Help! How do I manage my commercial oyster nursery?by Coline Arqué, Marylou Pourret and Robin Thibault
Published by 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.
Objectives:
- 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?
Crassostrea gigas (Thunberg, 1793)
- 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.
[Food]Nursery
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.
Temperature
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.
RESULTS
|
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.
ASSISTANCE TO MANAGERS
In order to promote widespread use, the model described in this paper for Pacific oyster nurseries is made available online: http://seaplusplus4.com/oysterspatbud.html. 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
MODEL APPLICATION TO FARMS
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.
TO CONCLUDE …
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.
Mytilus edulis (Linnaeus, 1758)
Ruditapes philippinarum (Adams et Reeve, 1850)
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.
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