster Crassostrea gigas cultured in Thau Lagoon. The oyster standing stock in the lagoon ranged between 10,000 and 15,000 tons a year. Two culture methods are presently in use in Thau Lagoon which are used in about the same proportions. At seeding, initial size of oysters is different among methods. The model was calibrated on (1) growth data accounted for both culture methods and (2) hydrobiological data (temperature, salinity, suspended particulate matter and chlorophyll a ), both recorded in several sites in the lagoon between March 2000 and October 2001. The lagoon is slightly eutrophic: total chlorophyll a and total particulate matter averaged 1.2 μg l −1 and 2.2 mg l −1 , respectively. Organic content accounted for ca. 40–50% of particulate matter. There was no seasonal trend in seston, whereas temperature and salinity were minimal in winter. Oyster growth varied among sites in response to spatial variations in seston. Growth was maximal in summer and minimal in winter because of temperature seasonality. For each location, we modelled growth as a function of particulate organic matter and temperature. Chlorophyll a was left out of the model because of a weaker fit with growth. Growth was modelled as G = a POM b T c Y d , where G is the growth rate in shell length (mm day −1 ) or in mass (g day −1 ), POM is particulate organic matter (mg l −1 ), T is temperature (°C) and Y is either shell length (mm) or mass (total individual mass or dry flesh mass in g). Allometry ( Y d ) allowed us to use the same model for both culture methods. The model yielded a good fit with actual size, either as measured by shell length ( R 2 =0.96) or total individual mass ( R 2 =0.93).

Ecophysiological model of growth and reproduction of the black pearl oyster, Pinctada margaritifera: potential applications for pearl farming in French Polynesia

A model of bioenergetics of the black pearl oyster (Pinctada margaritifera) was built to simulate growth, reproduction and spawning in suspended culture at field sites in Takapoto lagoon (French Polynesia). This model was based on allometric scaling of physiological functions and scope for growth (SFG) calculations. The input functions were clearance rate (CR, 1 day(-1)), retention efficiency (RE, %) for each kind of particle encountered in suspended matter, pseudofaeces and faeces productions (PF and F, mg C day(-1)), excretion and respiration rate (U and R, mg C day(-1)). The assimilated carbon (i.e., SFG, mg C day(-1)) was partitioned to the three internal state variables (somatic tissue, shell and gonad) according to the asymptotic increase of the reproductive effort (ER, %) with the age. Given organic and mineral particulate matter in suspension in lagoon water (POM and PIM, mg L-1) and assuming that the taxonomic composition of POM was fairly constant throughout the year, the model predicted annual evolution of total tissue weight (W-Tissue, g dry weight), shell weight (W-Shell, g DW) and gonad weight (W-Gonad, g DW) of pearl oysters at various ages. Data on tissue and shell growth, but also on gonad development of cultivated pearl oysters, acquired in 1997-1998 in Takapoto lagoon, were used to validate the model outputs. Results of the simulations indicated that the P. margaritifera growth model provided realistic growth trajectories for shell, somatic tissue and gonad, for pearl oysters aged from 1 to 4 years.

Application of a population dynamics model to the Mediterranean mussel, Mytilus galloprovincialis, reared in Thau Lagoon (France)

Abstract A population dynamics model developed for oysters cultured in Thau Lagoon was applied to the mussel, Mytilus galloprovincialis , that are raised in the same area. This model is based on a continuous equation of mussel abundance as a function of mortality rate, individual growth rate and inter-individual growth variability. The growth model was calibrated with growth, water temperature ( T , °C) and particulate organic matter concentration (POM, mg l −1 ) data that were measured in the lagoon between March 2000 and October 2001. The daily growth rate ( G in g day −1 ) was modelled as a function of POM, T and total individual mass, w ( G = a POM b T c w d ). A diffusion coefficient (value of 0.01) simulated the inter-individual growth variability in the general population dynamics equation. The population dynamics model used seeding and harvesting timetables (collected during interviews of mussel farmers in Spring 2001) for estimating inputs and outputs (i.e. mass of individuals at seeding and harvest, rearing density) for the basin; thus the cultivation strategies of mussel farmers were explicitly included in the model formulation. Simulations of long-term variations in the standing stock and the marketable production of mussels showed that predicted variations in the standing stock at equilibrium were between 3600 and 4300 t (equilibrium conditions reached in 3 years) and the annual production of mussels was estimated at 5400 t. The model results are particularly sensitive to parameters governing harvest. The results suggest that summertime is the best period for seeding to shorten the harvest duration.

Infectious diseases in oyster aquaculture require a new integrated approach

Emerging diseases pose a recurrent threat to bivalve aquaculture. Recently, massive mortality events in the Pacific oyster Crassostrea gigas associated with the detection of a microvariant of the ostreid herpesvirus 1 (OsHV-1µVar) have been reported in Europe, Australia and New Zealand. Although the spread of disease is often viewed as a governance failure, we suggest that the development of protective measures for bivalve farming is presently held back by the lack of key scientific knowledge. In this paper, we explore the case for an integrated approach to study the management of bivalve disease, using OsHV-1 as a case study. Reconsidering the key issues by incorporating multidisciplinary science could provide a holistic understanding of OsHV-1 and increase the benefit of research to policymakers.

Use of saline ground water for intensive rearing of Ruditapes philippinarum juveniles in a nursery system

Abstract Saline ground water was used as a thermal source for intensive rearing of juveniles of Ruditapes philippinarum and as a source of nutrients for phytoplankton production. In experiments concerning growth at different seasons, controlled factors were density of juveniles, flow of water, phytoplankton concentration, temperature and frequency of feeding. Their effects on growth and mortality were classified through correspondence analysis, analysis of multiway contingency tables and analysis of variance. An optimal stratery for summer and winter rearing was then defined. The different strategies are discussed in biological terms, with reference to the literature. Observed growth rates in this intensive culture were compared with available data for artificially fed and naturally reared populations.

Modelling the impact of a cultivated oyster population on the nitrogen dynamics: the Thau lagoon case (France)

The Thau lagoon (France) is an important site for the cultivation of Crassastrea gigas. The relationship belween the oyster population and the environment was assessed through a model of trophic relationships. The results represent the initial step lowards a more precise assessment of the biological fluxes in the lagoon. This preliminary model was based on the nitrogen dynamics among the following compartments: phytoplankton, zooplankton. oysters. detritus and dissolved inorganic nitrogen in the water column. Two other compartments were also considered in the sediment: detritus and dissolved nitrogen. The model considered the watershed input and seawater exchange belween the lagoon and the open sea . The parameters were estimated from experiments on oyster ecophysiology , in situ primary production and biomass measurements, and by calibration of simulations against data series. The importance of vertical exchange of material between the water column and the sediment due to sedimentation, biodeposition by the cultivated oyster populations, and nutrient regeneration from the sediment, was supported by the model. Therefore, the model emphasized the impact resulting from oyster culture practices and the sediment contribution to nitrogen dynamics. Oysters could be considered as a nitrogen well that stabilizes the ecosystem by removing nitrogen over a longer time scale than zooplankton. Since grazing was dominated by the oyster compartment, zooplankton had a limited effect on phytoplankton dynamics. Moreover, model calculation demonstrated the critical role of detritus in oyster food ration. For instance, the sedimentation rate of particulate matter was doubled by the deposition by oysters. The model was sensitive to parameters controlling the primary production. For example, modifying these parameter values resulted in large winter accumulation of dissolved inorganic nitrogen, triggering a first phytoplankton bloom at the end of winter. This sensitivity stressed the importance of using experimental data for calibration of the model.

Modeling approach of nitrogen and phosphorus exchanges at the sediment–water interface of an intensive fishpond system

Abstract The sediment–water interface in aquaculture ponds is both a sink and a source of various substances that are potentially toxic for cultured species. The sediment to water nitrogen and phosphorus exchanges were studied at the sediment–water interface in an intensive earthpond fish-farm on the French Atlantic coast. This was to define the contribution of diffusive fluxes to the total dissolved nitrogen and phosphorus produced during the 1997–1998 rearing period. Fluxes of particulate organic nitrogen (PON) and particulate phosphorus (PP) were modeled and validated using a set of observations. Diffusive fluxes were modeled using an empirical function of temperature based on in situ sediment porewater concentration profiles. An average of 15% of PON and 10% of PP, produced by fish food waste and fish faeces were sedimented in fishponds. Ammonia and phosphate diffusive fluxes (μmol m−2 h−1) were expressed as a function of temperature (T). J mod NH 4 + =(0.144T+3.49)×10 −6 exp (0.11T+16.81) J mod PO 4 3− =(0.086T+1.8)×10 −6 exp (0.09T+15.76) PON and PP stocks in the sediment decreased during the summer and increased during the winter. However, sedimentation and mineralization–diffusion processes were approximately balanced over the 2-year period. Ammonia and phosphate diffusive fluxes accounted for only 1–4% and 4–15%, respectively, of the total dissolved nitrogen and phosphorus components produced during the rearing period.