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    • Oysters with a plastic flavor in your plates for your celebrations!by Léa Bascoul, Vincent Dumontet and Sophia Lestringuez

      Published by Charlotte Recapet the November 8, 2021 on 5:52 PM

      For over half a century, global plastic production has increased exponentially. Plastics represent 50% to 80% of marine litter. Microplastics (MP) are derived from the degradation of larger plastic articles or directly produced as microparticles (< 5 mm). Oceans have a “funnel” effect concerning MP drained from rivers and streams, they concentrate pollution from fresh waters in the oceans. In pearl farming structures and equipment, the use of plastics is a source of pollution to the detriment of the lagoon ecosystem. Due to their ubiquitous and small dimensions, MP are likely to be ingested by many organisms and in particular by the oyster, Pinctada margaritifera (Figure 1), which is a species of ecological and commercial interest. So, the objective of the study is to assess the impact of these microplastics on the physiology of this oyster species from the pearl lagoon of Ahe atoll, in the waters of French Polynesia (Figure 2).

      Figure 1: Pinctada margaritifera

      Figure 2: Ahe atoll localisation

      In order to assess the effects of these molecules, IFREMER from French Polynesia, recreated microcosms [1] where oysters are exposed to polystyrene microbeads. Indeed, polystyrene is one of the most widely used plastic polymers and the most found in marine ecosystems.

      After two weeks of depuration, oysters, were divided into three treatments. For each, 24 oysters were exposed to polystyrene solutions for two months. These solutions were a mixture of polystyrene microbeads of different diameters (6 and 10 μm) at equal weight. A control was carried out without exposure to polystyrene. Each tank, kept homogeneous, was exposed for 12 hours a day to light and the physico-chemical parameters were kept constant. The microplastic solution was injected continuously into the tanks associated with a mixed diet of two microalgae (Tisochrysis lutea and Chaetoceros gracilis).

      Figure 3: Experimental exposure device of oysters to microplastics

      During the experiment (Figure 3), various parameters, like oxygen consumption, were measured to assess the different energy expenditure of the oyster. Expenses related to food (ingestion rate and assimilation efficiency), to growth (the scope for growth defined from the energy budget and the shell growth rate) as well as those allocated to reproduction (characterization gonad [2] size and analysis of gametogenesis [3]) are quantified in each experimental group and compared to the control. The measurements were carried out on eight oysters per treatment after one month and then at the end of the experiment.

      This experiment made it possible to demonstrate that at low MP concentrations, pearl oysters suffer adverse consequences for the sustainability of their populations. In fact, the metabolic responses of a stressed organism can lead to an increase in an individual's energy costs, forcing the organism to sacrifice some of its energy normally allocated to functions, such as growth or reproduction.

      In this classical and standardized experiment, the oysters exhibited a significant dose-dependent reduction in the “scope for growth” without however modifying the shell deposit rate. On the other hand, although the ingestion rates of the microalgae were not affected by the microplastics, for an equal volume of filtered water the assimilation of the microalgae is disturbed by the presence of the microplastics and the energy supplied by food is lower. This is either linked to interference in assimilation processes in the digestive tract or to indirect competition between algae and microplastics.

      The fact that metabolic (respiration) rates are not impacted suggests that the only possibility for the oyster to cope with a decrease in available energy is to modify its gametogenesis and / or its reproductive effort. However, reproduction in P. margaritifera is continuous and requires a lot of energy. The results of the study show that reproductive effort is not affected but that gametogenesis is strongly impacted. Thus “small holes” in the gonadal tubules [4] are observed in oysters exposed to microplastics. They could therefore be the start of the phagocytosis [5] of germ cells initiated to recover energy (Figure 4).

      Figure 4: Impact on P. margaritifera metabolism

      In a few words, the pearl oyster exposed to MP for a period of two months maintains its metabolism and its vital functions to the detriment of its reproduction, and therefore the sustainability of its future population.

      This study could also serve as a model for the MSFD [6] Technical Group Marine Litter to assess the effects of microplastic litter in biota (D10C3), because the effects observed on Pinctada margaritifera in the lagoon of French Polynesia must be the same as on other filter feeding bivalves in Europe. The European Technical Group Marine Litter could thus propose a regulation of waste at the European level and not only at the level of the lagoon of French Polynesia.

      The experiment results should support decision-making on plastic waste management measures in Polynesian lagoons, especially in atolls that have decades of pearl culture history without waste regulation or clean-up. This work would provide the necessary information to local authorities to regulate pearl equipment and establish a waste management policy such as the collection and recycling of this equipment.

      The aim of the study was to measure the impact of microplastics on the metabolism of Pinctada margaritifera. The team of researchers therefore sought to artificially recreate different levels of pollution in the laboratory. This methodology is suitable because it aims to recreate the pollution conditions possibly encountered in natural environments. To measure these different levels of pollution, the experiment is carried out at different concentrations. As for the various parameters measured to assess energy losses, they make it possible to identify, in a non-invasive manner, the functions (growth and reproduction) "sacrificed" to cope with the loss of energy. They also allow us to see if this loss of energy had an effect on the metabolism of oysters. The parameters monitored therefore seem consistent for this study and could be used for similar studies on other bivalve filter-feeding organisms. Experiments using this protocol could be considered in fresh waters with other types of bivalves such as the mussel pearl (Margaritifera margaritifera). Also, this study can be evaluated with a direct approach on oysters from pearl farms and over several generations.

      In several years, oyster populations on a global scale could be impacted by pollution from microplastics … so, if there are no more oysters, what will we eat with our white wine?

      Read the full study: Gardon T., Reisser C., Soyez C., Quillien V. & Le Moullac G (2018). Microplastics Affect Energy Balance and Gametogenesis in the Pearl Oyster. Environmental Science & Technology. 52(9), 5277‑5286.

      [1] smaller version of ecosystem

      [2] an animal organ in which gametes (a haploid germ cell, such as a spermatozoon or ovum) are produced

      [3] gamete formation process

      [4] organs of gametes excretion

      [5] cellular defense process, destructive function of phagocytes

      [6] Marine Strategy Framework Directive

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    • Help! How do I manage my commercial oyster nursery?by Coline Arqué, Marylou Pourret and Robin Thibault

      Published by Charlotte Recapet the September 6, 2021 on 4:16 PM

      Evaluation of food limitations in commercial oyster nurseries: an aid for managers

      Oyster production provides an important number of ecosystem services (nutrient cycling, providing habitat for other marine species, ...). Furthermore, shellfish farming is promoted and recognized as providing social and economic benefits, as well as ecological benefits. Oyster growth and production depends on several factors such as temperature, salinity, freshwater flow/rainfall, current speed, density, feed concentration and phytoplankton species composition, feed sharing with other species and disease outbreaks. For this reason, modeling can be useful in understanding the feedback between agricultural and environmental systems and the effects on production. Mass balance models can help estimate the food requirements of a given spat stock.


      • Develop and evaluate model for different culture structures using Pacific oyster spat
      • Make the model available online for wider use
      • Ensure it tackles two questions that arise when planning or managing an oyster nursery: how much food is required to sustain a given stock and for a typical range of food available in the surrounding environment, what is the maximum biomass that can be stocked

      Why use the Pacific oyster?

      • Strong Economic Interest: 4.4 million tons in 2003 (FAO)
      • The cultivation is well suited: to small family businesses, cooperatives or regional industry
      • The grow-out phase can be carried out by: relatively unskilled labor with minor investment in equipment and infrastructure




      The overall goal of this model is to estimate the food inputs for a given stock biomass; and the maximum stock biomass for a given external food concentration.

      Different parameters were considered in this model relied on the experiences of Langton and McKay (1976) ...

      Level of food supply

      Simulations of two feeding levels with an interval of 6 hours:

      • Exp A : Exp A: daily intake of 180 algae cells/μL x 250 L of tank
      • Exp B: 120 algae cells/μL x 250 L of tank

      In addition, to mimic the experimental setting, the model application includes only one class of oysters, so that at each run of the model, the spat size is set to the same size obtained from the weekly observations of Langton and McKay (1976) for the 6:00/6:00 regime.


      Key parameter used in the model as the optimal concentration to be maintained in the production unit. [Food]nursery: minimum dietary concentration that maximizes intake OR optimal concentration for growth.


      Chosen according to other references on the subject. The temperature for the maximum clearance rate is approximately 19°C. Thus, the lower limit of the model is set at 4°C and the upper limit at 30°C.

      The biomass of the stock was calculated by considering the density of 50 spat per liter, multiplied by the volume of the tank (250 L) and by the size of the seeds.

      Conceptual model for the oyster nursery.

      The choice of a model that takes several parameters into account allows a rendering close to reality. This application is a useful tool for managers who can limit as much as possible the costs that are not essential for the good development of a species. Indeed, the aquaculture environment is a field with high socioeconomic stakes. Therefore, it is useful in the long term to find new management concepts for sustainable resource management.


      Week 0

      Week 2

      Week 3

      Following weeks

      Exp A

      Exp B

      Exp A

      Exp B

      Exp A

      Exp B


      Feed supplied is much higher than the stock requirements

      Feed level supplied is still enough

      Oysters are fed less than the optimum

      Feed level supplied is near the threshold

      Feed level supplied does not meet the needs of oysters

      lower spat weight for experiment B than for experiment A

      These different growth rates measured in Exp A and B (Langton & McKay 1976) confirm the model's predictions of dietary limitation. The results of the model are also consistent with those of Langton and McKay (1976), which predicted that oyster spat are not diet-restricted during the first two weeks. The model outputs provide the feed requirements to ensure minimum concentration in the nursery. It also gives the maximum biomass that can be stored to ensure a minimum concentration in the nursery for optimized growth.


      In order to promote widespread use, the model described in this paper for Pacific oyster nurseries is made available online: It allows to carry out simulations on several types of nursery systems.

      This work describes the model user interface, including the menus for nursery setup (and seed characteristics), output for food requirements, output for optimum stock, and advanced settings (allows the user to modify the optimal feed concentration for oyster filtration). Examples on how to use the model for different case studies are also provided.

      Model limitations include the following:

      • Important effects that occur at smaller scale are not simulated in the model, e.g., changes in the water flow rate due to oyster size/densities or tank shape
      • The option with bloom tanks assumes these are interconnected with the oyster-holding tank, which together are the simulated unit. In this case, the water flow is the water that enters from the outside (an adjacent ecosystem for instance) into the bloom tanks forced by tidal height or pumped
      • The salinity effects on filtration rate are not simulated and thus it is assumed that water salinity is higher than 20


      In spite of the model simplification, it can still provide guidance for managing stock and food limitation in natural feeding oyster nurseries. It offers a wide range of possible scenarios in which the nursery operates. It provides guidance for the management of stocks and food limitation in naturally fed oyster nurseries. In addition, it also allows a quantification of the general rules concerning the spat holding capacity for a given nursery. Finally, the total biomass stock that can be maintained will depend on the quality of the spat.

      The cost of producing a species like oyster for commercial purposes needs to be limited. The technique applied in this paper to get there is sound for oyster farmers. It is important to know all the biological and ecological aspects related to the good development of this species. The food aspect is a primordial resource in the growth of a species. Therefore, the model proposed in this study is useful to better understand what the essential nutrient inputs for the good development of the oyster are. However, there are many factors influencing the growth of these organisms and further study to refine the model may be required.


      The model:

      • Presented provides an assessment of the seed stock limitations in an extensive commercial oyster nursery that can function with respect to food limitation.
      • Provide valid indications on the limits of the maximum stock in a given nursery or on the food requirements of a given spat stock for optimal rearing conditions.
      • Is intended for managers of commercial operations and can be used online.
      • Can be developed based on feedback from the growers regarding its usefulness.

      In addition, other features they consider important could be included, as well as other oyster species. We can also apply it on other biological models with a strong economic interest and whose physiology of the species is suitable for study in a controlled environment.

      This model is applicable to species whose production cycle can be controlled. Indeed, the model studied is a tool that tells us the maximum amount of food necessary for the proper development of the species. Therefore, it cannot be adapted to species that cannot be analyzed in a closed and controlled space. It should be noted that linking statistics to biology is a fundamental approach to evaluate and understand a species in the best possible way while including other parameters that may have a negative or positive effect on it. Furthermore, it would be interesting to extend this study to other oyster species and other organisms concerned by these culture systems. In addition, as the authors say, it would be interesting to consider the opinion, yields of shellfish farmers in order to improve the model for a better management aid.

      Read the full study: Nobre A. M., Soares F., Ferreira J. G. (2017) A Mass Balance Model to Assess Food Limitation in Commercial Oyster Nurseries. Journal of Shellfish Research, 36(3), 738-748.

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    • How raising snails allows to better understand the dynamics of a parasite?by Thomas Boyer and Thibault Dindart

      Published by Charlotte Recapet the July 19, 2021 on 9:38 AM

      The analysis of epidemiological dynamics depends on host and parasite interactions. But these interactions fluctuate because hosts and parasites are heterogeneous entities that exist in dynamic environments. Resource availability is a powerful environmental constraint of intra-host infection dynamics (temporal patterns of growth, reproduction, parasite production and survival).

      In this study, researchers developed, parameterized and validated an explicit resource infection dynamics model by incorporating a parasitism module in the energy balance theory. The mechanisms explained are the multivariate dynamic responses of the human parasite Schistosoma mansoni and its intermediate host snail to resource variation and host density. This parasite is widespread in Africa and inter-tropical America. Worldwide, more than 200 million people are infected with it, 9 million suffering from its symptoms, which cause more than 200,000 deaths every year. It tends to have erratic localizations (liver, spleen) and the accumulation in these organs of lost eggs makes the severity of the infection.
      The most common symptoms are diarrhea and even dysentery. Complications can appear such as rectal prolapse, fistulas, occlusion, appendicitis.

      To do this, they have parameterized the model using an experiment that manipulates food resources and follows the growth, reproduction, parasite production and survival of snail hosts. The model is then validated by simulating the dynamics of infection for host individuals undergoing different levels of intraspecific competition and comparing these predictions with the results of another experiment that manipulated host and resource density, and hence the intensity of resource competition

      This bioenergy perspective suggests that variation in resource availability and competition could explain the infective dynamics of this parasite. To begin with, total cercaria production could be low when snail densities are low (because there are few infected snails) or when snail densities are high (if competition limits per capita parasite production). This potential relationship between snail density and the risk of human exposure could be the reason for the success or failure of current and proposed methods of schistosome control, which depend on reducing the density of snail vectors by molluscicides or predators. If resource competition in natural snail populations is strong enough, then snail reduction programmes could backfire, as reducing intermediate host densities could free the remaining hosts from resource competition, thereby increasing parasite production rates per host.

      To conclude this bio-energetic model seems to indicate that the fight against the parasite Schistosoma mansoni begins with the regulation of snail populations (first host of the cycle). In order to better regulate this infectious dynamics, measures must be taken collectively between the countries concerned and must be directed towards a total reduction of snails or a limitation by intra-species competition. This model would establish priority levels of parasite infection for certain areas and would be followed by measures to control the parasite. A first measure would be the installation of pipelines to prevent the release of infected faeces into watercourses.

      Read the full study: Civitello, D.J., Fatima, H., Johnson, L.R., Nisbet, R.M. and Rohr, J.R. (2018), Bioenergetic theory predicts infection dynamics of human schistosomes in intermediate host snails across ecological gradients. Ecol Lett, 21: 692-701.

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    • Organisms and their environment: Dangerous liaisonsby Marius Dhamelincourt and Charlie Sarran

      Published by Charlotte Recapet the October 14, 2019 on 9:15 AM


              Whatever the incredulous think, organisms are necessarily linked to their environment to survive, science says. However, this relation, unstable, can be problematic for those concerned when variations overtake their acceptable thresholds. Although often natural, these variations can be exacerbated by anthropogenic actions, like putting fish on a grill.

              Global changes are often reduced to temperature increases, illustrated in the media with alarming news about ice melting and forest fires. While many people thus omit the complex mechanisms behind this black box, the necessity of a more “polar bear’s” respectful way of life is commonly accepted. In order to better understand how to respect such adorable creatures, scientists need to investigate their relationships with the environment.

      Chapter 1: Shells under investigation

              In order to scrutinize these relationships, selected species must be accessible, easy to catch/manipulate and in sufficient number. For instance, the study of the great white shark aggressivity over humans would require too many intern’s sacrifices. In response to this challenge, a valiant research team from Germany looked for the importance of these relations by studying in the Rhine a remarkable (body and soul), accessible, cheap and lovely species: Corbicula fluminea, a shell. This study is related to the mass mortalities events of this species, which occurred in the summer, especially in that of 2003. Their investigations aim to understand how these organisms are linked with their environment, and their reactions to changes.

      Chapter 2: Shells cooking in science

      Many tools exist to perform this type of search. Field searches can involve the scientist’s life (be bitten by a pigeon is a terrible experience…) and obstruct a long-term individual experiment. Now that researchers have selected the perfect organism, they must choose an appropriate way to analyse their problem. For this shell, scientists chose to use a modelling approach, a method dark and full of terrors. More precisely, they modelled several aspects of the metabolism of this organism at different scales: individual and population levels, using respectively DEBM (Dynamic Energy Budget Model) and PSPM (Physiologically Structured Population Model). This method, widespread in ecology, consists to “simulate the annual growth in length and mass and the reproductive success under different environmental scenarios”. This approach is suitable because an organism can respond differently relatively to their peers. Such fact can be proved by looking at many places and species, humans included… Ultimately, scientists aim to better understand the complex relation between the energy available in the environment and its utilization by shells.

      Chapter 3: Corbicula’s deadly summers

      Heat waves are often responsible for changes that every scientist can observe on living organisms. For instance, it is known that coral reefs are affected by increased temperatures, as shown in an article published in “Free Radical Biology and Medicine”. Many other examples such as lobster’s behavioural response to boiling water could be developed. Regarding our shellfish, scientists found an interesting pattern comforting our previous remark: temperature causes shell’s mortalities… Oh wait, no, it’s more complicated.

      In fact, mass mortalities events were probably related to a melting pot of many events like temperature increases and/or starvation. Moreover, these situations are also in relation with individual conditions. Indeed, researchers hypothesized that a combination of factors (biotic and/or abiotic), usually non-lethal under regular summers, can be problematic at high temperatures. Unfortunately, models developed were not able to explain completely the observed mass lethal events.

      Chapter 4: Life is not so simple

      Researchers finally enhance the comprehension of population dynamics, enlightening its complex mechanisms. However, in such cases, wishing to be exhaustive is useless and unproductive, like politics. That is why scientists look for compromise between easy-to-use and complicated (highly realistic) tools. For example, the authors of the Corbicula’s study proposed that it could be interesting to test other parameters, such as parasitism.

      To put it in a nutshell (you got it, right?), things are not always what they seem to be, even in environmental studies. Main hypotheses are not always validated, and measures considered can be only a part of a more complex system, or sometimes even unappropriated. On the flip side, model’s development can help to understand the life cycle of organisms like Corbicula, thus helping to manage populations concerned.

      Cited study: Petter, G., Weitere, M., Richter, O., Moenickes, S., 2014. Consequences of altered temperature and food conditions for individuals and populations: a Dynamic Energy Budget analysis for Corbicula fluminea in the Rhine. Freshwater Biology 59, 832–846.

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    • Identifying the optimal depth for mussel suspended culture in shallow and turbid Yann Godard and Louna Riem

      Published by Charlotte Recapet the February 1, 2019 on 4:52 PM

      Bivalve aquaculture is commonly carried out in shallow water where there is an important influence of winds, tide and currents. These factors can lead to the remobilization of particulate matter which is an important source of food for bivalves. However, the concentration of the particulate matter in water can be minimized by the filtration capacity of high density cultured bivalves. In order to improve the productivity of cultured bivalves, it is important to take into account these different parameters. Indeed, it will help to know where is the best place for bivalve’s growth.

      Skive Fjord mussel farms -

      A study conducted in a Fjord in Denmark suggests that the TPM (Total Paticulate Matter) in the environment of culture is very important for the growth of mussel. The autors have built two models to understand the mechanism:

      • The first model try to give a pattern of the vertical distribution of resuspended materiel by including, among others, the particles concentration at 1m above the seafloor
      • The second model allows seeing the growth mussel at different height in the water column and calculates growth rates. This is made under different scenario of food availability.

      Both of these models allow determining the optimal localization of mussel in the water column for the better production.

      Firstly, the authors have determined the characterization of seston in the long-term and in the short-term. They observe that there was important correlation between wind and TPM but with a lag of 9 hours. Thus, they say that the remobilization of particles is not depending of the wind in the farm but of the remobilization in another place in the fjord for the short-term seston characterization. These particles are then transported by water current until the Fjord. Moreover, a correlation is also observed between the chlorophyll concentration and the TPM but not between the water velocity and TPM.

      For the long-term, they were able to highlight that the repartition of seston, and particulary phytoplankton in the water column was not homogeneous. There is a difference in concentration at the bottom and at the top with more phytoplankton at the surface (because of the lightening).

      Secondly, the authors wanted to characterize the growth of mussels under different conditions of availability of food with the hypothesis that the concentration of phytoplankton is homogeneous in the water column. The results say that in reality, there is more phytoplankton in the surface, then, the growth in the top is not very well implemented in the model. They conclude that it is important to consider the position of bivalves in the water column for an optimal growth.

      The dynamic energetic budget model shows that it is important to have a lot of phytoplankton and less detritus in order to get a better growth whatever the position of the culture. Moreover, this model allows showing that the variation of growth between the top and the bottom is only 2.6% which is negligible. They conclude this part by suggesting that “The reduced impact of height above the seafloor on mussel growth is related to the small contribution of resuspended material compared to the high background concentration of detrital matter”.

      This study takes place in a Fjord where the conditions are highly variable (changes in the tide, current, wind…). This is not implemented in the model. Therefore, it could be interesting to add some hydrodynamics parameters in the model in order to adjust it. Moreover, to improve the model, it would be wise to include some biological mussel parameter like the energetic costs of pre-ingestive sorting and pseudofaeces production. This model could be experimented in different places and different moment to validate it.

      Cited study: Filgueira, R., Grant, J., and Petersen, J. K. (2018). Identifying the optimal depth for mussel suspended culture in shallow and turbid environments. Journal of Sea Research 132, 15-23.

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    • Global change and climate-driven invasion of the Pacific oyster along European coasts: a bioenergetics modelling approachby Mélanie Gouaux and Lise Guégniard

      Published by Charlotte Recapet the January 8, 2019 on 3:20 PM

      Environmental changes such as seawater warming, and coastal eutrophication have an impact on breeding, larval survival and recruitment of marine benthic species. Global change induces changes in the natural distributions of native species and facilitates the spread of non-native species. Nowadays, the spread of non-native species in marine ecosystems around the world is one of the most serious environmental concerns. In receiving ecosystems, consequences of biological invasions are readily identifiable when invasive species are ecosystem engineers such as polychaetes or bivalves.

      Scientists of the French Research Institute for the Exploitation of the Sea and of the university of Nantes investigated how global change relates to the invasion of European coasts by a non-native marine invertebrate, the Pacific oyster Crassostrea gigas.

      This species was introduced on the European coasts of the Atlantic at the end of the 19th century for shellfish culture purposes and is the main oyster species farmed in Europe today. In recent decades, the Pacific oyster has acquired invasive species status with the expansion of its biogeographic distribution along the northwestern European coast beyond its initial zone of introduction into sites breeding. Bourgneuf Bay on the French Atlantic coast was considered as the northern boundary of C. gigas expansion at the time of its introduction to Europe in the 1970s. From this latitudinal reference, variations in the spatial distribution of the C. gigas  reproductive niche were analysed along the northwestern European coast from Gibraltar to Norway.

      A bed of Pacific oyster Crassostrea gigas in the Netherlands - Bas Kers - CC BY-NC-SA 2.0

      Mechanistic models are valuable tools for this purpose, and modelling approaches are useful for gaining a quantitative understanding of the effects of environmental changes on marine communities, and predicting their responses to projected climatic trends.

      The use of the IBM and DEB models has shown results at different scales, at the individual scale, at the Bourgneuf Bay scale and at the European scale.

      At the individual level, the results showed interannual variability of the dry flesh mass (DFM). A loss of DFM is explained by a spawning event. There has been a significant increase in DFM and the number of oocytes in recent years. To explain these results, they studied the relationship between the environmental conditions in late spring and the characteristics of the oyster. Then they achieved positive relationships between DFM and phytoplankton. Likewise, between the number of accumulated oocytes and phytoplankton. Here, phytoplankton seems to be the cause of this increase of DFM and the number of oocytes in recent years for this species of oyster. Here, phytoplankton seems to be the cause of this increase of DFM and the number of oocytes in recent years for this species of oyster. Moreover, according to the individual model, the higher the temperature of the sea surface, the earlier the clutches (June-July). While a cooler sea surface temperature will result in late laying (August-September).

      Then, they applied the same models to the bay of Bourgneuf. The results showed an increase in the temperature of the sea surface in the bay, but also the effects of the temperature of the water on the laying. Indeed, as at the individual scale, the high-water temperatures lead to prose spits and vice versa.

      Results at the European level showed a change in the geographic limit of spawning habitat, regardless of phytoplankton concentration. In 1986, the limit was located at the level of the Loire estuary with a south-north spawning gradient, earlier in the south of Europe. In 2003, this limit moved completely to the north of Europe. This change can be explained by the global warming of the waters.

      Other studies have highlighted other results. Indeed, using a Degree / day model, Ifermer has demonstrated the importance of the nutrient pool for egg laying. Indeed, in recent years, laying is actually late because of the low nutritional value of phytoplankton. The warming of the waters would therefore cause the loss of the oyster's nutrient pool and thus a delay in laying eggs.

      Cited study: Yoann, T., et al. (2016). Global change and climate-driven invasion of the Pacific oyster (Crassostrea gigas) along European coasts: a bioenergetics modelling approach. Journal of Biogeography 43(3), 568-579.

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    • Use of dynamic energy budget and individual based models to simulate the dynamics of cultivated oyster populationsby Maxime Rochet and Jean-Baptiste Valerdi

      Published by Charlotte Recapet the October 8, 2018 on 12:45 PM

      This paper deals with a test of Dynamic Energy Budget (DEB) apply for predictions of the oyster Crassostrea gigas production in Thau Lagoon. The DEB model is based on physiological and environmental parameters, he predict the growth at indivual level. In the case of oyster production the prediction must be applicate at the cohort levels, its why they choose to integrate the DEB model into a population dynamics concept. Population model choose its the IBM (Indivifual Based Model) method, the equations are used for the predict the harvested production and the stocks in place (total number of individuals).  The advantage of this integration its to assess the effect of ecosystem changes on oyster production.

      Oyster farming in the Thau Lagoon - Olivier Pessin - CC BY-SA 3.0

      The models recently used (DEB) have been compared with a more common prediction tool. The partial differential equation (PDE) are empirical equations used for the growth prediction between different class and simulate by individual total mass. This equation are more straightforward than the DEB-IBM models but they use only a single variable to represent individual growth. The DEB model integrate two variable of calibration, the other parameters of the differents equations were estimated from independent datasets using comprehensive studies of oyster growth and ecophysiology under controled conditions. The calibrated parameters are the chlorophyll a concentration proxy of the phytoplacton biomass (principal food of oysters) and the température linked to assimilation and maintenance rates. This technique modelise by this way the indivdual capatcity of food assimilation and the allocation of energy between energical reserve, structural tissues ans reproductive structure and maintenance. To be more likelihood a growth variability showing variability between individuals have been implanted. Some variability have been implanted into the prediction of PDE method and the DEB-IBM model. This variability was integrate by diffusion to reproduce the variability between individual growth in the PDE and by Xk (half saturation coefficient) variability in the DEB-IBM case.

      The results of the differents simulations have proved a good capacity for the DEB-IBM model to predict the stocks and the harvest productions. The data estimated are close to the observed. He have to advantages to be generic, easy to etablish by the low number of measurables parameters. With the results showed in the study (see figure below) his capacity to take account of the environmment variables have been proved too . The limits are detectable in his sensivity to the variability and the large number of parameters estimated can induce in error.

      From Bacher & Gangnery 2006.

      The comparison of the two models have show the effect of the variability in the predictions values. The values predicted by the DEB-IBM model look closer to the observation than the PDE predictions. For exemple the harvestes productions have been estimated earlier by the PDE method than the DEB-IBM, so the modelisation of DEB parameters can influe strongly the dynamic population and the production previsions.

      Cited study: Bacher, C., & Gangnery A. (2006). Use of dynamic energy budget and individual based models to simulate the dynamics of cultivated oyster populations. Journal of Sea Research 56(2), 140-155.

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    • Effects of an exotic prey species on a native specialist: Example of the snail kiteby Mathieu Finkler and Hyppolyte Terrones

      Published by Charlotte Recapet the September 11, 2018 on 12:34 PM

      Exotic species have largely been studied over the years, their effects on native populations, their consequences... Most of the studies aim to see the competition between a native species and an exotic one. Here the study focus on the effect of an exotic species on a native predator.

      Florida snail kite (Rostrhamus sociabilis plumbeus) are endangered, their populations are drastically declining in recent years. It is important to study them and to determine why their numbers are falling to implement an adapted conservation strategy.

      The purpose of the study is to assess the effects of the recently introduced island apple snail (Pomacea insularum) on snail kite behavior and energetics comparing with the native prey (Pomacea paludosa).

      Juvenile snail kite - Cláudio Dias Timm - CC BY-NC-SA 2.0

      The authors determined different parameters such as the proportion of snail dropped, the searching and handling time, the consumption rate and proportion of time in flight. Caloric intake of both species has been determined by a model (Sykes 1987) and so is the daily energetic expenditure. Caloric balance seems to be perfectly suitable in this case because the difference in intake calories could affect all the life history traits and be the cause of the fast decline of the kites.

      Foraging on exotic snails led to a greater proportion of snails dropped, a lower consumption rate, a longer handling time and a lower energy balance (figure below). These conclusions are particularly true and worrying for juveniles. This results indicates that feeding on exotic snails will decrease their energy and so less energy will be available for others activities (like reproduction, growth, defence against predators...). Finally, lakes where only exotic species are present (Tohopekaliga) could form an ecological trap.

      From Cattau et al. 2010

      Even after this study, it will be hard to conclude on an optimal foraging theory because both snail species were never found together in a lake. Therefore it could be interesting to make the same study in a lake were both species are present. Furthermore, this study has been conducted during the breeding period. During breeding period, species will need more energy to feed their offspring, to protect them, potentially leading to a greater difference in energetic balance.

      Further studies may focus on the fact that kites feed on larger exotic preys (compared to native preys). Are the smallest individuals not available for kites or do kites choose to feed on larger exotic preys to compensate for their lower energetic content ?

      This method could be used in others studies of trophic relationships and not only on native-exotic conflict. For example if human overfish a species, the predator of this species will have to change preys. So it will be important to calculate the energetic balance with the new prey.

      Cited study: Cattau, C. E., Martin, J., & Kitchens, W. M. (2010). Effects of an exotic prey species on a native specialist: example of the snail kite. Biological Conservation, 143(2), 513-520.

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      This post is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.